Does Comfrey Really Improve Soil?

All of us who have studied permaculture have heard some impressive claims about comfrey. It is a dynamic nutrient accumulator; it improves the soil; it is “a slow motion fountain” of nutrients, bringing them up from the subsoil to improve the topsoil. We’ve heard lots of anecdotal evidence, but where is the empirical data for these claims? Peter Harper’s article in The Land last summer, “Permaculture: the Big Rock Candy Mountain,” made me want to find the scientific basis behind some of the anecdotal claims I’d heard other permaculturalists make, that I’d previously absorbed without question.

I ran to my books. From Introduction to Permaculture by Mollison and Slay to Gaia’s Garden by Hemenway to Edible Forest Gardens (Volume I & Volume II) by Jacke and Toensmeier, it is difficult to tell where any specific piece of information comes from, since all these books lack footnotes or endnotes. The Wikipedia article is peppered with “[citation needed]” and has been for years. The online database that comes closest to citing a source is Plants for a Future, which says comfrey’s value in compost was established by “a small booklet” (no indication where to obtain it) and the New RHS Dictionary of Gardening, which “contains a number of silly mistakes.” Clearly some newer and more conclusive evidence is in order!


I was feeling pretty foolish about planting so much comfrey around my small urban farm (about 20 plants in all) without any hard evidence for its effectiveness, when I realized I could just do a soil test and compare it to my past years’ tests, and then I would have at least one data point to support or refute the claims. The test results are pictured here, and they will benefit from some explanation.

Click for larger view

The first test is from February 2009, after our front yard had sat under sheet mulch all winter and our back yard was still mostly turf grass. The mulched front yard already shows a lower pH (we have alkaline clay soil here, so lower is better), more than double the organic matter, three times the nitrate, and about 50% higher phosphorus and potassium.

By the time of the second test in 2011, I had sheet mulched the back yard as well, but I distinguished between a “raised” bed where a sewer repair had exposed the heavy clay subsoil and a greenhouse area which had not been excavated. The greenhouse bed shows higher nutrient levels than either the front yard or the raised bed, with the exception of potassium, but even this is higher than it had been two years before.

So far so good, and I think I’ve made a good case for sheet mulching, but all these figures are blown away by the sample I took this year under the comfrey plants. After 5 years of comfrey, the topsoil in this sample shows a lower pH and higher percent organic matter than any of the previous samples, and the nutrient levels are practically off the charts – a 47 to 232% increase over the previously observed highs. I did not test for calcium or magnesium either before or after, but just on the basis of NPK the comfrey is completely vindicated.

It appears I owe comfrey and its promoters an apology, but still this is just one data point. To really make the case for comfrey, we will need lots of data from lots of soil types, and we need to publish the results in a reputable source that can be cited by Wikipedia, PFAF, and all the reference books we turn to for authoritative information. And we need to collect negative results (if any) as well as positive ones. Do you have any data on comfrey to share? Please let me know in the comments below!

Further Reading:



65 thoughts on “Does Comfrey Really Improve Soil?

  1. Lawrence Hills wrote several books summarizing his comfrey studies for the Henry Doubleday Research Center in England. I suspect that is what PFAF was referring to. There is indeed a lack of strong data on other “dynamic accumulators” though. Look for Robert Kourick’s updating to his classic DA chapter in a forthcoming Permaculture Activist.

    1. Yes indeed Eric. I have the books in question too. Lawrence D Hills carried out extensive trials, at his Henry Doulbeday Research station and Trial grounds, at Brooking, Essex, UK. His experiments and analysis were though and scientifically rigid in their methodology and presentation. His results were presented in the Comfrey Report (published by the HDRA in 1975). There is also a good summery of these experiments in Hills book ‘Fertility Without Fertilisers,’ also published by the HDRA. All of the results were laboratory tested by the Michaelis Nutritional Research Laboratory. The HDRA later changed their name to Planet Organic. They may be able to supply copies of the report to you. Hills has ben overlooked by members of the Permaculture movement, which is a great shame, as he pioneered so much of the early work which was the forerunner of permaculture. he has always been an hero of mine too (He was also the first person to introduce comfrey into the UK.

      1. Does anyone know if comfrey also collects lead from the soil. We have a low/moderate lead issue at our community garden and I am assuming that using comfrey as a mulch would only keep recycling the lead in the topsoil.

        1. All plants will typically accumulate heavy metals such as lead in the highest concentrations in their roots, they will accumulate smaller but still significant amounts in their leaves and shoots, and no detectable amounts in flowers/fruit. I would use this as a guide for what you plant in your community garden. Root crops would be a real issue, as would edible greens.

          In regards to other comments, this is probably the only valid use of chemical testing in my mind, determining contaminant levels. We know that if we do things naturally, things do what they’re supposed to do in nature – such as comfrey nutrient mining and adding beneficial nutrients to the soil, why test for what we know happens. It should be common sense that the nutrients accumulated in comfrey leaves go back into the soil, that’s where everything goes ultimately, even sceptical humans, that’s basic biology.

          When it comes to human created damage, lead poisoning of the soil and such, then it makes sense to use the technology to determine the extent to which we have poisoned our surroundings – something that is not natural, using our science to gauge our damage, nature takes care of itself in most circumstances, especially without humans.

        2. all accumulator plants should be harvested and comosted away from site as a guarntee that heavy metals are not being recycled. In a compost they can be reduced by fungi. regards pete

    1. G’day David,

      Forget seeds – go for a root cutting. I have many dozens of plants that all originated from a single cutting the size of my thumb. Whilst your conditions may differ from mine, I have had success in some of the toughest driest and inhospitable places. It seems sometimes as if nothing is going to happen, then when I have forgotten all about it it will send up a leaf or two and then it is there come drought or floods it will continue to survive and usually thrive. Keep in mind though – once it is growing, it is difficult to get rid of it as the smallest bit of root left in the ground always seems to shoot back up again.

    2. Hey David, I recommend staying a country mile away from any comfrey that reproduces by seed. It becomes unmanageable! Stick with the comfrey that only reproduces by being dug up and bits of roots being moved around.

  2. the book you are looking for is a book written on the bocking comfrey trials in Britain, they trialed many cultivars of comfrey for the effectiveness in each field(fertilizer, animal feed, medicine, etc..) they ended up with known varieties for certain jobs. the most common imo bocking 14 comfrey.

    don’t forget, everything is a dynamic accumulator, its just how much and of what does it accumulate.

  3. I just in the last few weeks planted a few comfrey roots. It’s a good opportunity to get a baseline soil test in that spot with this acidic red clay to make future comparisons.

  4. I wholeheartedly concur with the need for documentation. But this is where I end up thinking about invisible structures once again – in this case, economics, in particular – something for which I’m often chastised. I say this as people on the ground who are realising gardening successes have little to no incentive to document the details of those successes (it takes time – for which they are not paid, and too few have any spare time for such things), let alone to share that documentation in a form suitable for inclusion in an appropriate database. In contrast, we have universities which are being sponsored by large agribusinesses, where they expect a certain result, and thus are returned the results they’ve paid for, so as to continue receiving funding – that result being their documenting the ‘need’ for products those agribusinesses happen to provide (fertilisers, gmos, etc.).

    In an ideal world (one I work towards as best I can) I’d like to see publicly funded research stations in all kinds of climate niches, whose sole purpose is to trial all kinds of methods (and not just in gardening, but also building methods, energy systems, etc.), with the goal of sharing the results of those trials with the communities in their vicinity, and in similar climes across the world. This is where common needs can fund common solutions that can be ‘freely’ shared. Instead, we live in a world of competition, where any kind of solution must be patented, and protected lest someone else gets hold of it…. Privatisation, rather than publicly owned, has a lot to answer for, and goes a long way towards explaining why we have insufficient data to bolster our own personal observations.

  5. Thanks for sharing your project & documenting the changes with the soil analysis.
    This book is available for download from the online Soil and Health Holistic Agriculture Library:
    Hills, Lawrence D. Russian Comfrey: A Hundred Tons an Acre of Stock or Compost for Farm, Garden or Smallholding. London: Faber and Faber Limited, 1953.
    I have grown and used comfrey in my forest garden & garden as a mulch, sheet mulch and compost and also as a tea extract for over 20 years. Some of my fruit trees in the forest garden mostly and sometimes only receive a mulch of comfrey as annual nutrient input. I have also witnessed the fertility of the soil increase but I haven’t documented it through the years. Although I have a soil analysis from the first year and I could do another analysis now; I would need to mention that some of the trees might have received other input of mulch & compost. So, I couldn’t claim that comfrey did it all; although I know that comfrey did most of it.

    1. Hi Greg, just starting landcare funded tests on comfrey inc soil analysis on my French Island farm – did you end up doing soil tests – love to understand more

  6. Brilliant, cheers for this its definitely much needed to see some data to back up the anecdotal evidence even if it is only done with the limited resources available to the most of us. You mention there was increases in N,P and K but can you let us know what the increases were for each particular one as its good to know what different plants accumulate depending on your particular needs

  7. Very glad to see people referencing Hills in the comments as I would have been very surprised if Doubleday did not have detailed studies on Comfrey. My grandfather was a member of Doubleday and there was not a day that did not pass where Comfrey came up in the conversation such was his opinion of it and my memories are that there were books on the topic.

    1. I was a member too Dean. members would share information and topics ad Lawrence would print many of the trails up that members had carried out on behalf of the association. His
      work is still very important, I reread his books often. And I bought my first comfrey plants fro the HHRA. Back it those das it was considered a good plant to use in cooking too, until later tests revealed it could carry links carcinogenic cancer.

    2. Love to hear more Dean as just starting some landcare funded trials on comfrey at my 50 acre farm on French Island, one hour from Melbourne

      1. Hi Graeme reading your article in the Weekly Times and have a dear old friend who lives near me who would like to try your cream you have produced. I live in Allendale outside of Creswick or half hour from Ballarat. Hope to hear from you soon as to where we can purchase this cream and how much.
        Cheers Jenny

  8. Wow your stats are impressive. I grow comfrey and make a liquid foliar feed from the leaves, but as with most gardeners that is not all I do. I make and add lots of my own compost, I add lime, manure and minerals. This year I added in ground worm buckets as well. Every year my soil gets better and better, but in order to isolate what the comfrey itself does someone would have to do a controlled experiment. I look forward to seeing what happens as a result of this discussion.

  9. Nice writeup, but I would also love to see soil results from areas which had been sheet mulched for 5 years but NOT had comfrey added, and/or spots which got comfrey but no sheet mulch, for real control plots…as it is, it’s hard to tell how much of the gain can be attributed to sheet mulching alone?

    In my own experience, on my dry, sandy soils (SW Michigan, zone 5b, ~34″ rainfall/yr., flat terrain), I have a hard time just keeping comfrey alive. Even in areas where it’s planted into a sheet mulch near a young tree, it is hardly thriving after 3 years in the ground (nor are most of the trees putting on a lot of growth; not sure if that’s causation or just correlation—in most cases there are 2 comfrey plants per tree, each planted maybe 2′ from the trunk on opposite sides in a 4-5′ ring of sheet mulch). However, where I’ve planted comfrey into hugelkultur beds, richer soils, or spots that get watered more, it has done better and I’m able to chop a decent amount of it for biomass and mulch a few times per year.

    A friend in our local PC group did several test plots with young fruit trees and found that in each case, the trees WITHOUT comfrey planted near them put on significantly better growth, all other conditions being the same. He attributes the stunted growth of the comfrey-paired trees to competition for water or other soil resources. I know it sounds like heresy and flies in the face of conventional wisdom, but is a reminder of the importance of trying things out on our own sites at a small scale before mass-implementation. Maybe planting it a bit farther out from the root zone is key on difficult sites in the first years?

    Similarly heretic is Mark Shepard’s claim that he’s seen little-to-no positive effect from planting woody Nitrogen-fixers between his (100+ acres of) tree crops—though he says that he’s seen some benefit from herbaceous N-fixers like clover. Hard to believe, as most of the literature advises it, and I’ve seen studies showing that N-fixing nurse crops definitely speed growth for others, but I trust Mark’s many years of experience, and he’s planted (and observed) more trees than many of us probably ever will. There are no one-size-fits all solutions, so many things are site-specific. Just food for thought, thanks to all who contribute to this amazing site.

    1. Really fair points re comfrey planted near young fruit trees and also woody n-fixers. You have to In The case if young fruit trees and young comfrey plants, look at where they are competing……obviously both at this stage are going to be shallow rooted and narrow in their spread, therefore competitive….. Given time they would end up in different niches (wide and shallow for the fruit trees and narrow and deep for the comfrey. It is therefore a matter of timing one must go first and the other later…either wait until the fruit trees are established enough to accommodate the comfrey until it moves out of the competitive horizon or let the comfrey establish and prepare then go with the fruit trees that may well benefit from the increased nutrient, organic matter and mulching properties of the comfrey….. Of course the transplants themselves must be tall enough to cope with the potential height of the comfrey in ideal conditions.

      As far as woody n-fixers, it is my belief that they only provide real benefit as regards nitrogen deposition once cut back causing root shedding, although they can provide a quick growing shelter for younger, slower growing plants from wind/strong sun etc.

      1. Just planted lots of comfrey around new 60 tree espalier fruit garden with help from Workaway visitors – some ignored my instructions to plant 3 comfrey at 120 degrees apart some metre from new tree – so some are very close — after one year the trees are going well and got first peach from tree with comfrey right next to it – early days of observation so would love to see how you are using the comfrey

    2. >>Similarly heretic is Mark Shepard’s claim that he’s seen little-to-no positive effect from planting woody Nitrogen-fixers between his (100+ acres of) tree crops—though he says that he’s seen some benefit from herbaceous N-fixers like clover. Hard to believe, as most of the literature advises it,…<<

      You don't say whether he chops and drops the foliage from these woody N-fixers or they are just growing there. Legumes only fix N from the air if there is a deficiency in the soil, and the fixed N is taken up in the roots of the bacterial host plant (i.e. the legume). It only returns to the soil if all or part of the plant is allowed to decompose on-site. Of course natural leaf drop will (slowly) achieve this, but chopping and dropping the foliage will (probably) introduce N to the soil more quickly. In addition, if the legumes are not pruned there could be root competition between them and the tree crops (heavy pruning will cause reduction in the roots of the legumes, which can potentially keep them away from the roots of the tree crops).

      Chopping and dropping the foliage of woody N-fixers has the advantage of providing a more of a balanced dose of carbon and nitrogen to the soil than annual legumes would, and also the woodiness results in slower decomposition which, among other things, helps to ensure that the N stays available in the soil.

      I do have some doubts about the practice of chopping and dropping onto the surface, rather than putting the prunings under a mulch layer, where logically the moist environment would facilitate quicker breakdown of outer layers of leaves and of the woody material, whereas I'd assume that material lying and drying on the surface could lose N before it is broken down. But I haven't yet been able to find any literature on this – which is of course one of the reasons for consulting the scientific literature: to find answers to things we aren't able to research for ourselves, as well as helping to avoid making fools of ourselves by spreading poorly understood "facts".

      Regarding Craig's assertion that
      "universities which are being sponsored by large agribusinesses, where they expect a certain result, and thus are returned the results they’ve paid for, so as to continue receiving funding – that result being their documenting the ‘need’ for products those agribusinesses happen to provide (fertilisers, gmos, etc.)."
      Well, maybe in some instances, but if he thinks that this means that the majority of published articles on matters that could be relevant to permaculture contain dishonest or skewed conclusions, all I can suggest is that he spends a bit more time in the literature using a good bibliographic search engine.

      1. Gordon, if we look at what nature does, it clearly ‘chops & drops’ on the surface of the soil. Understanding biology, the class of organisms termed decomposers serve the purpose of taking that material into the soil. The top layer is the mulch, like on a forest floor. It’s a first-in, first out decomposition process where the last addition serves as mulch, to be later covered by additional organic matter on the surface. It has worked for over 400 million years! Do people really need journals to tell them this or a basic grounding in the first principles of biology and ecology to be able to see this? I’m fortunate to have studied the life sciences, but you can understand this stuff without having to get degrees in the subject.

        If you doubt any permaculture articles and their facts, don’t adopt the ‘learned helplessness’ posture of the majority of the general public, and await the dogma of the high priesthood in white lab coats to hand down the holy writ in a published journal, that’s the secular substitute religion of scientism, empower yourself and go out and try it, if you don’t trust others findings. All good science is repeatable, you just need the method. If it works for you, that’s all that matters ultimately…

        1. So that’s what people learn in the life sciences Angelo. Coming from a generation where these things were called ecology, zoology, botany, microbiology, biochemistry, etc. I’d always assumed that “the life sciences” were about useful knowledge and skills for making ones way through life, like philosophy, logic, methods of inquiry, modes of cooperative action, humility, …. Clearly I was wrong.

          So what you are saying, if I may summarise, is that what’s on the ground is mulch, and chop & drop (C&D) prunings go on top of that and they become mulch and the original mulch layer is decomposed, then another layer of prunings goes on top, etc. etc.

          That’s good as far as it goes, and works really well in a rainforest, or any fairly closed forest in a relatively wet environment. But we don’t (and can’t) grow most of our food requirements in closed forests, it has to be more like open woodland, savanna or even (god forbid) grassland. And let’s not forget that Australia is the driest inhabited continent, and most of the country is drier than the near-coastal areas and wet forest areas where the majority of our permaculture gurus tend to live.

          So the environment for C&D material in the average enterprise geared to producing a basic diet is rather more exposed to drying forces (e.g. wind and high temperature) and sunlight, all of which are fairly lethal to soil organisms, including a lot of the “class of organisms termed decomposers”. And in addition, these conditions will further harden the already tough layers of leaf cuticle and stem bark – making it more difficult for surviving decomposers to begin the breakdown process.

          Further, there will not be a more or less constant rain of C&D’d vegetable material onto the ground. The typical Australian climate does not lead to rates of growth that allow this – the addition of C&D material will be intermittent, and a lot of the time the new material will dry out before more is added. And anyway, do you remember the guideline that C&D should be done only when precipitation exceeds evaporation? That’s only for a very short time each year over most of the inhabited part of Australia. So unless C&D’d material is put under the mulch, most of the time it is going to dry out, as described above.

          So, if you are still with me Angelo, we have moved away from your simple “damp mulch layer on layer” scenario, and have arrived at the much more common C&D situation that I referred to in my previous comment, where I suspect that “material lying and drying on the surface could lose N before it is broken down” – and possibly other nutrients as well. This isn’t some far fetched scenario. I’ve had the experience of cutting huge heaps of grass and seeing them lie, virtually unchanged, on the surface for four years when I first came to the Lockyer Valley. Because it’s dry most of the year. Because it is exposed to sunlight, high temperatures and winds. Because in this condition, though it has been C&D’d, it is not attractive to decomposers.

          Now I can’t find anyone who can authoritatively tell me whether the nutrients remain in the dried C&D material or not. I can, thanks to what I remember of the aforementioned ecology, zoology, botany, microbiology, etc. make an educated guess, but I really want to see peer reviewed research results so I can rely on the information I get. I want this kind of source because not only is it important as a supporting component of my family’s food production, I also want to teach permaculture, and I refuse to teach things that I don’t know with some authority are true.

          This brings me back to the life sciences. I’ve always found that one of the hardest things for most of us is to understand the limitations of their knowledge base. This bloke isn’t a permaculture guru, but I thoroughly recommend absorbing his explanation of the nature of this problem as a way of advancing permaculture methodologies and knowledge:

          1. Hi Gordon, I too once hoped that the ‘life sciences’ were about the wisdom of life, but I too found out in the western world such a term just refers to the discipline of describing living things! So I now read Eastern philosophy as well!

            You summarised my point correctly, and yes I do live in a coastal area in Australia so it works well for me, sounds like you’re further inland, with a more hostile environment. I can say from experience, which might help people as practical advice, when I chop and drop and spread things out thinly, like you have found, things dry out fast. If the materials are piled thick in one spot, even seemingly dry materials such as berry prunings yield enough moisture to create a damp, decomposing pile that draws earthworms. With this method, you only enrich small areas at a time, but that makes it more manageable.

            I checked some statistics, Lockyer Valley is the driest part of South East Queensland with average annual rainfall of 780 mm, in Melbourne where I am, the average annual rainfall is around 650mm, but being a city we’re not as exposed to drying winds like rural areas. We’re pretty dry down south too.

            To answer your question about drying plant matter and nutrients, nitrogen is definitely lost as green plant matter dries, increasing the C:N ratio of the material, therefore requiring additional nitrogen to get it to decompose. Other more stable nutrients stay in the plant matter, they’re not volatile. Minerals such as potassium stay even if you burn the material, hence the high level of potash in wood ashes, that’s where the term comes from.

            I understand your concerns, and I can help out in respect to sources of information – you don’t need journals to get the information you’re after, most of it is common knowledge in agriculture, horticulture and botany, you can find the information in agricultural extension resources put out by universities, and in general textbooks in these fields. If you understand mobile and immobile nutrients in plants, you can figure which nutrients will stay in naturally dry plant matter after the plant has reabsorbed the mobile nutrients, and what nutrients you’ll have if you chop and drop. By understanding the various nutrient cycles, you can understand where the nutrients go, and which ones are easily lost to the air or due to water solubility.

            An example of such useful resources is “Basic Concepts of Plant Nutrition, NRCCA Resources, Cornell University” :

            If you look at this resource, it explains a bit about nutrient mobility, and nutrient levels in dry matter.

            1. “I can help out in respect to sources of information” … “An example of such useful resources is “Basic Concepts of Plant Nutrition, NRCCA Resources, Cornell University” :

              Angelo, the document has nothing to do with the loss of nutrients from plant matter drying on the ground surface.

              You say “If you understand mobile and immobile nutrients in plants, you can figure which nutrients will stay in naturally dry plant matter after the plant has reabsorbed the mobile nutrients, and what nutrients you’ll have if you chop and drop.”

              This would be useful information if we were chopping and dropping only the material that has dried on the plant, and if the nutrient in question was one which was mobile from old leaves to new (not nutrients are mobile within plants). But most of us chop and drop green material containing fresh living branches and leaves. So I am still looking for an authoritative source of information on what happens to the nutrients in chopped and dropped material when it is left on the surface in typical Australian conditions.

              I know you also told me “To answer your question about drying plant matter and nutrients, nitrogen is definitely lost as green plant matter dries, increasing the C:N ratio of the material …” but as I said in an earlier comment, I am looking for an authoritative source because I want to pass this information on to others with some confidence that it is correct. I’m afraid that information provided by someone who thinks that peer reviewed science is the “dogma of the high priesthood in white lab coats to hand[ed] down [as] holy writ in a published journal” just doesn’t cut it as far as authoritative sources go.

              Something I’d be interested in hearing you explain though is how you think that this “tainted” material in peer reviewed journals is transformed into credible information when it is relayed via agricultural extension materials and textbooks. What is contained in “agricultural extension resources put out by universities, and in general textbooks” isn’t, as you suggest “common knowledge in agriculture, horticulture and botany”. It is actually material drawn from published results of research and presented in “digest” form using language and terminology suitable for the target audience and good extension materials usually cite the sources of the information. This also applies, in general, to textbooks. Exactly what is it about peer reviewed publications that attracts your ire?

              If those in the permaculture community who consistently denigrate science, researchers and research journals would instead direct their efforts to pointing the rest of us to authoritative material useful to permaculture practitioners and, where necessary and where they have the skills, reporting it in everyday language, permaculture might move ahead much more quickly and effectively and we might build some much needed bridges between permaculture and the scientific community.

              I’m not saying that it is not important for permaculturists to carry out their own trials and to report their experience to the wider permaculture community. Of course that is important. But as others have suggested in this and other threads on this site, we don’t all have the time or expertise to do this, and anyway, there is a need for both anecdotal information and formal research results.

        2. Do people really need journals to tell them this or a basic grounding in the first principles of biology and ecology to be able to see this?

          No and yes. Much of the work done in journals is reductionist and has to be viewed carefully. However, one of the problems in so much of the permaculture literature is the lack of footnotes and references so you have to sometimes turn to the journals. The classic example is the one cited by Eric Toensmeier: Robert Kourick’s DA “work”. None of Kourik’s references lead to anything that supports the data in his tables. This info has been repeated by Hemenway (including Kourik’s “references”) and Jacke and Toensmeier(no references).

          There is much that is appealing in the idea of dynamic accumulators and therein lies the danger of unreferenced information.

          Lest anyone think that I’m attacking any of the authors I’m mentioning, I’m not; I’m simply calling for a higher standard in permaculture writing.

      2. dear Gordon, chop and drop can be accelerated if you stir up the organic matter from time to time, especially before a rain. It all rots down a lot quicker like that, especially if you don’t have pigs or ground birds doing the stirring for you. regards pete

  10. This is a great discussion opening up …. so important !
    I would just like to add an observation that comfrey is very effective in mopping up potentially escaping nutrients .Not only mining for deep minerals . It’s great for absorbing grey water . I have also used it as a plant barrier to prevent the spread of one invasive natured plant from one area to another . A “hedge” of comfrey leaves throws a deep shade and a “curtain” of comfrey roots beneath is very discouraging even for the most invasive of plants .

  11. I might add that, while Dave and I provided hundreds of footnotes to our sources in Edible Forest Gardens, the dynamic accumulator section is one that needs an overhaul. Most of our data came from Kourick who is about to release an update to his research. In fact I don’t really teach dynamic accumulators in my courses any more at all, until we get some more data. Several folks in academia are looking at the topic here in the states.

  12. “A tropical alternative to comfrey is Mexican sunflower (Tithonia diversifolia). It has been shown to accumulate phosphorus in some studies, and grows and resprouts very rapidly.”

    Yes – I was going to comment on it. I use it extensively in both of my Florida food forest projects.

  13. Hi, I live in Lanzarote Canary Islands, and if I could get the stuff to grow from seed, I´d let you know ….. how can it be so difficult to grow comfrey?

  14. I don’t have any soil tests by the year (aside from my handheld meter) but I do know the sections I plant comfrey on an acre sector of mine, the earthworm population is far greater than the rest. That in and of itself is good for me and my little slice of heaven. The redworms *accumulate* around the root system and take advantage of the scluffing that occurs. If NPK and micronutrients come out double and triple and sometime quadruple as good the posterior end of earthworms I believe we’re on the right track.

  15. Comfrey is great for composting yes but be careful it is invasive. It’s comming up all over the place in my yard now. And I use the roots madincinaly but even after digging loads of them up the plants come back.

    1. This may well be the problem….in my experience, sterile comfrey such as the bocking hybrids only spread uncontrollably when the roots are disturbed through digging or other means of disruption. In fact I often do this deliberately to proliferate and spread comfrey plants….to the extent that I get a double whammy by digging a section of root to transplant and then also increasing the spread of the comfrey in the original location…….because of this be certain you want it where you plant it lol!

    2. Brooke,

      Given all but the harshest of climates, comfrey will regenerate from even the smallest piece of root. 1cm in length is more than enough, given time, for a new comfrey plant to emerge.

      Every time you dig up the roots, you are spreading small bits of root throughout the soil- resulting in it spreading quite quickly.

      My experience is mainly with S. x uplandicum Bocking 14 and is overwhelmingly positive. It should be treated, as Lawrence D. Hills pointed out, as a permanent crop.

      If you want to kill it, you have to either dig the roots in deep part of winter (hard in some places with the snow) to expose them to freezing temperatures. Afterwards, the plants need to be cut very, very often in order to exhaust them. Turning pigs loose on exhausted roots might get the better of them.

      Even then, however, some will probably survive.

      To really ensure that it dies out, you can plan for deep shade. Comfrey will tolerate partial shade quite well (even thrive in the right climate, say in places where summer temps reach and exceed 30C. So deep shade is a must if you want to remove it from the ecosystem entirely, IMHO.

      Site comfrey where you are willing to allow it to live for quite some time.

      I don’t have any Bocking 14 here in Finland with me, but hope to soon. We will see how the S. asperum fares at the summer cottage. Should be good too!

  16. OP, Thanks for posting this, it’s great to see real numbers when dealing with any statement whether its a widely accepted one like comfrey, or anything else.

    “…I would also love to see soil results from areas which had been sheet mulched for 5 years but NOT had comfrey added, and/or spots which got comfrey but no sheet mulch, for real control plots…as it is, it’s hard to tell how much of the gain can be attributed to sheet mulching alone”

    THIS. The more I look around, the more I see ‘stacked’ attempts to modify/increase fertility that compromise the ability to gauge success. Leaving a control, unappealing as it is, is the only way to provide conclusive proof of the impact of whatever flavor of soil amendment you prefer. The stacking thing may be successful from a yield standpoint, but being unable to quantify these yields is a setback to understanding.

  17. What an outstanding article and discussion. As a disclaimer I have 100’s of cuttings of comfrey about the place and am accumulating more with each passing year (through cuttings). As an observation at this location, fruit trees which have a comfrey companion have more growth and are far more drought tolerant than those without them.

    Comfrey can out compete the herbage too so it is a very handy plant to have around the drip line of fruit trees. It will also happily regrow after having been thoroughly chopped and dropped and/or fed to the chickens. I cannot recommend this plant highly enough.

    Hi Craig. I thoroughly endorse and second your comment. Observation and sharing are some of the remaining “free” activities that we have available to us. I work with a couple of local groups towards this end and even this very evening had a couple of people up visiting having a look around to see what is possible with a bit of imagination. There is so much that we can do.

    1. I read your blog post on the advantages of comfrey under fruit trees some time ago Chris and resolve to give it a go.

      It definitely works with citrus for me. Not surprising, I guess, given that citrus are typically shallow-rooted, so the deep-rooted comfrey is extracting deeper nutrients (probably including those leached from the shallow soil layer) and depositing them via leaf breakdown.

      I’ve got my comfrey under the drip line too – partly because they don’t like the lack of sun if they are further under the tree, but also because it means that the water falling from the drip line of the canopy is absorbed under the comfrey, whose overlapping thick leaves then reduce evaporation.

      The fact that three rows of closely planted comfrey seem to stop almost all creeping grasses and similar weeds is a nice bonus.

  18. I do not want to sound like the voice of dissent after so many positive replies but I want to point out a few things about soil testing that we’re not taking into account with this discussion. I would also like to add the disclaimer that I do use comfrey around my fruit trees and bushes and such, and I haven’t read all the replies above my own.
    A typical soil test uses chemicals that plants do not use to determine the levels of minerals available in the soil. These levels are not indicative of what is physically plant available. Plants do not use strong acids, for instance like in the mehlich III test to release nutrient. While the snap shot examples above show a strong case for using comfrey, we need to know what other inputs, if any, were used (although I assume none), what companion crops were grown, and the times of year are very important for nitrogen especially as it’s highly leachable and volatile.
    Soil tests, we have to remember, are a snap shot of ONLY the chemical analysis of the soil. It would be my assumption that using more diversity (such as comfrey planted among other plants) are going to release many more diverse root exudates into the soil ecosystem, supporting more soil life, enhancing nutrient cycling, water infiltration, etc… all other things being equal. I would urge you, Ben, to consider doing the testing again, over the course of the year on each plot, to see how N, P, K and SOM change over the course of the season. A simple spring, summer, fall would be fine to show how much N availability varies. Also, excessive soil P can shut down mycorrhizal sporulation (effectively negating the benefit of soil fungi of other nutrients) as phosphorus is their main bargaining chip, of nutrients, effectively trading P for Ca, amino acids (containing N), etc. I agree that a deep rooted plant that grows as fast as comfrey and has a low C:N ratio will release deeper nutrients, I think we should really look into this “study” a bit more deeply, as has been mentioned above.
    Finally, I would urge Ben to look into the Haney Soil Health Test, out of ARS, Temple, TX, to look not only at the macronutrients, but at other aspects like not only Organic matter, but active organic content, burst CO2 showing total life, etc, which will give you a much better understanding of what your soil is doing. Dr. Haney is looking for samples from around the country to prove this test’s effectiveness, and your site might be something he wants to look into for certain.
    Here is a link to an archived conservation webinar that details the test. I think everyone on here will benefit from reviewing it.
    I hope this long winded reply benefits some.

  19. ive found root cuttings take a long time to establish really you are better of buying crown cuttings those things really start thriving within 3-6months!!!

  20. Thanks everyone for the wonderful discussion. It actually takes me forever to read through all the comments as there is so much research done in between. Love the links, the suggestions and the ideas. I was wondering if there was anyway to channel comfrey (I don’t like the word control). Is there anyway to create a barrier for its roots not to spread to much. Digging bricks or planks? I guess one can never have enough comfrey but just thinking what would contain its expansion…naturally and as effortless as possible.

  21. Keveen, look up rhizome or root barrier you will find a lot of information about containing roots. Its especially used on bamboo and other really vigorous plants. It can be a good thing to do on a small individual plant scale in a herb garden too so vivacious plants like mint or lemon balm don’t take over. You can do this by simply getting a bucket sawing the bottom off and burying it in the ground so that a few inches of the top of the bucket are pretruding above the surface, like this

  22. Thank you for all the comments on this article! I would be especially glad to see any similar soil tests from other people who have established comfrey plants. A few specific replies below:

    PJ Chmiel: The comfrey soil sample was taken from beneath plants that were not in the sheet-mulched beds, so I am confident in stating that the improvement there is exclusively attributable to the comfrey. At US$15 per sample I chose not to test the sheet-mulched beds this year, since I had already satisfied myself of the benefit. FWIW the sheet-mulched samples were not taken from directly under comfrey, though for all I know they may have benefited at a distance. Which leads me to…

    Outlaw: I’ve had a couple people ask me about the size of the area enriched by comfrey, but I don’t have a handheld meter. Since you do have one, could you provide some measurements at various distances from your comfrey?

    Ray: I appreciate your words of caution about soil testing. The plants I took the samples from were not planted with anything else, besides grass; they are occupying areas of the yard that are not conducive to other plants. And although they do receive preferential nitrogen supplementation from our dogs during the growing season, it’s been months (November) since the comfrey was visible enough to attract the dogs’ attention. Aside from that there have been no fertilizer inputs. Thank you also for the link to the Haney Soil Health Test — I’ll look into it!

  23. Great discussion! With regard to the experiences mentioned in the comments above of fruit trees and comfrey not working well together, Michael Phillips recommends in his book The Holistic Orchard, that comfrey should not be planted right up against a young tree as it would be too competitive in the early years when the tree is trying to grow wood structure. He waits to plant it until a tree is about 4 or 5 yrs old and then plants it in the “anticipated outer diameter” of the tree – the dripline – which can be about 6 – 8 feet from the trunk.
    I’m getting ready to start true comfrey from seed and will try to set up a test plot based on recommendations made in the comments.

  24. Hi All! So I went to my university library to see what peer-reviewed journals I could find on comfrey and its benefits (or lack of) to soils. There is a ton of studies on medicinal properties by the way. I didn’t find much on soil building or restoration in relation to comfery (symphytum)…but I’m still looking. One very interesting study I found was on an endophytic fungi from comfery used as a biological control of a serious pathogen of legumes. I was trying to find a published version outside of my library. You all might find this interesting. This could be a trail to other research as well.

  25. Good on you for your research, documentation & inquiring mind.
    Don’t waste your breathe on Wikipedia.
    I do research & editing for a small publishing company on the side. Absolutely no references are allowed to be cited from Wikipedia, they are considered unreliable. No reputable professional would cite them.
    spend your efforts on more professional sources.

  26. Regarding Gorse’s question about the N, P, and K increases, I’m sorry the scanned image is not fully readable. Nitrate increased from 5 (in the worse of the two samples) to 83 ppm, Phosphorus from 20 to 135, and Potassium
    from 294 to 760.

    Kim, I think even the most diehard Wikipedians would agree with you that Wikipedia is not a “source” and should never be treated as one. You might as well call a stained glass window a source of light. But as a *repository* for information, it is probably the most convenient and expedient medium ever devised, so I found it telling that no one has added reputable sources to the article on comfrey. I’ve downloaded the Hills book and plan to cite it when I get the chance!

    1. Many thanks for that Ben, much appreciated. Just thought that as well as testing the soil under the comfrey we need to test the soil where we’ve been applying the comfrey, be it as mulched leaves or as a liquid feed, so we can see how much that translates into increased nutrition for our crops as well as the soil where the comfrey is growing.

      On the theme of DIY testing, refractometers are great tool that can potentially be accessible to the average Jo…

      It might not be able to give specific results for different nutrients but it can record an increase in nutrient density, so is a way to test plants before and after applications of comfrey to see if there is any marked difference.

  27. Seriously, do we need cited references and soil tests? Maybe if we’re preoccupied with convincing stubborn academics…

    If we’re concerned about growing food, we can observe the results in our yields, two plots nearby, one with comfrey, one without, results speak for themselves, and like 99% of people who grow food on this planet to live, they know the methods (the technology), and don’t bother with the scientific explanations. Technology for the greater part of humanity’s existence has existed without the explanations of science, and for the most part outside the western world, still does!

    It all depends on what you’re trying to achieve.

  28. Angelo, anecdotes without evidence are just gossip. If we’re going to say permaculture is based on science, we need to test hypotheses and share results, because that’s how science is done.

  29. The first benefit of comfrey is as a compost accelerator – I use the leaves regularly during the summer composting phase (open air, let the rain on) and this summer an entire 90cm * 90cm *100cm bin was satisfactorily decomposed, if not yet ripe, within 3 months. I was also adding Yarrow but comfrey definitely works as an accelerator.

    Secondly, comfrey can be turned into either a straight tea (putting the leaves in rainwater and letting it decompose over a few weeks) or a concentrate (rotting the leaves under a crushing weight produces an odourless black liquid within 4 – 6 weeks in summer), both of which can be used diluted as a high potash fertiliser for tomatoes, beans etc.

    Thirdly, comfrey leaves defiinitely improve potato yields if you surround your seed potato with leaves at planting time and/or layer freshly cut leaves around the growing plants as you hill up. I got yields of up to 4lb per tuber this summer doing that, the best ever. This was my first season with 18 comfrey plants supply, so I’m glad I replicated Hills’ experiments of 50 years ago.

    As for improving soil, what comfrey does have is a very deep tap root so it is likely that the subsoil will be slowly broken up and aerated by the comfrey growing downward.

    I suspect that a mulch of comfrey leaves mixed with dried grass/hay would also work well, but I’ve not yet done it.

  30. Thanks for the data. Though I don’t have soil tests to back up my observations, the soil around comfrey plants shows a remarkable increase in organic matter, soil life, and soil structure. This is in heavy clay alkaline soil. Established plants can survive 5-6 years with10 inches of rain, but require supplemental irrigation to really thrive. I have noted a slowed growth of young trees planted symultaneously as comfrey nearby. But distinct health and flavor improvements of mature trees when comfrey is added around the drip line. Comfrey is about the only plant I have that can stop running grass from spreading.

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