From the Bottom Up – A DIY Guide to Wicking Beds

by Rob Avis

Wicking beds are a unique and increasingly popular way to grow vegetables. They are self-contained raised beds with built-in reservoirs that supply water from the bottom up – changing how, and how much, you water your beds. In this article, we’ll talk about how wicking beds work and why we love them. We’ll also show you some great examples and leave you with ideas and instructions for creating your own.

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How Wicking Beds Work

A wick works through capillary action – the same force you observe when you dip a piece of tissue paper partially into a glass of water and watch the water climb the paper. Wicking occurs in many materials; cotton, wool, geo-textile, soil, gravel and even wood to some degree. Every material has different wicking properties which you can test by placing that material into a glass of water and watching the water “climb” up. When one end of the wick is saturated and the other end is dry, it creates a moisture gradient, which drives the wick until the gradient no longer exists or you run out of water. With the earth box, one of the more popular examples in North America, the soil is suspended above the reservoir with wicks dangling into the reservoir pulling up moisture. As the plants use the moisture in the soil, it creates a moisture gradient (the soil is drier than the reservoir) which drives moister through the wick into the soil.

Advantages of Wicking Beds

Wicking beds have a lot of advantages over standard raised beds and in-grown swale-based gardens:

  • They are water-efficient! Watering from the bottom up prevents evaporation of surface water (which occurs when you water beds from the top).
  • They are self-watering! Wicking beds are an especially great system to use in community gardens because they save people from driving every day during hot weeks to water their beds. A full wicking bed should irrigate itself for about a week.
  • They can be placed close to the house without risk of flooding your basement, since the water is contained in the bed. This makes wicking beds a great alternative to swales on properties with sump pumps or basement water issues.
  • No evaporation means no salting of soil. If you are watering your soils from the top with hard water, you risk accumulating salts, because the water evaporates and leaves the minerals behind. Eventually your soil will struggle to support plant life.
  • They provide a lot of drainage in the event of a large downpour.
  • Since they’re raised, they will warm up quicker in the spring.
  • You can easily attach cold frames to them.
  • They are great for people with less mobility and strength as you don’t have to haul heavy water containers.
  • By using an intermediary tank, you can automate the watering process… but more on that in a future blog.

Disadvantages of Wicking Beds

Wicking beds do have some disadvantages as well:

  • They cost more to install than in-ground swales and standard raised beds.
  • They will freeze sooner in the fall than non-raised beds.
  • There are additional freeze/thaw considerations that need to be taken into account, which is not required for conventional gardens.

Types of Wicking Beds

Reservoirs with Media

Most of the DIY sites for wicking beds focus on building beds that use media, a layer in between the soil and the water reservoir, as their wick. This is an easy and cheap way of supporting the soil on top of the reservoir. Gravel is the most common medium, but there are a number of materials that do the trick. Here’s a good DIY blog on media wicking beds.

Media-less beds

Beds without media require a false bottom that will allow the soil to be suspended above the water reservoir. Again, this wick system can be made from a variety of materials. Here are some examples of media-less wicking beds:

Design Considerations for Media-filled Reservoirs

When designing your wicking bed, it is important to keep the depth of the media-filled water reservoir at or below 300mm as the capillary action struggles to lift the water higher than that. The soil above the reservoir acts as a wick as well, so it is important that the soil layer stay between 300 – 320mm. The soil could technically be deeper than this, however, the soil at the top will likely be much drier than the lower soil, so you’d want to make sure to that the plants you use can access this deeper soil moisture, like tomatoes, which can buried deep into the bed.

Once you have determined how deep your soil and media is going to be, you need to create a containment device. There are many ways of doing this, for example, in our last blitz we experimented with four different techniques: the global bucket, a food-grade plastic tote, above-grade planter boxes, and an in-ground wicking bed.

A key element of the containment device is the overflow pipe. This pipe allows water to escape once it reaches the top of the media, ensuring you don’t drown your plants with too much water.

Above-Ground Planter Box

Because saturated soil is so heavy, we designed the beds the same way that concrete forms are designed. A carpenter friend of mine recommended that we use “whalers”, which are 2x4s with the thick end perpendicular to the plywood, and bound together using a lap joint. See the photo below. These whalers were spaced around the box to resist bending and bowing of the heavy soil. Pressure treated plywood was used to prevent rotting on the inside of the whalers and increase longevity, and then the whole box was clad with cedar fence boards. Ordinarily I don’t like to use pressure-treated wood in gardens, but since this bed was to be lined with plastic, the wood will not come in contact with the growing medium. Originally I was going to use pond liner, but it was cost-prohibitive at $0.90/sqft, so I decided to use 6 mil builder’s poly instead.

Layers in The Bed

  1. Landscape fabric stapled to the wood which protects the poly from sharp edges.
  2. Poly liner.
  3. Landscape fabric on bottom of bed to protect poly from punctures from the gravel.
  4. Weeping tile to increase rate of water communication in bed as well as reservoir capacity. If you have a long enough weeping tile you can bend it up the side of the bed and use it as a water fill pipe.
  5. Drainage pipe the length of the bed to encourage even drainage from the bed. This pipe is connected to the bulk head fitting and has holes drilled in one side facing down.
  6. 300 mm or less of gravel. Note you want to make sure that you have enough gravel to cover the weeping tile as you want to make sure that the gravel is in contact with the soil, not the weeping tile.
  7. Landscape fabric to segregate the soil from the gravel and preserve the pore space in the bed.
  8. High carbon soil

Other considerations:

To account for the freeze/thaw issue in this climate, I set my drainage hole to the bottom of the beds so that I can drain the bed before winter. The amount of water held in the bed is determined by an elbow and stand pipe which can rotate on the outside of the bed. I like this method of water control as it allows me to infinitely control how much water the bed can store.

ICU Totes

When it comes to raised wicking beds, cheaper alternatives to the raised wood boxes are food grade 1000 L totes cut in half. We decided to use this method for our passive solar greenhouse as these tanks were inexpensive (they cost me roughly $100 each, and one tank cut in half can make 2 wicking beds). This is far more affordable than the wood variety, which are roughly $600 each, not including the time they take to build.

Layers in the ICU

  1. Weeping tile to increase the rate of water communication in the bed as well as reservoir capacity. If you have a long enough weeping tile you can bend it up the side of the bed and use it as a water-fill pipe.
  2. 300 mm or less of gravel. You want to make sure that you have enough gravel to cover the weeping tile so that the gravel is in contact with the soil and not the weeping tile.
  3. Landscape fabric to segregate the soil from the gravel and preserve the pore space in the bed.
  4. High carbon soil

In-Ground Wicking Beds

For our blitz we built one in-ground bed. The in-ground bed is cheaper than the other two because you use the earth as the support for the water reservoir. This means you only need a containment device for the soil above grade. For our bed we chose to use cedar planks to build the above-ground bed and we made a small dugout to contain the gravel. To allow the excess water to spill out, you need to make sure that the water can leave the bed either with a designated spillway as we did, or just raise the bed up on shims so the water can leave the periphery.

Layers For The In-Ground Bed

  1. Landscape fabric on the soil to protect the poly from holes.
  2. Poly liner.
  3. Landscape fabric on the bottom of bed to protect poly from punctures from the gravel.
  4. Weeping tile to increase both the rate of water communication in the bed, as well as the reservoir capacity. If you have a long enough weeping tile you can bend it up the side of the bed and use it as a water fill pipe.
  5. 300 mm or less of gravel. You want to make sure that you have enough gravel to cover the weeping tile so that the gravel is in contact with the soil and not the weeping tile.
  6. Landscape fabric to segregate the soil from the gravel and preserve the pore space in the bed.
  7. High carbon soil.

A Neat Blog on In-Ground Beds

Milkwood Permaculture has pioneered an in-ground wicking bed using builder’s plastic and a round galvanized culvert ring. We have also seen people use stock watering tanks.

Materials

Soil

We chose to use a garden mix from Western Canadian Compost which is combination of loam, compost and peat. I was really impressed with the quality of the soil and I will give updates on the results over the course of the summer.

We selected gravel as our resevoir/wicking material. Generally speaking, gravel has about a 33% pore space which means that 1 cubic meter of gravel in a container will only have enough room between the gravel to hold 333 litres of water.

We also used 4-inch weeping tile in the bottom, which increases the amount of water that the bed can hold (because it is hollow) and increases the rate at which the gravel bed disperses water. The dispersement action of the weeping tiles ensures that one side of the bed does not get initially over-saturated. The weeping tile also doubles as the watering pipe.

Other Adjustments

There are all sorts of design elements that can be added or modified to change how the beds work. One popular tweak is to insert a worm composting tube into the soil portion of the bed. Food scraps can be added to the tube for the worms to process, and the resulting vermipost and worm juice will be distributed throughout the bed keeping the nutrient levels high in your soil. The worms also help to keep the system aerated and therefore prevent the system from going anaerobic. I do not think that the red wriggler can survive our cold winters, so I would recommend having an indoor worm system ready when it starts getting colder out, so that you can keep them alive until the next growing season.

As my students know, I am a big fan of cover crop systems. Typically I recommend nitrogen fixing legumes that build carbon and nitrogen into the soil through their root systems. Since these wicking beds are segregated from the subsoil, strategies are needed to keep up soil fertility. This could include cover cropping or the addition of compost, blood and bone, and rock dust. Cover cropping also reduces weeds, shades the soil, and provides a built-in mulch system.

As you can see, the sky (or the soil) is the limit when it comes to wicking beds. They are an effective and water-efficient DIY gardening implement that can be created and adjusted according to your budget, materials, space, and garden plan.

Even though we’ve built our fair share of wicking beds now and have gained a great deal of knowledge from other people’s designs and experiences, we are still learning as we go. Stay tuned throughout the summer for updates on our wicking beds, and hopefully you can learn with us!

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18 thoughts on “From the Bottom Up – A DIY Guide to Wicking Beds

  1. I wonder if biochar mixed into the gravel bed would be a useful addition? And might this be a useful container garden if stretched along the roof drip line of homes (like mine) without gutters? Roof top gardens for those flat roofs on many developing countries?

  2. hey rob great job mate ,how do you think a ball cock valve/float valve on the outside of the frame
    would go?

    could be a good system for schools or public places

    take care mate and well done

  3. Dave, I was thinking about doing that so that you could directly hook it up to a rain tank have have it self water. The one issue with these beds is that if you keep them wet all the time the soil does not dry out and breath. So I think the auto watering would have to be batched to allow the beds to dry once in a while.

    Thanks everyone.

    Rob

  4. Globalbuckets.org has some good info on what Dave Spicer is talking about. I have been experimenting with “shelves” on the sides of the container above the wicking water level. This gives some vertical variability to the surface evaporation/transpiration gradient as well as allows variable rooting depths to water level, enhances growing area and allows more gaseous exchange through the soil and through plants growing out of the sides of the container. I did this to a 210L (55gal) plastic barrel and a 68L (18gal) plastic tote. Both seem to be working well except for the plants in the very bottom row of the barrel. A worm tube might also help with gaseous exchange as well as provide a vermicomposting, fertizing option.

    Another thought, is that by having a float valve in a small container that is easily moved up and down, it would be very simple to adjust the water level of as many wicking systems as are hooked to that water level. So if that float valve was tied to the rainwater collection tank, it would automatically water the wicking system and adjust the water level as desired. Now to automate moving the float valve container up and down…new experiment…

    1. Hey Shaun,

      Rather than move the float valve up and down I’ve opted for a different solution. My float valve have a tap in line just before it so I can control the inlet rate of water. I have it set very low as it doesn’t need to be flowing in fast to keep the resovior full.

      At the base of th ibc bed I have a drain port with has a 24v watering solenoid connected. I have a watering control unit that opens for 4 hours every fortnight and drains the water out of the beds into my duck pond (sump for the whole system). The float valve is open the whole time the drain is but its flowing so slowly it makes no difference. It’s gravity feed out of my ap tank (which has its own float valve to stay full of fresh water)

  5. Keep up the wicked work you willing wicking bed workers, great design for arid land urban food production.

  6. I’ve recently built a raised bed with two layers of 8 inch concrete block. Approximate dimensions are 3 by 10 internally. The bottom layer of block lined with plastic is the reservoir. Overflows vent between the bottom and top layer of block. Sitting inside the reservoir are 12 plastic milk-crate style file boxes individually wrapped in landscape fabric. The crates contain the bulk of the water volume in their open lower portion while providing an air space between most of the soil and water above. The top of the crates are a couple of inches above the overflow level. The garden soil lies on top of and between the crates down into the water. The bed is easily drained as needed by removing the blocks on one end and letting the plastic liner drop to ground level. I’m hoping to grow a variety of salad greens as far into the cooler months as possible with a canopy of wire and plastic.

  7. Could an old bath be used to build a wicking bed ? Are there any disadvantages in this? thanks, Barb

  8. Hey,

    So I was wondering exactly how one would manage the Vermicomposting. It’s new to me. Would you have a tube directly into your soil where you put food scraps into and put the worms into your soil, then hope they find the tube? Would you put another sort of Weeping Tile system in the soil? If you’re doing an in ground bed, when (if ever) would you need to tear up and start over?

    Thanks,
    Lindsay

  9. few question
    – with a media-less beds who act as wick?
    + in the global bucket they have dirt in a cup with slid
    + in the earth box what they use?
    – with reservoir with media
    + what is the size of the gravel suggested for 20/30cm (8/12″) deep reservoir ?
    + is finer better for capillarity action (sand?)
    + organic material (sawdust) could work or it will be a mess (mold, anaerobic decomposition etc)
    + is just soil a possible option (1% of the surface something similar to the 2 buckets or constant wet soil can bring probelm of mold etc
    i saw ventilation a key in media-less beds no mention in media bed. I guess you lose some moisture for evaporation…
    – i read that to reduce salt and silting reservoir has to be flush (empty) once a month; dose anybody has experience of that?
    – doese make any sense to have a reservoir high more that 20cm (8″) ? when reservoir is almost empty the wicking effect has to raise the water 20cm + anothere 10 or 20 to get to roots system…
    thanks

    1. Alvise, you ask: “is finer [than gravel] better for capillarity action (sand?)” Yes. clean (aka well sorted) gravel can only wick about 10 mm. I see a lot of folks posting pictures of wicking bed builds are using poorly sorted (“dirty”?) gravel and sand. That will work because the fine sand fraction provides the capillarity. I learned a lot from “bench testing” capillarity the various materials I had to choose from. I observe that it takes a medium to fine sand (0.2 to 0.5 mm particle size) to wick the full 200 to 300 mm lift height often suggested in wicking bed designs.

    2. Alvise asks: – “I read that to reduce salt and silting reservoir has to be flush (empty) once a month; dose anybody has experience of that?”

      Silting shouldn’t be a problem if there is a fabric barrier between the soil and the main storage volume.

      I checked the water pH and TDS (total dissolved solids aka salts aka solutes) in my wicking bed and SIP reservoirs this spring. TDS was less than 1000 ppm, pH was neutral, all good. Apparently, precipitation has provided adequate salt flushing for my purposes. If you have high TDS in your irrigation water, or in your soil amendments, flushing from precipitation may not be enough. My TDS could build over time. If my reservoir water TDS went over 2000 ppm I would think about a soil/reservoir flush. If reservoir went over 4000 ppm, I would expect to see some impaired growth during hot weather.

  10. Like Charly, I’m thinking about the possibility of putting a hay bale garden atop a wicking resevoir. I’ve got lots of invasives and was considering leveling slightly below grade, lining with some plastic I have onhand, filling that with gravel and throwing hay bales on top. I’ve even been toying w/the idea of replacing gravel with scattered hay 20-30 cm thick as I could then toss the spoiled hay into a nearby sheet mulch when I want to renew everything.

  11. My beds are built, I’m ready to go. Looking for a cheaper alternative to gravel, and something easier to cart up hill, I was wondering about using old tyres in the reservoir laid flat to provide a floor for the soil. Would there be a problem with them contaminating the water?

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