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This Saturday, June 21st, is the fourth annual International Ragweed Day. For those who are allergic to ragweed pollen, the various varieties of ragweed (Ambrosia ssp.) can be a real bane of life. When someone you care about is swollen up like an itchy tomato and popping antihistamines and decongestants just to get out of bed in the morning, you might be tempted to want to eradicate the plant entirely. But is it all bad?

From an ecological perspective, and particularly in the Americas where it is native, we must first think of ragweed’s significance as a food for wildlife — notably quail, but also including some now-rare butterflies and moths. However, for those of us living in cities and towns where these creatures rarely venture anyway, any value to hypothetical wildlife is moot. Some herbalists may be inclined to follow up on ethnobotanical evidence that the Cherokee used ragweed to cure insect bites and pneumonia.

But for the rest of us permaculturalists, I think the most exciting way to simultaneously dispose of ragweed and put it to use is as a compost activator, particularly in sheet mulches. I should say here that all of my experimentation has been done with giant ragweed (Ambrosia trifida) and not with common ragweed (Ambrosia artemisiifolia), for a few simple reasons: it’s more common here in eastern Kansas, it’s easier to spot at a distance, and it’s much easier to harvest in quantity. Given a good thick stand of the stuff, one person can fill a pickup truck with giant ragweed in an hour!


This photo was taken before the top layer of brown matter (dead tree leaves) was applied,
showing the 6″ depth of the ragweed layer. This is one pickup truck load of ragweed.



This was my first ragweed sheet mulch, and
the one whose soil test results are reported.
Note that the soil was already raised due to
excavation, and okra plants were already
established on the bare ground before the
sheet mulch was applied around them.
The brown matter layer is corn husks.

Giant ragweed is a close relative of sunflowers and sunchokes, and when its leaves first appear, they can be mistaken for the leaves of these more friendly plants. But as the plant matures, the leaves first divide into three lobes, then into five, and finally into seven, developing the characteristic ragged appearance that gives it its name. Also, the leaves are always parallel on the stem, so that from a distance the plant has a distinctive pagoda-like appearance. In my experience the leaves are also reliably a deep hunter-green color, even when neighboring foliage is yellowed from lack of nutrients. Once you’ve familiarized yourself with giant ragweed, you’ll find you can easily spot it at a distance.

The key thing to know about using ragweed in the farm or garden is when it blooms in your area. Here in USDA climate zone 6, it blooms in early September. That means that I can plan to harvest its copious green matter anytime from June through August without fear of spreading seeds or bringing home pollen. As an added bonus, pulling it up just before it blooms sets it back so much that even if it does grow back from the roots, it will not be able to bloom and set seed. Those who are allergic to the plant may want to protect their skin from its hairy leaves and stems while harvesting, though I haven’t heard of anyone having a skin reaction. I find that a firm two-handed tug is enough to uproot even the most stubborn and established plant.

Over the past two years I have done a comprehensive survey of the area around my house and found not a single giant ragweed plant in anyone’s front yard, since even those who don’t know what it is know that it’s undesirable, but it can be found in abundance in alleyways, where no one cares if you pull it up and haul it away.

In June of 2012, after completing this survey, I harvested the largest stands for a mulching project, taking care to leave no survivors to set seed. Even so, when I returned the following year, the same stands had returned. This implies that seed can remain dormant for two years or more before germinating, which means any attempt at eliminating the plant must be persistent. Unfortunately I was not able to continue the experiment by pulling up the same stands the second year, so I leave that to you.


The yellow spots on this map indicate stands of giant ragweed, and the orange spots near
the middle indicate where I returned to eliminate the stands.


Again, the yellow spots indicate stands of ragweed. The take-home point is that they were
not noticeably smaller than the previous year.

Once harvested, the ragweed stems and leaves can be used in a traditional hot compost, but given their large size (some well over two meters) and their fibrous nature, they are better used as green matter in a sheet mulch, where they can be laid down whole to minimize labor. I have had great success with this technique year after year. The earthworms love it, and it breaks down completely by spring, if not earlier. After allowing a 3-6” (7-15cm) deep layer of ragweed to wilt to ensure it is dead, thoroughly rehydrate the green layer with water before adding a brown layer on top; otherwise it will mummify and take much longer to break down.

My soil tests before and after sheet mulching with ragweed show a doubling of organic matter (1.5 to 2.9%), a modest increase in nitrates (5 to 6 ppm) and more significant increases in phosphorus (20 to 81 pm) and potassium (294 to 516 ppm). These figures are from the same location, with ragweed and dead tree leaves as the only amendments, applied to the surface with no tilling. As I reported in an earlier article, comfrey produces much more impressive results, but ragweed has the benefit of being free for the taking, and the more you harvest, the less noxious pollen will be in your neighborhood!

I hope that my success with putting this unwanted and medically harmful plant to constructive use will inspire you to do the same. Please let me know if you’re giving it a try!


This sheet mulch was built in late June of 2013, so it has only had 3 months to decompose,
but the fibrous stalks are already breaking down.


This is the same project as above, but the
following May. Only a few fibers are left.


Same project as the above, planted for the summer. Only a thin layer of straw remains
from what was fully a foot (30cm) high pile.

4 Responses to “Ragweed: Curse or Blessing, the Choice is Yours”

  1. Ben

    I’m glad to see your appreciation for my old pal, ragweed. It’s my naturally existing smother crop! I read that seeds have been found in caves several times larger than natural and used as a pseudo cereal. Thx.

    Reply
  2. Ben Stallings

    Thanks for the note, Ben! And thanks to Craig for publishing the article. I should note that the photo Craig added at the top is not of giant ragweed but one of the other varieties. For those who aren’t familiar with giant ragweed, here are some photos of the 3-lobed leaves: http://essmextension.tamu.edu/plants/wp-content/gallery/giant-ragweed/giant-ragweed_228_img_5125.jpg 5-lobed leaves: http://courses.missouristate.edu/pbtrewatha/Giant_Ragweed2.JPG and fully mature leaves: https://www.icmag.com/ic/picture.php?albumid=29549&pictureid=1063027 . The “pagoda” silhouette: http://www.toronto-wildlife.com/Plants/Aster_family/ragweed/ragweed_giant_7sh25_072010_640x480.jpg , a typical stand: http://essmextension.tamu.edu/plants/wp-content/gallery/giant-ragweed/giant-ragweed_228_img_5096.jpg and an example of how tall it can grow: https://news.uns.purdue.edu/images/+2007/giant-ragweed.jpg

    Reply
  3. Linnie

    “I should note that the photo Craig added at the top is not of giant ragweed but one of the other varieties”. Ah, that makes sense, I was wondering how people culd mistake even the immature leaves for sunflowers or Jerusalem artichokes (I’m guessing that’s what sunchokes is? :-)), because ours just look like small ragweed leaves on a small ragweed plant. We have the one that the other Ben showed in the image. We have bucket loads of it… semi-trailer loads of it! Along our country road, it is THE dominant weed, amongst lantana and various other weeds.

    Thank you for this article, Ben. I will definitely have a go at using it in this way and eventually trying to eradicate it, because I’m fairly certain it is the culprit behind my own itching eyes and sinus headaches. It also grows in an area where the already narrow roads become dangerously narrow when council refuses to slash the roadsides, and it grows tall enough to bend over the road verges causing real visibility issues on blind corners! So, out with the stuff, I reckon… Maybe if I sprinkle local species Lomandra (tough as anything) or similar plants around they’ll eventually smother it and create a low road verge planting. :-) I will wear gloves, though, as even if the ragweed leaves themselves don’t create an allergic reaction, they smell as if they would! :-)

    Thank you again for the idea, and I’ll mention it to all my other local permaculture-type friends. Maybe we can, united, squash this beast! :-)

    Reply
  4. Dana Brunson

    Ragweed is also excellent rabbit food. After it goes through the rabbit, it makes even better garden mulch!

    Reply

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