This is a guest post by Ben Stallings (brother of ‘Becca), adapted from this post at Blue Boat Home.

There are two troublesome things I have in abundance in early summer on our urban farm in eastern Kansas: overgrown weeds and ideas for what to do differently next year!  As is often the case, adding two problems equals a solution.

If you have an organic garden, you probably have a compost pile, and that means you need equal parts green matter and brown matter.  Dead leaves are easy to stockpile from the previous autumn, or you can buy straw by the bale, but green matter (fresh leaves, veggies, and fruit) doesn’t keep.  If you need a quick burst of fertility for your nitrogen-hungry summer crops, you need a lot of green matter that will break down quickly and completely.  You may not have seriously considered weeds as a source of food for your garden–a handful of weeds scattered around the garden is a nuisance–but a wheelbarrow-load growing all in one place is a resource! I start to eye the roadsides and alleyways for lush groves of unwanted plants.

I never paid much attention to giant ragweed (Ambrosia triffida) before I met my wife because I’m not allergic to its pollen, but Jessie is, and it makes her miserable throughout most of August and September.  I try to do whatever I can to minimize this, which means leaving the house closed up even on nice days.  But maybe I can do more than that . . . maybe I can get rid of the ragweed before it blooms!

I tend to believe that tilling garden soil does more harm than good, so when I want to start a new garden plot or revive a tired one, I use a technique called sheet mulching.  A sheet mulch is basically a compost pile built right on top of the garden bed, with no need to turn it or otherwise fuss with it–you just pile on the ingredients and wait.  Fortunately, this time of year when the ragweed is growing prolifically is also the best time of year to start a sheet mulch for planting in late fall or spring.

After learning to sheet mulch by working on an organic farm, I built my first in my own garden in 2008 and had some ideas about how to do it differently next time.  Starting in July 2009 I tried using ragweed as the green matter, and I was astounded at how well it worked.  It had decayed completely by October.  We’re talking a 3-4″ deep layer of whole stalks, completely gone in 3 months, without turning.  I’ve never seen a sheet mulch work so quickly.  There were a few volunteer ragweed plants in the spring, but no more than I’d expect to blow in from elsewhere; for all I know, that’s where they came from!

The garden I prepared this way in 2009 was extremely productive for the following two years, and the patch of ragweed I’d pulled up did not grow back.  I was ready to try again.

In 2010 I put out the word that I’d be hosting a “work party” (as is the permaculture tradition) to teach folks about sheet mulching and eliminate ragweed from the neighborhood.  Unfortunately the only date that worked for me didn’t work for any of my guests, so it was just me and my mother-in-law.  Together we piled a good 3″ of ragweed onto the partially-completed bed I’ve been using for okra, and I added a neighbor’s unwanted corn stalks for good measure, as shown in this photo.  (The bed is raised, so you’re seeing several inches of soil on the near side below the mulch, and the live plants growing in the mulch are okra, not ragweed.)

Ben’s okra bed, sheet mulched with ragweed and corn stalks

This covered only about half the area I intended to mulch, so a few days later I returned to a neighbor’s yard and harvested enough ragweed to finish the bed. The okra loved the ragweed mulch and gave us a bumper crop! Since then we have established asparagus and rhubarb–both nitrogen-hungry crops–in that bed with good results.

In 2011 I tried to do a comparison test using all the ragweed I could find in part of a plot and iris and comfrey leaves and other excess green matter for the rest. I had meant to demonstrate that the ragweed broke down quicker than other green matter, but we had a dreadfully dry summer and I gave up trying to keep the plot moist, so that by the time it decomposed the results were inconclusive. I did satisfy myself that by harvesting the ragweed a few weeks earlier (late June, early July) I reduced the risk of accidentally planting ragweed in my garden while still eradicating it at the source. If you’re composting in a traditional hot pile that you turn every few weeks, starting this early would give you finished compost in time to top-dress your late summer crops.

The Wikipedia article on ragweed is discouraging about the chances of eliminating the allergen.  Pulling it up is best done in the early spring, it says, before the root system
develops.  My experience is that it doesn’t come back once pulled up . . . but even if you do eradicate it locally, the pollen will drift for miles, and I know there’s plenty of other ragweed around town.  So my small, local efforts are probably not effective at reducing Jessie’s hay fever symptoms.

This year (2012) I commented to Jessie that there should be an official Ragweed Day so that it wouldn’t be just me pulling the stuff up.  On a whim I Googled it, and what do you know–this coming Saturday, June 23, is in fact the first annual International Ragweed Day!  So why not take an hour or two to pull up all the ragweed you can find in your neighborhood (with permission, of course)?

Even if we don’t succeed in eradicating enough ragweed to relieve our loved ones’ hay fever symptoms, we’ll get a lot of organic matter for free, and we’ll feel better knowing that we stopped our own neighborhood ragweed from blooming.  Wouldn’t it be great to get everyone on board and pull up all the ragweed in your whole city and put it to good use?

How to sheet mulch with ragweed:

• You can start with either a patch of lawn or an existing garden with plants growing in it–no need to till.
• Aerate and loosen (but don’t turn) the unoccupied soil with a spading fork.
• Sprinkle live compost and water the ground thoroughly.
• Cover the ground (grass, weeds and all) with a thick layer of cardboard or newspaper; cut slots to fit around any plants you want to keep.
• Thoroughly wet the cardboard.
• Surround the bed with 2′ poultry netting (chicken wire) to prevent mulch from blowing.
• Pile on ragweed until you can’t see the cardboard anymore (3-5″ deep). It’s easiest to put down a thin layer of ragweed oriented one way, then another layer oriented the opposite way.
• Optional: wait 12-24 hours for the ragweed to wilt before continuing, to be sure it’s dead.
• Sprinkle live compost again and water thoroughly.
• Pile on last year’s dead leaves or straw until the mulch is 1′ high (to the halfway mark of the poultry netting). Water thoroughly.
• Wait 3-6 months before planting seeds or new transplants. In extremely dry weather you may need to water the mulch to keep it moist.

Composting ragweed works for me!  Visit Your Green Resource for more environmental articles!

Check out these Earthling’s Handbook articles about composting:
Lazy Composting
Composting in the Office