Sheet mulching with ragweed

Submitted by Ben on Wed, 07/28/2010 - 15:08

ragweed and cornstalk layer in an okra bedMidsummer poses a number of challenges here at our urban farm.  It's when I've finally gotten around to preparing garden beds for sheet-mulching, but that means I need a large volume of green organic matter, as well as the leaves or straw I've saved from the previous fall.  Grass clippings are easy to come by, but they tend to turn slimy and smelly under sheet mulch conditions.  Meanwhile, all over town, even right on our block, ragweed is getting ready to bloom.

I never paid much attention to ragweed before I met Jessie because I'm not allergic, but Jessie is, and it makes her miserable throughout most of August and September.  I'd like to do whatever I can to minimize this, which means leaving the house closed up even on nice days.  But maybe I could do more than that... maybe I could get rid of the ragweed before it blooms!

So last year (late July 2009) I tried using ragweed as the green matter in my backyard sheet mulch, 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.  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 last year has been extremely productive all this year.  I was ready to try again.

This year I put out the word that I'd be hosting a "work party" (in the good permaculture tradition) to teach folks about sheet mulching and eliminate it 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 (pictured at right -- 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).  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, when I get around to leveling it.  (The empty wire cage in the background is the temporary home for the ragweed stalks.)

The Wikipedia article on ragweed is discouraging about the chances of eliminating ragweed.  Pulling it up is best done in the early spring, it says, before the root system develops.  But even if you do eradicate it locally, the pollen will drift for kilometers, and I know there's plenty of other ragweed around town.  So my efforts are probably not effective at reducing Jessie's hayfever symptoms.  However, I got a lot of organic matter for free, and I feel better knowing that even if I didn't kill the roots, I stopped one city block of ragweed from blooming this year.  Maybe by next year I can get other folks on board, and we can take it to a larger scale!  Wouldn't it be great to pull up all the ragweed in the whole city of Emporia and put it to good use?

One of the people who had wanted to attend the work party asked afterward for instructions, so here's what I wrote up:

How to sheet mulch with ragweed:

  1. aerate and loosen (but don't turn) the soil with a spading fork
  2. sprinkle live compost and water the ground thoroughly
  3. cover the ground (grass, weeds and all) with a thick layer of cardboard or newspaper, thoroughly wet
  4. surround the bed with 2' poultry netting (chicken wire) to prevent mulch from blowing
  5. 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.
  6. optional: wait 12-24 hours for the ragweed to wilt before continuing, to be sure it doesn't re-sprout
  7. sprinkle live compost again and water thoroughly
  8. 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.