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  • From: yarrow@sfo.com
  • To: permaculture <permaculture@lists.ibiblio.org>
  • Subject: Re: [permaculture] Question regarding sheet mulching
  • Date: Thu, 31 Aug 2006 13:52:46 -0700

Dawn, I have successfully used sheet mulching at my community garden to control bermuda grass and bindweed in the paths. I'm in the San Francisco Bay Area, with no summer rain (though the neighbors' garden plots, where the weeds still thrive, are watered at least weekly). I first used this method several seasons ago, and it has since been used throughout the 2.5-acre garden. (procedure below) At my garden, it has reduced bermuda grass and bindweed within one season from up to 25 percent cover in the paths to one weed every week or three. I imagine that if that one occasional weed were not pulled, the weeds could take over again, so with weeds as tenacious as bermuda and bindweed, persistence and vigilance are required.

I have also attended a presentation by Ted St. John where he described controlling weeds (at a site that had been weeded by volunteers for 20+ years) using mycorrhizae. I believe the site was a public park on the SLO coast.
http://www.mycorrhiza.org/restoration.htm

Another method worth investigating is the Bradley Method of Bush Regeneration: http://en.wikipedia.org/wiki/Bradley_Method

Finally, for more details on weed control, see Steve H. Dreistadt, 2004, Chapter 7: Weeds (p. 297-340) in Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, 2nd edition, UC-ANR.
You will find detailed information on what methods have proved useful for specific weeds. (In particular, solarizing is not recommended for bermuda grass, and I have seen it encourage its growth.)




Sheet mulching for weed control

1. For something as tenacious as bermuda grass or bindweed, I remove as
much as possible first. At my community garden, where bermuda grass and
bindweed were infesting the paths, I dug down 6 inches to remove every
last bit that I could see on the surface, and after a season the few weeds
that
did get through were weak and easy to pull (but this is adjacent to a plot
where weeds are allowed to flourish, so the roots are still being fed).

2. Then put down overlapping layers of cardboard (typically 1-2
thicknesses, or you can use 5-10 sheets of newspaper instead, but you may
have to moisten the newspaper as you lay it down so it doesn't blow away).
The weeds may still come up at the edges and, if you haven't overlapped,
between pieces of cardboard.

The minimum to use is 2-5 sheets of newspaper, which studies have shown is
the weed-blocking equivalent of 2 inches of mulch. The newspaper must be
covered with an inch or two of mulch so that it does not blow away.

Mulch alone is also good, but in my experience the cardboard/newspaper
multiplies the effectiveness of the mulch, rather than just adding a bit
more weed-blocking power. In my opinion, if weeds are a big problem, it's a
waste of time to use mulch without the cardboard/newspaper.

The purpose of the cardboard or newspaper is to block the light, so you can
use any organic material that will feed the worms and enrich the soil. Cotton
sheets, old carpet made of natural fibers, paper bags, etc., are other options. I
would avoid plastic or landscape fabric, however, which create anaerobic
conditions in the underlying soil and also help nurture bermuda grass -- at
the
community garden, the bermuda grass was thriving under the landscape fabric
and growing up through it, making it harder to dig up all the pieces.

3. As you lay down each piece or so of cardboard, put 2 to 6 or more inches
of mulch on top, depending on when and what you want to plant. I use
whatever free mulch the arborists deliver --- can be wood chips alone, or
chips mixed with leaves or conifer needles. If you are planting right away,
pay attention to what's in the mix. Eucalyptus and walnut may contain
allelopathic compounds (which may retard growth of other plants).

If you will not be planting for 6 months or so, then you don't need to
worry about the constituents, and you can also pile the mulch on more
thickly. That's what I usually do. All the mulch brings worms in to work
the soil, so if you're starting with clay, this is the best way I know to
get beautiful soil with hardly any digging.

Mulch holds water in, moderates the temperature, and keeps the worms happy,
so when it comes time to plant, you will not need to water as much.

4. If you are starting with a weedy area, you will not be able to plant
seeds anyway, because the weeds will overtake them before you learn to
recognize the seedlings you want. In early spring (until June), slugs and
earwigs and isopods may eat whatever seedlings remain. So I would mulch the
area (as above) to control the weeds, then:

Plant seeds in flats or 4-inch pots, and then when they are big enough to
survive on their own, dig into the mulch to plant them. You can add compost
to the planting holes (controversial to use compost with natives, I know)
or not. Annuals don't need holes punched into the cardboard layer; trees
and larger shrubs do.


I'm an intern with the City of Santa Barbara Creeks Division in Santa
Barbara, CA. I'm doing research into the use of sheet mulching against
Bermuda and Kikuyu grasses, specifically methods that involve layering
cardboard, mulch, newspaper, etc. I'd like to know if this is an effective
method, and if so, the conditions under which it was used. Any help would be
appreciated.




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