grass clippings

Alex McGregor waldenfarm at
Tue Jul 11 14:15:55 EDT 2000

Jim & Jo,

I have been composting mowings from our fields with leaves for years.  Our mower leaves
the mowings hay-like, they're coarse, not finely ground up like lawn mowers.  This is
better for composting since the finely chopped grass will mat and anaerobic bacteria will
dominate.  Research has shown that fine grinding is only necessary for woody things like
branches and corn stalks.  Breaking the outer layer of plant material is all that's
needed for the bacteria to colonize the inside.

Also, grass is an excellent mulch.  I let it dry on the field a few days before using as
mulch so it won't heat up when I place it next to the plants.  Experiments have shown
that straw and hay mulch keep potato plants from attracting Colorado potato beetles.  And
it's free!  (Just a little work, but not much more time or labor than hauling manure and
hay from off-farm.)  Straw and hay are also excellent habitat for spiders, the frontline
of insect pest management.

About "pulling nitrogen from the soil as they decompose."  This will only happen if you
plow the material in.  They call it "tying up the nitrogen."  No, there aren't S&M
creatures living in the soil.  What this means is that the soil life can out compete
plant life for nitrogen.  When you plow material into the soil, you increase food for the
bottom of that food chain and there's a population explosion on all feeding levels.  The
soil life uses all available nitrogen for amino acids and proteins.  These stay in the
foodweb until the population starts dying off.  The N is still there, just in living
animals.  I call it an artificial N shortage.

When plant material decays (is fed on on the surface) the decomposition is slower and
very little of the N in the soil is utilized.  It's a different ecology that feeds on the
dry material at the surface- fungally dominated as opposed to bacterially dominated
within the soil.

As for optimal thickness-  if you're using mulch to suppress weeds, the weeds will tell
you how much is needed.  If you're using it for soil moisture retention, your experience
is the best judge.  Observation is a farmer's best tool.  It will depend on what kind of
plants you're mowing (how stalk-like), what you're using it for, the crop you're mulching
(don't want to bury them), how much it rains, your soil texture and percolation...

The secret for avoiding weed seeds in the mulch is to mow before they set seeds.  But,
that's not always possible.  you would have to keep it looking like a suburban lawn.
But, take consolation in the fact that there are already billions of weed seeds in the
soil and adding a few more are not going to make that much difference.  At least you
won't be importing new ones with manure or hay.

Alex McGregor
Walden Farm

five springs farm wrote:

> We are considering investing in the mowing equipment that would allow us to
> gather up large amounts of grass clipping (and leaves) to use as mulch. The
> intent is to heavily mulch much of our 1/2 acre of intensive raised bed
> veggies. We would hope to be able to reduce or eliminate animal manures (we
> use horse poop mostly) as it brings in so many new weeds each year, is a
> hassle to obtain and haul, and seems such excellent grub habitat. It would
> also substitute for much of the straw mulch that we purchase. We would also
> compost the material (with garden scraps and leaves).
> What experience do folks have with grass as mulch? Are there any particular
> down-sides (not counting the possibility of contaminated yard clippings from
> lawns that use chemicals, since it would all be from our own land). Will the
> clippings "pull" nitrogen from the soil as they decompose? Is there an
> optimal thickness for the mulch? And so on...

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