Re: grass clippings as mulch

At 10:05 AM 6/29/99, Damian Martin wrote:
>Dear Med-planties,
>
>Can anyone comment on the relative benefits of raw grass clippings as
>mulch?

Let me introduce another concept, one that many of you may be familiar
with;  allelopathy.
Everyone knows that the leaves of *Juglans* species contain toxins
injurious to other plants.  It's a readily understandable genetic advantage
for one plant to be able to suppress the growth of potential competitors
for scarce nutrients.  A researcher at the University of Rhode Island in
the States in the 70's discovered that one of the most potent allelopathic
producers were cool season grasses;  rye, bluegrass, fescue, et. al. and
that the most common effect of these allelochemicals was on woody
plants......

 I have recently lost some newly planted Celtis
>australis

I used to take classes to local places where, by chance, architects had
designed in bunches of trees, some in turf, some in bare soil.  It was
always an eye-opener.  There would be a line of trees, all obviously
planted at the same time, yet the turf-encumbered trees were generally half
the size of those in bare soil.  Competition accounted for some of the
difference but allelopathy produced most of the contrast.  The grass was
clearly secreting substances (still unidentified) that were killing
roothairs of trees.  The same effect can be observed if you grow several
trees with varying amounts of grass-free areas maintained at the base of
each tree.  The bigger the grass-free patch the faster grows the tree.

I should add, however, that allelochemicals are highly soluble.  I.e., the
harmful effects usually leach away after a couple waterings.  *However*, if
one were conscientious and adding new clippings all the time you could
easily kill off a young tree.

Ironically turf allelochemicals are a public hazard in the US.  California
passed a law some years ago mandating that municipalities reduce their
waste stream by 50% by the year 2006(I confess I'm vague on those numbers,
someone more knowledgeable should correct me) and the principal target for
most cities has been lawn clippings.  Not only do they form one of the
largest components of what people throw away but they poison waterways with
concentrated leachates.  Most counties in California have funded public
agencies designed to encourage backyard composting;  again the idea is to
reduce waste, especially, ta da, lawn clippings.

>Damian Martin
>Talavera, Central Spain (hot dry Summers, cold Winters)
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