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Re: composting/mulching (trees in deserts)

(Original post - author unknown):
>I live in the low-intermediate desert area in Arizona, USA, which is
>similar to some areas in Australia. I wonder if you could help with info
>about composting in areas that get some soil moisture, but not enough to
>support a garden. I tried putting grass & weed cuttings under a tree
>this summer to help with nutrients & trying to cool the roots, but the
>tree died from lack of water volume, because the tree well was full of
>weeds & didn't hold as much water.
>It is too hot here to grow most things in summer, so we plant when the
>temperatures get into the 90's in October.

Desert soils are often very different than others, even in an area like
my own Mediterranean california climate, which is often considered a
desert environ.  Check on the following points:

 - was your mulch deep and applied widely beyond the drip-line of the
   tree (the area of shade cast at 'high noon').
 - did you watering reach the lower areas of subsoil where the tree
   roots were (a slow drip over a couple of days is the best way to 
   water this deeply for trees and shrubs)
 - did you plant at the correct time - Fall is pretty much the best
   time to plant everything except the most tender tropicals that might
   be marginally hardy in your area - in this way the cooler fall/winter
   temeratures and rains help establish the roots of even deciduous
   trees deeply in the soil - important in a desert climate.

Deserts usually have a slightly different soil stratification than
other parts of the world.  The organic material is small, and there
is often what is called 'desert pavement' - a mulch of medium to small
rocks on the surface of the soil.  This is caused by the climate itself,
and plants adapt to it.  Using a rock mulch is a good way to keep the
soil cooler and facilitate decompostion of organic mulches underneath.
They also help retain water.  Rock mulches can be 'mimicked' using
concrete pavers, brick, or other artificial material that behave in the
same manner as rock.  But be sure that your subsoil is being irrigated
fully by whatever technique you are using (drip is perhaps the best
for you) - you may need to take core samples or dig holes to be sure.

In Article Re: Composting,
Harry Bouckley <hwb@nospam.iinet.net.au> wrote:
> Do not forget to keep the mulch clear of the trunk, so that it does not
> cause other problems with humidity and the bark.
> 
> Large particle mulch is said to be best, as it lets the water into the
> ground, some mulches sold actually stop water from getting to the ground -
> heard it on the radio, so it must be true :-)

Actually, in our area, some mulches are being observed to cause a problem
with shedding water.  The largest culprit is shredded Redwood tree back,
commonly called 'gorilla hair' mulch.  This sheds water the most easily.

In my experience, very dry mulch laid on previously unmulched ground is
likely to have this problem.  This can be mitigated by a light roto-till
before the mulch is applied, possibly mixing in a small amound of
organic material to create a 'transitional' zone.  Also, overhead
sprinkling for a period of time after the mulch is applied helps to
saturate the mulch material and start decompostion/stratification.  I
ALWAYS prefer to use a diverse mulch product - not just one type of
shedded material (i.e. leaves, branches, different types of shredded
trees/plants, lawn clippings, etc.) whenever possible, as this tends
to prevent this problem.  Since I almost exclusively use tree-chippings
from local tree companies, there is usually a varied mix of material
in my mulches.  If purchasing material, go for less uniform mixes or
add different materials together.  Over time, it is useful to 'refresh'
the mulch with different material.

Large particle mulches may allow for easier penetration of water, but
they also allow for weed growth and erosion.  'Bark chips', common in
my area, are a poor type of mulch, even though they are very popular.
They require landscape cloth underneath to prevent weed growth, and
this often causes water shedding as well.  You also don't get the
benefit of a mulch's 'sheet composting' effect.
 
 Sean A. O'Hara                     sean.ohara@ucop.edu
 710 Jean Street                    (510) 987-0577
 Oakland, California 94610-1459     h o r t u l u s   a p t u s
 U.S.A.                             'a garden suited to its purpose'
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