western-queensland-gardening-guide.pdf - South West NRM

Laying the mulch
Mulch should be spread from the stem to 1 m to 1.5 m all around the seedling. It
should not directly touch the stem as wet mulch may rot the stem and decomposing
mulch may burn it. Ideally, mulch should be applied directly after the tree has been
planted and watered, over weed-free soil. Initial weed control can be achieved
through cultivation or use of herbicides. Although mulch does reduce competition
from weeds and grasses, residual herbicides used in conjunction with mulch can
greatly improve performance. A good covering of mulch should be in place before
each growing season. Mulch is most important in the first years of growth, and
organic mulches will benefit a tree throughout its life.
Potential pro blems with mulch
The properties desirable for mulch will depend on the soil type and climate. A thick
layer of mulch will reduce extremes in soil temperature on a daily and yearly basis,
while a thin mulch might act quite differently. Black plastic will encourage heat
absorption and increase soil temperature, while white plastic will do the opposite.
Organic mulches which are largely wood or bark based often draw nitrogen from
the soil as they decompose. This should not affect fertile soils or "nitrogen fixing"
plants such as acacias or casuarinas. For less fertile soils, however, a small quantity
of nitrogen fertiliser should be added to such mulch. This may increase the rate at
which the mulch decomposes, but should ensure that it does not compete with the
growing tree for nitrogen. Fertiliser meant for the tree should be incorporated into
the soil, not the mulch, otherwise it will be used by the mulch in decomposition,
rather than getting to the tree.
In frost prone areas, some mulches may actually increase the risk of frost. A thick,
dry layer of organic mulch does not store much heat during the day. At night, it will
cool down much more quickly, thus reducing the risk of an induced frost. Mulches
can harbour a variety of insects. Most of these will be a sign of healthy soil, but if
they appear to be damaging the tree, push the mulch away from the tree and seek
advice from your local DPI officer.
Mulch can also act as a fuel pile (especially straw and hay), concentrating fire around
the tree, so remember to maintain fire breaks.
As they decompose, many mulches will require additional nitrogen. Freshly-cut
vegetation such as grass will produce heat as it decomposes, and produce heat
which could burn the roots and trunk of the tree. Wait until it has dried before using
it as mulch.