"Although cheaply available, caution should be given to its use since it doesn't act like landscape mulch. Users should be aware that it also results in nitrogen draw down problems and growth inhibiting properties derived from the phytotoxin chemicals (polyphenols) it contains."

Nitrogen draw down is rarely if ever a problem for native plants as very few draw significant sustenance or moisture from the top few centimetres of soil, under the mulch layer, where nitrogen draw down occurs. The vast majority of native plants are well adapted to low nitrogen and phosphorous levels and have very deep root systems. At any rate any N lost from the soil is returned as the mulch ages. This temporary deficit of N in the surface soil can also help to suppress many weeds that thrive in rich soils.

"ULM composed of fine and coarse particles initially soak up all the rain and irrigation rather than allowing it to flow freely down into the soil and onwards into the plant's root zone. Hence, due to its sponge like effect, there is a substantial volume of precious water lost to the soil especially during light showers. Ultimately a lot of the water held in the mulch layer is wasted since it evaporates back into the dry atmosphere.

ULM and especially the uncomposted tree mulches are notorious for their tendency to become hydrophobic (water repellent)."

Water absorption and water repellancy is often a problem with green mulches for the first several months. But this almost always resolves itself after 12 months, at least in clay soils, as the green matter in the mulch breaks down and earth worms incorporate the organic matter into the undelrying soil thus obliterating the interface between mulch and soil. It may be a more long term problem in dry soils with a low clay content.

"After a few months they compact down causing the smaller particles to fill up the air spaces between the larger particles resulting in an impervious layer over the soil. Coupled with this they form an ideal environment for rapid growth of fungi mycelium and the white actinomycetes (the white powder colour in mulch), which tend to strongly bind the soil particles into a very compact hydrophobic mass. Consequently, it is nearly impossible for water to penetrate into the soil below. The only recourse is to regularly "fluff up" the mulch. Interestingly, as a temporary solution, this technique is used on the fine mulches spread under play equipment as soft fall to maintain its impact absorbing qualities."

This is only a problem with light and infrequent rainfall which is unlikely to benefit many plants significantly even if the mulch was not there. As with any other medium water will eventually pass into the soil once the mulch becomes saturated. This will be aided as earth worms break down the interface between soil and mulch in several months.

This temporarily impervious layer, containing phytotoxins and fungi, is also very effectve at suppressing weeds for about 12 months....