Issue Number #37
Issue Date Summer 2007
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Author Phil Watson

A satisfying sense of achievement can be enjoyed by enthusiastic bush carers, gardeners and landscapers alike, following a planting which features a fresh blanket of neatly spread mulch. These, committed folk take it for granted that by spreading mulch they will not only enjoy an attractive landscape, but will be rewarded from an array of water-saving, weed reducing, disease minimising, fire retarding and nutrient releasing benefits. However, recent scientific studies, reported in the "Gardening Australia" magazine suggest that the anticipated outcomes don't eventuate, primarily due to poor selection of mulch ingredients and an inconsistent range of particle sizes. This article aims to provide a solution to the frequently experienced grower's frustrations associated with loss of plants as well as planting and growing time, due to the poor performance of the mulch.

Three types of mulch:

Soil conditioner mulches (pea straw, Lucerne hay, compost, sea grass etc) are composed of a mixture of coarse and fine particles which will decompose into humus over short periods. Here they contribute significant amounts of nutrients to the soil as well as improving soil structure by clumping together soil particles to form peds. This in turn enhances its water holding capacity and air flow into the soil (air filled porosity). However, since they rapidly break down, they are of little use as a landscape or revegetation mulch.

Landscape or revegetation mulch (pine or gum barks, composted recycled organic mulches, woodchips, various grades of gravels etc) are a specialised group of mulches composed of carefully graded chunky pieces which are slow to break down.

Green mulch These popular green mulches are produced by the tree pruning contractors from tree and shrub pruning or by the home gardener with their domestic mulching machines. Although cheaply available, caution should be given to its use. It results in nitrogen draw down problems and growth inhibiting properties derived from the phytotoxin chemicals (polyphenols) it contains.

Landscape mulches require uniform sized chunky particles

Many landscape mulches leave a high maintenance legacy simply because they are composed of more than the surprisingly small figure of 5% fine particles. The quality mulches (wood chips, chunky barks etc) are screened to a uniform particle size (15mm, 20mm, 30mm etc). This sieves out the finer particles, but does impose a significant increase in production costs.

Many types of mulch are water wasters not water misers

Unscreened Landscape Mulches (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, there is a substantial volume of precious water lost to the soil especially during light showers.

ULM are notorious for their tendency to become hydrophobic (water repellent). 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.

Chunky mulches promote air flow and limits microbial nasties

Chunky Landscape Mulches (CLM) allow air to flow freely through the network of spaces between the chunky particles, ensuring the air in the soil is maximised. A constant airflow allows the good microbes including the nutrient postmen of the soil mycorrhiza and nitrogen fixing rhizobium bacteria to dominate at the expense of the bad ones.

Chunky mulches are weed and fire retardant

The uniform grades of chunky particles in CLM mulches produce the advantages of restricting the germination of any wind borne weed seeds. The seed falls freely or is easily flushed by water, deep within air spaces of the mulch. If it does germinate the seedlings' first shoots would have a fight on their hands to reach the surface, and the first rootlets would find it difficult to permanently attach to the chunky particles. This contrasts to the ideal seed germination environment provided by the fine particles making up the surface of most ULM mulches.

Similarly in explaining the fire retarding properties of the chunky mulch, the presence of uniform chunky particles ensures limited flammability.

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