Mulches Aren't Just Mulches

Phil Watson

A satisfying sense of achievement can be enjoyed by enthusiastic bush carers, gardeners and landscapers alike, following a revegetation or landscape 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

As a brief background, mulches are grouped as either landscape mulch (sometimes referred to as groundcover or revegetation mulch), soil conditioner mulch or green mulch (uncomposted shredded trimmings).

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. As humus, these mulches can be dug directly into the soil. 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).

They are excellent for veggie patches and mulching around fruit trees or flower beds. 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 This is treated separately as a problematical mulch type although it is normally considered to be landscape mulch. These popular green mulches are produced by the tree pruning contractors in their tub grinders from tree and shrub pruning or by the home gardener energetically stuffing pruning into their domestic mulching machines.

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.

Landscape mulches require uniform sized chunky particles

Although many landscape mulches are spread with all the best intentions, they 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. This screened finer organic material is not wasted as it supplies an ideal ingredient for soil conditioning compost.

Summarised below are the benefits derived from Chunky Landscape Mulches (CLM) in contrast to the inherent problems associated with the Unscreened Landscape Mulches (ULM).

Many types of mulch are water wasters not water misers

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).

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.

Chunky mulches promote air flow and limit microbial nasties

CLM allow air to flow freely through the network of spaces between the chunky particles, ensuring the air in the soil (air filled porosity) is maximised. Air flow is an essential ingredient to inhibit excessive growth of unwanted disease micro organisms including infectious fungal diseases such as root and stem rot fungi (pythium, phythphora, armillaria etc.). ULM acts like a wet sponge during rainy periods which sits on the soil forming wet scarf-like bands around the stems and crowns of plants allowing these nasty fungi to dominate.

CLM on the other hand provides a constant airflow which 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 seedling's first shoots (plumules) 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. As a result, this eventuates in proliferation of weed maintenance problems.

Similarly, in explaining the fire retarding properties of the chunky mulch, the presence of uniform chunky particles ensures limited flammability. These chunks are very hard to ignite by abandoned cigarette butts or matches in the wrong hands. In contrast, the ULM's fine organic materials are extremely flammable and they are hard to extinguish, smouldering for long periods.

Select your mulch carefully

In the light of the knowledge that mulches aren't just mulches, careful consideration should be given to ensure the mulch selected provides the anticipated benefits. This hopefully will lead to the right mulch (rather than being swayed by cheaper alternatives) and consequently a vigorously growing landscape made up of healthy seedlings.

From "Growing Australian", the newsletter of the Australian Plants Society (Victoria); December 2007.

Australian Plants online - 2008
Association of Societies for Growing Australian Plants