Organic weed control - the use of mulch and 'living mulch'

Most mulches are required to fulfil both of the main functions of mulch, that is;

1. to limit moisture loss from the soil profile, and

2. to prevent weed germination.

Whether the mulch performs well as a weed barrier is a function of both the nature of the material itself and the environment into which it is introduced. Environmental factors which will affect performance of the mulch may include:

The seed bank of weeds present in the soil.

The introduction of new propagative material by wind, water, animals (in dung or attached to wool/fur etc) or gravity.

Extension of underground growth from the standing crop of weeds or growth from remnant fragments of stolons and rhizomes which still remain after the site has been prepared for planting.

The effect of topography or exposure on all of these factors.

Mulches can work to prevent germination and establishment in a number of ways, including the following:

They may insulate soil from diurnal and seasonal temperature variations which can stimulate weed growth.

They may prevent light from reaching seeds (many plants are stimulated into germination by light, eg Poa annua).

They may release allelopathic or phytotoxic compounds from the decaying mulch material.

They may prevent new seed or propagules introduced by wind etc from establishing contact with soil.

They may keep soil under the mulch in a loose and friable condition so that young weeds can be easily pulled.

Mulching materials can include a wide range of plant debris or vegetative material, mineral aggregates or a variety of natural and synthetic fibres.

Natural fibre mulches are commercially available. They include coconut husk mats, waste wool mulchmats and compressed cardboard or paper products and compost.

Synthetic materials include black plastic and woven weed mats or "mulch-mat". Plastic sheeting limits oxygen and water movement into the soil.

Natural mulch materials must be selected on the following characteristics:

Be within the budget of the project (considering the cost benefit of reduced plant loss, watering and weeding).

Last a long time to avoid the cost of reapplication, or be cheap enough to replace.

Break down quickly enough to allow natural regeneration.

Look good, usually by blending into the landscape. In some situations mulch may become a feature, such as in formal horticultural environments utilising scoria or marble chips.

Be free of weed seeds and propagules.

Provide a barrier for evaporation and weed growth.

Mulches are seldom adequate to control weeds which grow from vegetative material (stolons, rhizomes etc). Couch grass (Cynodon dactylon) and kikuyu (Pennisetum clandestinum) are particularly aggressive invaders of mulch.

Because of cost and efficiency, large wood chips or bark are the most frequently used materials for urban use. If used in sufficient depth, these materials are generally very good at limiting growth. Follow up control of germinating weeds is usually necessary, with hand pulling being an option where growth is not very dense, as the weed roots release their grip on loose soil under the mulch easily. Use caution with large quantities of pine bark, as the resins and turpenes which they exude can inhibit germination and growth of sensitive plants.

Mulch should be used to ten centimetres in depth.

More recently, composted "green waste" mulches have become available. They are made from recycled municipal green waste and green industrial waste. These materials must have been subjected to hot composting processes to be considered safe. Incorrect composting could introduce weed seeds and plant diseases and cause offensive odours. A certified product, such as the Jeffries Garden Soil Organic Compost or Forest Mulch is ideal. It meets organic growing standards and conforms to the Australian Standard AS 4454, for compost, soil conditioners and mulches.

Alternative weed control options

Alternative weed control options should always be considered as part of the weed control program, before resorting to herbicides.

Control Method - Cultural techniques

Hygiene: avoid transfer of weed propagules (clean equipment etc), quarantine, avoid seed set, prevent establishment, decrease weed seed reservoir

Ecosystem manipulation: uses changes in soil fertility (nutrition, pH), drainage, irrigation (eg use drippers to restrict water), planting density and depth, planting date etc to control weeds

Physical barriers: mulching-synthetic and natural

Materials brought in from other sites are quarantined

Control Method - Mechanical control

Hand pulling: slow but very selective and can prevent damage to vulnerable plants nearby

Slashing or mowing: may not be suitable for some weeds eg creeping oxalis (Oxalis corniculata)

Brushcutting: expensive, but can give good control, especially suited to linear features such as fences

Cultivation: may cause soil erosion or create seed bed for new weeds to establish

Control Method - Thermal weeding

Burning will break dormancy of many weeds: which can then be destroyed (eg Apple of Sodom Solanum hermanni or Gorse Ulex europaeus). 55 minimum (80 - 100 for persistent weeds or thick seed coat)

Can stimulate germination of some weeds leading to new problems - may be dangerous and need to be restricted to certain times of the year to avoid wildfire hazard

Solarisation: cover soil with clear polyethylene sheet (40 -100  thick) and seal:

secure the edges of plastic sheet

soil must be moist

can kill weed seeds and diseases

allow a minimum four weeks treatment, depending on daytime temperatures

Control Method - Use of vegetative cover

The best way to exclude weeds is to use dense plantings of hardy competitive species which will suppress weed growth. Once established, they should require little maintenance. Closely planted trees may achieve this effect once the canopy closes over, but the technique is more often used for groundcovers.

Use of vegetative cover for weed control

Use of vegetative cover is the most desirable form of weed control, from an ecological perspective. This strategy includes the use of "living mulch" grown to out-compete weeds. Vegetative cover has the following advantages:

It prevents new weeds from germinating or smothers weed seedlings.

It keeps the soil covered and protected from wind and water erosion.

It protects the soil ecology, by insulation, temperature control and continuous addition of small quantities of organic matter.

It keeps the soil active - plant roots are continually exploring the soil profile and stimulating soil biology as they grow.

It provides food sources or habitat for small animals, birds, insects and other organisms.

If appropriate species are selected and well grown they will require little maintenance and will compete very little with crop plants.

Factors which influence the economics of this choice include the following

Location and seasonal variation which will influence the number of times per year that grass swards need to be mown or groundcovers replaced.

Aesthetic requirements of the site.

Slopes, such as steep batters, where machinery access is difficult.

The ratio of edge to planting area (ie the length of boundary that needs to be maintained against weed infestation).

Weed resistance of the species chosen for living mulch or groundcover.

The weed species present (eg rhizomatous or stoloniferous perennial weeds such as couch or kikuyu may be very difficult to remove from groundcover or massed shrubs).

The growth rate of species used.

The requirement of species used for additional inputs (fertiliser, irrigation etc).

The quality of planting stock.

Longevity of living mulches or groundcovers.

Generally, the groundcover species used should be selected to be self replacing, or to have a long life expectancy, but should not themselves become weedy, for the technique to be competitive.

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