Most mulches are required to fulfill 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
• 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:

• insulate soil from diurnal and seasonal temperature variations which can stimulate weed growth
• prevent light from reaching seeds (many plants are stimulated into germination by light, e.g; Poa annua)
• release allelopathic or phytotoxic compounds from the decaying mulch material
• prevent new seed or propagules introduced by wind etc from establishing contact with soil
• 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.