MULCH - The good, the bad and the dirty.

By Geoff Connellan

It takes just a few hot days and our attention turns to the well being of the garden and if the plants have enough water. The natural reaction is to apply copious amounts whether it is actually required or not. Sometimes this is more for our own consolation rather than to satisfy the plant's needs.

There are several strategies that can be used to prepare the garden to cope with hot, demanding conditions. The use of mulch is an important one of those strategies and it is the cornerstone of a water efficient garden. Over the past two decades it has probably been the single most important technique or product has had an impact on Australian gardens.

The term "mulch" can include many different materials, such as plastics, paper, wool, stone, crushed rock, screenings and wood chips. Organic mulch, which consists of shredded or fine broken down vegetative matter, will be the focus of this paper.

The claimed water savings by using mulch range up to 50% however to achieve the benefits of mulch it is important to recognise that it does have some limitations.

The benefits of mulch can be categorised as:

a. Aesthetically enhancing qualities
b. Improved soil characteristics
c. Water saving properties

Organic mulch can be very attractive as a landscaping technique (see above) as it can cover up a lot and provide a textured and uniform surface which is aesthetically pleasing. Being an organic material, it will break down in time and be incorporated with the upper layer of soil. It generally improves the properties such as water holding capacity and encourages microbiological and worm activity.

Water saving is achieved in a number of ways. A good covering of mulch prevents weed germination and growth and so eliminates a wasteful source of water use. Mulch also minimises evaporation from the soil surface and hence reduces losses from bare soil areas.

It has a good reputation as a water saving technique and rightly so! However, if it is used in conjunction with inappropriate irrigation systems it does not provide the savings expected. In fact, it can encourage more water use rather than less. It does have some bad points.

Two properties of organic mulch adversely affect the water needs of plants. These are:

a. High water holding capacity
b. High impermeability to water droplets.

Mulch does not readily allow water to pass through it. It acts as a barrier. Both rainfall and irrigation water can be prevented from entering the underlying soil by a layer of fine mulch. Both low precipitation rate or light rainfall and water applied as small droplets from sprinklers and sprays are trapped by the mulch. Organic mulches that consist of fine particles are very good at holding water. Tests carried out at Burnley College show that shredded pine based mulches, for example, can store 20 mm depth of water in an 80 mm mulch layer. This represents 20 litres of water for each square metre of the test mulch. The water stays in the mulch and does not drain through to the soil. With some fine microsprays it can take two hours or more just to wet the mulch. Light rainfall also just wets up the mulch. The soil does not benefit as much of the water is evaporated back to the atmosphere from within the mulch and so is lost and not available to plants.

The method of application of water should be carefully considered when selecting irrigation applicators or outlets for mulched areas.

Irrigation equipment can be placed either above the mulch or below it. Drip irrigation systems can be covered by the mulch and will work very efficiently. The irrigation system is out of the way and the mulch maintains moist soil conditions without water lost by evaporation from the soil surface.

If above mulch irrigation equipment is used (see below), then spray outlets with very high precipitation rates should be selected. High precipitation rate sprays (greater than 30 mm depth per hour) or bubblers which produce a localised wetting by using high flow rates and small areas of coverage are recommended. Drippers can be used above mulch layers however it is important to select drippers with flow rates greater than 4 litres per hour so that there is enough flow to encourage water to drain through the mulch.

After the next rainfall or irrigation gardeners should dig up the mulch and see how far the water has progressed through the mulch layer. Don't assume that all the water applied to the surface of the mulch is beneficial to the plants. It could be wet on top and still drought-stressed underneath.

About the Writer: Geoff Connellan is a Senior Lecturer in Plant Science and Engineering at Burnley College, University of Melbourne. His current urban irrigation research includes soil moisture sensors, irrigation scheduling and improved water use efficiency.

Copyright protected - refer all copyright enquiries to Global Garden 
Global Garden http://www.global-garden.com.au

More "Down to Earth" Articles 

   RETURN TO GLOBAL GARDEN HOME PAGE