People’s awareness of the potential benefits to gardens of using mulch has increased markedly over the past few decades. With the recent limitations imposed on domestic water resources and the implications for water use in the garden, plant moisture stress caused by drought is becoming an increased cause of tree decline. Mulch is gaining attention as one strategy that can assist gardens to cope with hot dry conditions.
Mulch is a covering placed over the ground. Numerous materials are used as mulch and these can be broadly divided into organic products, such as wood and bark chips, shredded leaf and branch matter, recycled pallets, leaf litter, saw dust, straw and inorganic products, such as crushed rock, gravel, plastic and shredded rubber. Organic wood-based mulch is the focus of this fact sheet.
Benefits of mulch
Many beneficial claims are promoted regarding the use of mulch; most common is the ability to save water and add nutrients to the soil. The benefit of any particular mulch will be limited by several factors including existing site conditions, mulch type and application, site preparation and irrigation methods. Broadly speaking, mulch can assist in the conservation of soil water and can improve soil characteristics.
Mulch forms a layer over the soil surface that can prevent soil moisture losses by evaporation from otherwise bare soil. This can potentially reduce overall garden water consumption. Depending on the particulate size of mulch used, it can also reduce runoff. By replacing turf and other plants beneath trees with mulch, more water is available for the targeted trees and shrubs.
As mulch breaks down, organic matter is incorporated into the upper soil layer, providing an environment conducive to beneficial soil organisms. The increase in soil microbial activity results in the addition of nutrients to the soil and increases soil aeration and water movement through the soil. The addition of organic matter into soil generally enhances soil water holding capacity. The insulative property of mulch has the ability to moderate soil temperatures and in conjunction with minimising fluctuations in soil moisture, can induce root growth.
Other benefits of mulch include reducing runoff and soil erosion particularly on sloped sites. Eliminating grass from around the base of trees and shrubs can potentially reduce mechanical damage caused by lawn mowers and weed trimmers. Mulch can also inhibit weed germination.
Limitations of mulch
It should be noted that mulch could have detrimental properties. The type of mulch used and the thickness of the mulch layer are important considerations. Some mulch can reduce the amount of moisture available to plant roots by having a high water holding capacity and high impermeability to water droplets.
Moisture from low rainfall events, light rainfall and surface irrigation systems that deliver fine droplets of water, can be trapped by the mulch and lost to the atmosphere through evaporation. This effect can be exacerbated by an excessively thick mulch layer. Mulch types with fine particles in particular may form a crust, further inhibiting moisture infiltration.
Before installing or using any irrigation, be sure to check for water restrictions that may limit how and when water can be applied.
Efficient irrigation is essential for increasing the efficiency of mulch. The placement of an irrigation system above or below the mulch layer will depend on the irrigation volumes and spray types used. The most efficient way to irrigate a tree is to use a drip irrigation system placed beneath the mulch on top of the soil. This system has very low evaporation rates and avoids the potential of delivered water being held within the mulch. Installing drip irrigation beneath the mulch can also help to suppress weeds germinating in the top layer of the mulch if it remains relatively dry.
If above mulch irrigation is installed, then the droplet size and irrigation volume should both be sufficient to penetrate the mulch layer and adequately wet the soil beneath.
Guidelines for mulch application around trees
- The greater the area of root zone that is mulched and free of other plants, the less competition for water, air and nutrients and the more benefit to your tree. The area to be mulched should be a minimum 1m radial spread of mulch for small trees, 2m for medium sized trees and 3m for large trees.
- Remove or kill grass, ground covers and unwanted plants from the area prior to mulching. It is important that the soil has reasonable moisture content before the mulch is applied and that arrangements for adequate irrigation are made thereafter.
- Mulch should be applied directly to the soil surface. Apply a layer 7cm to 10cm in thickness depending on the tree species and type of mulch used. Organic matter produced as mulch decomposes, which is conducive to root growth. To avoid root disturbance, mulch should generally not be removed.
- For better cultural practices, keep mulch a minimum of 20cm away from the trunk. This space will allow for air circulation around the base of the plant and help avoid potential disease problems that can arise from excessive moisture against the trunk.
To purchase mulch from the Riversdale Depot, call 9278 4444
Some information within this fact sheet has been obtained from published sources. For further reading, refer to the Trees – Further Reading Fact Sheet.