We are told that mulch is good for the garden because it helps retain moisture and can be aesthetically pleasing, but is that all it does? Mulching actually provides a wide range of benefits that are not so obvious.
What is Mulch?
Mulch is any material placed on top of the soil; this can include many different materials, such as paper, wool, stone, crushed rock, organic matter and wood chips.
They can be cheap or expensive and come from either organic or inorganic materials.
Quick rotting mulches, such as pea straw and compost, have the benefit of adding organic matter rapidly, but need to be replaced regularly. Long term mulches, while lower in maintenance, do not give much benefit beyond reducing evaporation.
Mulches also prevent or delay the formation of water repelling crusts that often form on the soil.
Mulch Type vs Benefit
Evaporation
While saving water is the most obvious benefit of mulching, the best type of material for minimising evaporation from the soil is one that is coarse and chunky as it traps air to create an insulating blanket. Materials such as coarse eucalypt chips, pine bark chunks and coarsely chopped straw will all help save water.
Soil Temperature
Mulch also assists in keeping the soil warmer in winter and cooler in summer. Again, as above, coarse materials that trap the air will provide the best result.
You can also influence affects of temperature changes by choosing different coloured mulch. For example, dark coloured mulch can be used in colder climates to try and maximise heat absorption, which assists early spring root growth and can lengthen the growing season; especially for vegetables. On the other hand in hot climates light coloured mulches such as straw will help reflect heat and help avoid high soil temperatures that could cause damage to the roots.
Control Weeds
Although a good covering of mulch helps prevent weed germination and growth, consideration must be given to the type of weeds you are dealing with. Weeds that grow from seed will usually be stopped by most mulch materials provided they are applied to a depth of 5-10cm. However for creeping perennial or bulbous weeds such as kikuyu, or clover try using a continuous underlay of several sheets of thick overlapping newspaper or woven fabric weed ‘mats’. Note: be aware that straw or manure used as mulch may contain weed seeds.
You will find that seeds which do germinate in mulch are easier to remove.
Feeding Plants
If you are using mulch to also feed your plants, it is important you select the right material, such as lucerne hay, pea straw, lupin mulch and fully matured composts. Note: woody materials such as eucalypt chips and pine bark can actually draw nutrients out of the soil and compete with plant roots for food. Ideally it is best to use a high nutrient material such as pea straw underneath a coarse woody mulch to get all the associated benefits.
Over time organic mulch will break down and become incorporated into the upper layer of soil, which will improve the soil structure as well as encouraging microbiological and worm activity.
More about Mulch Types
Grass clippings – Are high in nitrogen content, these are best mixed with leaf litter, straw or twiggy material to prevent it forming soggy, water impenetrable layers. It is always best to compost grass clippings before using.
Paper or cardboard – Ideal to use under bark or stone. The paper should be several layers thick and have holes punched in it to allow the water through to nearby plants. Note: Wet the paper or cardboard before laying it.
Bark or wood chips – Some bark and wood products contain substances that inhibit plant growth, and should be left for several months in the weather to leach out toxins prior to using as mulch. Also see A Guide to Laying Mulch.
Mushroom compost – This has great moisture holding ability and makes a fantastic soil improver, as well as good mulch. Generally it is weed free, but be aware that it has lime added to it, and so is no good for lime sensitive plants such as Camellias.
Stone, pebbles or rocks – Stone, pebbles or rocks are examples of a good long-lasting mulch material, especially for storing heat through the day and releasing warmth at night; which is great for frosty areas. Try to avoid using unsustainable materials and choose by-products or recycled materials like crushed bricks. Note: Inorganic materials do not contribute to the health and fertility of the soil.
Compost – Compost is the best mulch you can use as it has plenty of nutrients, good evaporation control, and quickly improves soil texture and water-holding. It also encourages worms, and is easy to produce.
Pea straw or lucerne hay – Both pea straw and lucerne hay have plenty of nitrogen and break-down well to supply nutrients to plants. They are ideal for heavy feeders like roses and vegetable gardens, especially where their quick decay improves soil texture.
Shredded green waste – Council landfills and private operators often have shredded green waste available. Be aware that this material can often contain weeds or seeds and it is essential that you compost it first before using it as a mulch or soil improver.
Plastic – This is best avoided as it reduces the ability for the soil to breathe, often making the soil turn ‘sour’. While plastic does reduce evaporation it also sheds water, often meaning that less water gets into the soil. If you are worried about weeds it is better to use a weed mat as it lets air and moisture through. Note: Inorganic materials do not contribute to the health and fertility of the soil.
Mulch Pitfalls
Fresh wood chips and barks can contain toxins such as tannins that can damage plants. When using this type of material it is important to age it by leaving it in a pile for several months to allow the toxins to dissipate.
Nitrogen drawdown is the technical term for the process where organic material saps the nutrients from the soil as it breaks down. For this reason woody mulch material should never be dug into the soil. An underlay mulch of manure or some other nutritious organic material such as pea straw will help avoid this problem.
Tips and Tricks
Mulches should be kept away from the trunk or crown of garden plants to minimise the possibility of collar rot occurring.
Some mulches can also become water repellent if they mat down (eg lawn clippings) or are allowed to dry out completely (eg composts). In these cases a good soil wetting agent will overcome the problem.
Mulch and Irrigation
During summer you may find the fine particles of mulch may not allow water to readily pass through it. In fact it may act as a barrier. Light rainfall or small droplets from sprinklers and sprays are trapped by the mulch and much of the water can be evaporated back into the atmosphere from within the mulch.
It is therefore not safe to assume that all the water applied to the surface of the mulch is watering the plants. It could be wet on top and still drought-stressed underneath.
Note: After the next rainfall or watering, dig up the mulch and see how far the water has penetrated the mulch layer and soil.
The method of application of water should be carefully considered when selecting the type of irrigation applicators or outlets for mulched areas.
Irrigation equipment can be placed either above or 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 loss by evaporation from the soil surface.
If using sprayers then spray outlets with very high precipitation rates should be selected i.e. greater than 30mm depth per hour. 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.
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