How to choose the best type of mulch for your garden.

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There are many benefits that come with mulching.

A thick layer of mulch can reduce the evaporation rate of moisture in the soil by up to 70 per cent. It also helps to suppress weeds that compete with other plants for moisture and nutrients. Then, as it breaks down, it becomes humus and that helps to improve the soil structure and increase its ability to absorb and hold moisture.

There are many different types of mulch available and each has its benefits in a chosen situation.

Compost
Compost makes excellent mulch because it has good evaporation control and quickly adds humus to the soil. Good compost in large quantities is more expensive to buy and less accessible than other forms of mulch.

Lucerne or pea straw
These forms of mulch break down quickly and need to be topped up regularly. They are ideal for poor soils, as well as vegetable and rose gardens where regular applications of nutrients and humus are required. Lucerne and pea straw are generally sold in bags or bales, and they are relatively expensive.

Barley straw and sugar cane mulch
These are also sold in bales, are easy to handle, less expensive and more readily available. They also break down quickly and are good for vegetable gardens and areas where soil improvement is required. Top them up regularly.

Mushroom compost
This is a very good mulch and soil improver, but it can often contain lime, so don’t use it on lime-sensitive plants such as camellias and azaleas.

Barks and woodchips
These are slow to break down, so are generally long lasting. They are best used around established plants like shrubs and trees where immediate soil improvement is not required. Be aware that some woody mulch can cause a nitrogen deficiency that shows up as a yellowing in plant leaves. This is referred to as ‘nitrogen draw down’. It’s caused by bacteria taking nitrogen from the soil in order to break down the woody materials. If you notice yellowing of foliage when using woody mulches, add some extra nitrogen to the soil in the form of blood and bone.

Forest mulch
Landscape suppliers will often sell chipped green waste from tree loppers as forest mulch. It is generally the most economical of all mulches and should be used in the same way as barks and woodchips.

Pebbles and gravels
This is the longest lasting of all mulches. It is great for reducing soil evaporation, although it will not improve soil structure. These materials store heat in the day and release it at night so they can be of great benefit around frost-sensitive plants and are the best types of mulch to use around succulents and Mediterranean-type plants like lavender.

How to mulch

The best times to mulch or top up mulch are in early spring and mid-summer. This will ensure that the soil is well covered throughout the warm growing seasons. Before laying mulch, remove weeds and open up the surface of the soil with a garden fork. Deeply water any plants in need.

Whenever possible, apply your mulch straight after good rainfall. The new layer of mulch will act like a blanket, trapping the moisture in the ground for much longer. Most mulch can be laid up to 70 mm thick. If you are using compost, make the layer 30 mm thick. For best results, try to maintain your mulch to its optimum thickness. Do not mulch right to the stem of plants otherwise fungal rot may occur.

Extract taken with permission from Down-to-Earth Garden Design by Phil Dudman, $35, ABC Books.

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