10th January, 2004

GRAHAM LEWIS

A recent edition of Burke’s Backyard left a few people wondering about the value of mulching when Don Burke (apparently) made the statement that mulching was not always good for your garden.

I believe that Don was talking about the effects of a chemical called tannin which is contained within some garden mulches. Tannin consists of several chemical compounds found within the bark of most trees. It is also found within oak-galls and of course in any cup of tea (just ask someone who has to wash the dark stains from their Royal Doulton tea cups). This chemical is chiefly used in tanning and dyeing of material.

The question remains as to whether or not too much tannin can cause damage to plants after mulching. My research has not been able to determine the negative effects of tannin on plants. However, like most things in life I would anticipate that when used in moderation, those mulches containing tannin (such a pine bark or any tree by-product) should not be disadvantageous. So too much mulch replaced too often may be the source of damage. If you are really concerned then use an alternative such as mushroom mulch or pea straw which should both be tannin-free.

The Benefits of Mulching
The value of mulching is invaluable in a country such as Australia, where heat and dryness are a part of our every day lives.

While the use of organic mulches is the most common practice, you can also mulch with inorganic substances such as gravel, stone, granite, coarse river sand or blue metal chips. The choice will depend on the location of your garden in terms of climatic conditions, the type of effect you are attempting to create and the type of plants to be planted.

In general, most gardens utilising Australian native plants will not require a rich organic mulch. These plants are adapted to the nutrient poor soils of the Australian continent, so too many nutrients will not be good for them. However, if your garden consists primarily of exotic/English/European/American species, then an organic rich mulch such as mushroom mulch will benefit your garden greatly. Pea straw is rich in nitrogen, a mineral which many Australian soils lack, so it will also be good for exotic gardens.

Stones, gravel and river sand are great in areas subject to severe fire risk as they act as a fire suppressant. They are also becoming very “trendy” with the return of succulents, and plants with strong elongated leaves such as New Zealand Flax and Native Grasses. Many gardeners utilising these plants are seeking to create a garden with a strong emphasis on the texture and shape of foliage. As such gravel and stones provide a neutral contrast to the colour and shape of many plants, providing a striking visual effect. For a really in vogue style, those with coastal gardens could try using coarse seashell pieces to provide a “beach house” image. At approximately $75 per cubic metre it is not cheap. However, I personally like the effect. Once again the choice is yours (or your wallets).

Mulch is best applied in either early spring or autumn. If applied too late in hot, dry areas, mulches may not retain the moisture contained within the soil from spring rains. Alternately, if applied too late in autumn, mulches may keep the soil too cool over winter.

In summer, mulches may need topping up after two to three months as the materials may have broken down. In such cases, garden areas should be recovered with a thin layer of mulch and watered in.