WEEKEND GARDENING | Mulch MUCH ADO ABOUT MULCH Protect<strong>in</strong>g the soil surface with a mulch layer is the gardener’s secret weapon <strong>in</strong> the fight aga<strong>in</strong>st heat, drought, heavy ra<strong>in</strong> and weeds. Here are tips on how to select and best use mulch CanStockPhoto 38 | <strong>Good</strong> <strong>Organic</strong> <strong>Garden<strong>in</strong>g</strong>
Mulch | WEEKEND GARDENING Leaf litter makes good mulch Words Jennifer Stackhouse Mulches keep soil cool on hot days, suppress weeds, protect soil from damage from heavy ra<strong>in</strong>s, reduce water loss and, as organic mulches break down, add nutrients to the soil. Many gardeners believe that the deeper the mulch layer, the better. But, while deep mulches may suppress weeds and add organic matter as they break down, it appears that deep layers of mulch can lead to soil problems, especially <strong>in</strong> times of low ra<strong>in</strong>fall. Too much mulch Soil expert Kev<strong>in</strong> Handreck has conducted studies <strong>in</strong>to mulches. His research has revealed that deep applications of f<strong>in</strong>e, waterabsorb<strong>in</strong>g mulches <strong>in</strong>hibit water reach<strong>in</strong>g the soil and contribute to water repellency <strong>in</strong> soils. In cottage-style and permaculture gardens, deep mulch<strong>in</strong>g also <strong>in</strong>hibits selfseed<strong>in</strong>g of annuals and perennials. In his book <strong>Good</strong> Gardens with Less Water, Kev<strong>in</strong> expla<strong>in</strong>s that f<strong>in</strong>e mulches, especially where they <strong>in</strong>clude an added wett<strong>in</strong>g agent, hold water. If there isn’t much water fall<strong>in</strong>g on the garden, the mulch may stop water reach<strong>in</strong>g the soil and hydrat<strong>in</strong>g the plants. Mulches with high levels of dust can also absorb moisture and prevent it reach<strong>in</strong>g the soil. “A 100mm-thick layer of f<strong>in</strong>e mulch with a wett<strong>in</strong>g agent added to it could trap up to 60mm of ra<strong>in</strong> or spr<strong>in</strong>kler water before any water reaches the soil,” Kev<strong>in</strong> expla<strong>in</strong>s. “Mulches with high proportions (greater than 80 per cent) of particles larger than 5mm are best at lett<strong>in</strong>g ra<strong>in</strong> reach the soil.” He also warns that <strong>in</strong>creas<strong>in</strong>g the depth of coarse mulches above 35mm has no benefit <strong>in</strong> reduc<strong>in</strong>g water loss. The best coarse mulches reduce water loss by up to 63 per cent. These f<strong>in</strong>d<strong>in</strong>gs have led Kev<strong>in</strong> to recommend us<strong>in</strong>g a 5–10mm layer of f<strong>in</strong>e mulch overlaid by a 35mm layer of coarse mulch as the best mulch<strong>in</strong>g regime for gardens. Total mulch<strong>in</strong>g layers should not be greater than 50mm <strong>in</strong> depth. The f<strong>in</strong>e mulch could be compost or well-rotted manure, which breaks down to feed the soil. Natural systems Kev<strong>in</strong> also recommends us<strong>in</strong>g mulches that best reflect the natural grow<strong>in</strong>g conditions of the plants that are be<strong>in</strong>g mulched. For example, cacti and succulents grow naturally <strong>in</strong> sandy and rocky areas so are best mulched with <strong>in</strong>organic mulches. Native woodland plants (<strong>in</strong>clud<strong>in</strong>g many vegetables) grow amid a natural mulch of fallen leaves and bark and so do best with coarse, organic mulches. This type of mulch also improves the beneficial bacteria and other organisms <strong>in</strong> the soil. To read more of Kev<strong>in</strong>’s mulch<strong>in</strong>g recommendations, visit books.google.com.au and search for mulches. Select<strong>in</strong>g the best mulch There are many coarse, water-repellent mulch<strong>in</strong>g products. Chopped or shredded bark, green prun<strong>in</strong>gs and gravel all fit the bill. For an organic gardener, it’s important to select and use mulches that are free of contam<strong>in</strong>ants, <strong>in</strong>clud<strong>in</strong>g pesticides and Prun<strong>in</strong>gs can be mulched ... deep applications of f<strong>in</strong>e, water-absorb<strong>in</strong>g mulches <strong>in</strong>hibit water reach<strong>in</strong>g the soil and contribute to water repellency <strong>in</strong> soils ... deep mulch<strong>in</strong>g also <strong>in</strong>hibits self-seed<strong>in</strong>g of annuals and perennials. <strong>Good</strong> <strong>Organic</strong> <strong>Garden<strong>in</strong>g</strong> | 39
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