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The “ins and outs” of mulching your garden.

Mulching around the Bushes

There are many important reasons to mulch; but probably the most important reasons are to prevent soil erosion and for water conservation. This is especially important in our hot climate because mulching can reduce watering by about 60%. Mulches can either be organic or inorganic, but organic mulches have the added advantage of slowly breaking down and adding nutrients to the soil. This improves soil structure and drainage, and encourages earthworm and soil microbial activity. Mulching will also give your garden a neat, tidy appearance.

Weeds compete with plants for both water and nutrients and a thin layer of mulch will help suppress the germination and growth of weeds, and therefore reduce the amount of time you need to spend on weeding. Mulching also keeps the soil temperature more constant during hot summer days and cold winter nights.

If laid correctly there is generally no problem with mulches, however, improper mulching materials and practices may have a negative impact on plants in your garden. For example, if organic mulch is too thick and is piled up against the stems of your trees or specimen shrubs, it can cause the stems to soften, making them vulnerable to rot and other diseases.  Therefore, mulches should be no thicker than 75mm, any deeper can cause problems. Also, only replace organic mulch when it has almost completely rotted away, because mulch which is too thick can produce a hard layer which is difficult for water to penetrate. Another reason to keep your mulch thin is that slugs, earwigs, cutworms, and other insects love cool, moist, dark places to hide away.

Mulch bark nuggets.  Picture courtesy Stone & Bark

Mulch bark nuggets. Picture courtesy Stone & Bark

It is also vital to remember that before putting down wood based organic mulches, you need to cover your soil with a thick layer of compost or manure before laying the mulch; and if you are renewing your mulch, you first need to remove any old mulch which has not decomposed, before adding the fresh compost. Adding a layer of soil before mulching with bark chips will ensure that what is called a “nitrogen drawdown” does not occur.  Nitrogen drawdown occurs when micro-organisms like fungi and bacteria start breaking down the mulch; and because nitrogen is the major nutrient they need to do this, and they need to draw it out of the soil; this results in a competition for nutrients between plant roots and the mulch that’s sitting on top. To combat this, you need the “manure or compost sandwich” between the soil and the mulch. The manure will provide nutrients for the plants directly, while providing a buffer for your plant roots. For this reason it is always best to use medium to coarse grade bark mulches. Coarser mulches also don’t absorb much moisture, while still allowing water to penetrate to the soil easily. The disadvantage of using finer based mulch is that it absorbs moisture and can form a seed bed for weeds.

Black Slate Pebbles. Picture courtesy Stone & Bark

Black Slate Pebbles. Picture courtesy Stone & Bark

Also worth considering is the colour of your mulch. Dark coloured mulches will absorb and retain more heat from the sun than light coloured ones. This is an advantage in cooler regions but a disadvantage in hotter climates. Light coloured mulches, and especially white stones, will reflect light and heat and in very hot climates can dangerously overheat surrounding plants.

Old newspapers can also be recycled and used as mulch by laying the sheets thickly over the soil, before weighing it down with compost or manure. Newspaper is great for suppressing weed growth and decomposes quickly, adding valuable structure to the soil, and earthworms also love it!

Because mulch, whether it’s degradable or not, will be down for a long time, it is really worthwhile to prepare the ground thoroughly before laying down any type of mulch. Ensure that the area is completely weed and debris free, and for non-biodegradable mulches lay down several layers of woven landscape fabric on the surface of the soil before applying your decorative mulch. This material is permeable, allowing water and nutrients to reach the soil; but remember that over time, soil may build up on top of the fabric and any weed seeds which fall on top may germinate and will need to be weeded out. Nevertheless, these weed fabrics certainly help a lot to suppress weeds.

Picture courtesy Stone & Bark

Picture courtesy Stone & Bark

Some types of mulch just won’t stay put; gravel and stones creep onto lawns; light husks like Cocoa hulls can easily blow away; and small bark chips can wash away in heavy rain. It will be necessary to edge some beds and pathways to prevent this.  Generally mulches with heavy or large pieces are more likely to stay put.

Apart from a few plants that don’t like mulch, such as the bearded iris, whose rhizomes actually need to bake in the sun, mulching will encourage the garden to thrive and help to conserve precious water.

Biodegradable mulches break down gradually to release nutrients into the soil and help improve its structure. Layers will only need only replacing when the material has fully rotted down. Among the best materials are; leaf mould, compost, well-rotted manure, bark chips, nut husks, and even pine or other cones.  If you live in a rural area you may be lucky enough to be able to procure straw based mulches, or spent hops (poisonous if eaten by dogs), lucerne, cane type mulches, or even seaweed and sea shells. Plant based mulches like straw are most economical and are usually available by the bale, or chopped and bagged for ease of handling and a neater look in the garden. Cube type mulches like palm peat are also great to use in small areas and for potted plants. When water is applied to them, they swell out considerably and are light and long lasting. Palm peat is also a good soil conditioner and can be worked through the soil.

Blue Sea Shells. Picture courtesy Stone & Bark

Blue Sea Shells. Picture courtesy Stone & Bark

Bark and nut-shell based mulches are great organic choices and freely available from garden centres and some hardware stores. Macadamia, pecan nut, and peanut shells look stunning in the garden. Milled bark is also most attractive, and available in various grades, from fine through medium, and coarse.  An advantage of bark and nut shell based mulches is they take longer to break down, so you don’t have to apply them as often.

Non-biodegradable mulches don’t break down to boost the fertility or structure of the soil; but they will suppress weeds, conserve moisture; and are almost maintenance free. Beautiful finishes and textures can also be achieved, depending on the medium used. These types of mulches are perfect to use in areas like pathways, near patios, water features, swimming pools and entranceways, where their beauty can be admired. There are so many decorative aggregates to choose from; like tumbled glass in an array of wonderful colours, river pebbles in all sizes and shades, slate, shingle; and even gravel. These materials can also be used in containers; and if you love to re-cycle, even crushed CDs can be used on the surface of containers.

If using non-biodegradable mulches in the garden, bear in mind that they are not easy to clean and cannot be raked, so try not to use them underneath trees which drop their leaves or near other shrubs which are messy, dropping their flower or fruits. The answer for such areas would be to invest in a garden blower to do the job.



In conclusion, if done correctly, mulching your garden will not only beautify it, but also greatly improve your soil, reduce soil erosion and the need to weed; and save you a fortune on water bills. Mulching can be done at any time, but is usually done in spring or autumn, so if you have not done so yet, get mulching!