Mulch: What is it?
Any takers? I thought that I’d better clarify a few things about mulches and why we mulch so that everyone in the garden is on the same page.
Mulch is a protective ground cover that is placed on top of soil. Its primary uses are conserving soil moisture and preventing erosion. Because the soil is covered with an additional organic layer water doesn’t evaporate as readily and soil is not exposed to winds which might whip it away.
In addition to these qualities mulch can also provide nutrients to the soil. As the mulch material is broken down by soil microbes this releases trace nutrients that may be contained in the mulch material. Often times the factor limiting mulch breakdown (as in compost) is Nitrogen availability so it can be useful to place Nitrogen rich material beneath your mulch layer if you aim to feed the soil.
An additional benefit to mulching is it is useful in suppressing the growth of weeds and weed seeds. By covering the soil and preventing light through mulch can block the access of weeds to light. By mulching thickly around the plants that you want this keeps volunteer plants at bay. Our tactic when we first revitalised the food forest was to sheet mulch the cooch grass. When sheet mulching the idea is to make an impenetrable layer that is so thick that light doesn’t make it through and the plants you covered die off. You can use anything organic that comes in a sheet, such as newspaper or cardboard. Easier said than done when dealing with cooch. To ensure that the grass doesn’t run it is important to place the sheets at least 6 layers thick and to have overlaps of 15cm when placing the sheets down. With species such as cooch I’ve been told to give up on Newspaper and just go for sheets of cardboard with a 15cm overlap. From what I saw when the cooch came back in spring, I’m inclined to give up on paper. Another good idea when embarking on sheet-mulching is to mow or cut the unwanted plants as much as possible and to add manure to bring the Nitrogen ratio up. This will assist in decomposing the mulch and generating an environment unsuited to plant growth.
Different kinds of mulch have different benefits. A thick, coarse mulch like bark chip is good for an area like the food forest as we want the mulch to stay around for as long as possible. Woody mulch is mainly broken down by fungi so this process is slower than bacterial breakdown that happens quickly for non-woody material. For an area like the swale bed it is better to use old straw or leaf litter as this is not as thick and breaks down faster. However, mulch must still allow the soil to breathe. Grass clippings and similar materials are no good for mulching as they all press together and rot without air which stinks and is no good for the plants. Leaf and grass mulch is usually chopped up before use to allow air spaces in the mulch. Hence, the advent of the mulcher.
One aspect of mulches that shouldn’t be forgotten is that of “Living Mulch”. When we have a living ground cover this provides the benefits of a mulch. Ground cover plants keep the soil cooler and alive and often generate benefits for other plants in the soil. I’m not a huge fan of Ivy but it grows in my backyard as a living mulch underneath a pine tree that nothing else seems to live under.
So whats the drawback? Well mulch made from some eucalyptus bark and leaves is “Aleopathic” this means that it inhibits other plants from growing. This is why we don’t use the bark mulch on the veggie beds. If you know what trees your mulch is coming from then this isn’t so much of a problem but its very hard to get specific mulch from supply companies.
Another drawback of mulches is that they don’t have the same thermal mass as the soil. This is why we want the mulch in the summer so the soil doesn’t heat up. However, in the winter the presence of mulch keeps the soil cooler and may lead to frost damage. As a rule I remove mulch from the soil surrounding trees in the winter and don’t replace it until a ways through spring. Speaking of which, as good as mulch is, it is never a good idea to have it right up against tree trunks. The decomposing of mulch is rife with microbes and fungus and these are not friends of a tree’s trunk. If mulch is right up against the trunk it can let invaders into the tree trunk. This is especially a worry with many fruit and nut trees which are grafted. The graft point is a weakness in the plants immune system and mulch against this part of the plant is practically begging for infection.
So thats all the who, where and why of mulch that I can think of at the moment. Any questions, I’d love to answer them.