Mulch is essential to gardeners who want to grow plants well and it helps to reduce garden maintenance. Mulch is a layer of organic or inorganic material that is placed on top of your soil and as well as serving practical purposes, it also has an aesthetic benefit, adding a finishing touch to a garden. While many people think of mulching by the time the dry weather starts, I suggest applying it beforehand when the ground is still wet to help retain the moisture which is still in the soil.
Mulch acts as an insulation layer, helping to regulate soil temperature. It keeps the soil cooler in the warmer weather, keeping plant roots cool and keeping earthworms and soil microbes active.
It conserves water by helping to stop the soil drying out as quickly, reducing the watering required by 50 to 70%. It reduces evaporation of water from the soil surface and coarse mulches also allow rain or any water applied to penetrate and wet the soil more effectively without run off.
It reduces weeds by smothering them and by reducing weed seed germination. Weeds compete with your plants for water and nutrients in the warmer weather.
Organic mulches have the added bonus that they add extra organic matter to our soil as they break down. This helps to improve our soil structure and increase its water holding capacity. However many people spread compost on top of their soil thinking that this will act as a mulch.
Compost is finer than mulch and while it is a great way to add organic matter to the soil, after applying it make sure you always add a layer of mulch on top. A layer of fine compost will absorb a light shower of rain and not let it penetrate down to the soil. Wind passing over the fine compost will wick water up and out of the soil, causing the soil to dry out quicker. Finally weed seeds will germinate in the fine compost layer using it as seed raising mix.
There are basically three types of mulch –organic, inorganic and living
Inorganic mulches include gravel, stones, scoria, crushed brick and roof tiles. They are a long term solution that is great in a fire prone area where organic mulches are too flammable. The main drawback with these mulches is that they don’t add any goodness to the soil and they make it hard to do any further on going soil improvement. Whatever inorganic mulch you use make sure that it comes from a local sustainable source. River pebbles were very popular a few years ago however many of the bagged pebbles were actually imported from active waterways in third world countries.
Woven weedmat could even be classified as an inorganic mulching fabric. Usually it is covered with mulch to make it look more attractive. Be aware that over time, weeds seed into the bark mulch and end up growing in on top of the weedmat.
Organic mulches include straws, bark based mulches and garden derived organic material such as chopped prunings, fallen leaves and lawn clippings.
Living mulch is simply when you let other plants such as groundcovers act as a mulch layer in your garden, insulating the soil.
How thick should mulch be and when should you apply it?
The thickness of mulch you apply depends on the sort of mulch you use. If using a light mulch such as straw, a 10cm layer can be used as it is full of air, however for heavier products like bark or scoria mulch only needs to be 2-3cm thick. Avoid leaving a thick layer of mulch up against the base of your plant so that they don’t develop ‘collar rot’. In my garden where the wind is drying even in winter, I make sure that my soil is always covered with mulch. This also helps to control my weeds all year round too.
Where to buy mulch
Mulch is available from garden centres, hardware and fodder stores, or direct from the farmer. It is most economical to use unprocessed products such as bales of straw rather than chopped processed straw. As far as bark based mulches go, you can sometimes source it cheaply or even for free if you see a tree lopping crew working in your area. This freshly chopped mulch should ideally be aged for three to six months to prevent nitrogen drawdown. I have however often used this mulch immediately, simply by spreading some pelletised chicken manure around first, to compensate for any nutrient required in the break down process.
You can also make your own mulch by putting garden prunings through a mulcher. If you don’t own one, make mower mulch by simply throw your raked leaves and leafy garden prunings on the lawn and run your mower over them.
Common mulch types
Scoria, gravel, pebbles and crushed bricks or roof tiles last forever however they do require some maintenance to remain weed free.
Straws such as pea, bean, lucerne and lupin are available in bale form or processed and compressed. For larger areas bales are great value and it is always best to use what is local to your area. Most straw mulches will require replacing each year.
Sugar cane mulch is similar to straw based mulches in that it is effective for around one year.
Bark based mulches come in a range of grades which can remain effective for two to three years. The coarser the bark, the longer it takes to break down.
My favourite mulches and where I use them
In the vegie patch I use a light layer of pea straw only once new seedlings are established. If I use it up against tender new plantings, creatures like slaters which work to break down the straw, can damage the seedlings.
In the garden close to our house where I am actively trying to improve the soil to grow roses and other special plants, I lay biscuits of pea straw and hardly even fluff them up. Very few weeds gets through the biscuits apart from a few pea shoots and I handle these by simply lifting the biscuit and turning it over face down when the pea shoots appear.
In the areas of the garden where I am growing fruit trees and hardy climate compatible exotics and natives I use bark based mulches which only need to be topped up every two to three years.
There are some plants that prefer not to be mulched such as bulbous plants like bearded iris, belladonnas and Josephine lilies which like to bake.
If you want annuals to self-seed, scrape mulch away while their seed is falling and germinating.
And finally leave some area of your garden un-mulched to allow ground dwelling insects like native bees to take up residence.