Mulch is a broad term for anything that covers and protects the soil. Usually it refers to organic material that is fairly finely cut and spread over gardens to prevent water loss through evaporation or runoff.
Most folks think of the various types of straw – pea, barley, lucern, wheat -when they think of mulch. Sugarcane and chipped woods are other popular mulches carried by local hardware stores and garden centres.
Mulch can be inorganic too. Gravel can count as a mulch as can recycled and granulated rubber or plastic. Plastic sheets are used as mulch in commercial strawberry operations too. This page only deals with organic mulches for simplicity’s sake.
Mulch is used for several purposes. Some mulches are aesthetically pleasing while others are more ‘functional’.
Mulch helps soil retain moisture so the plants have access to it for longer. It can protect the soil and plants from too much Sun and heat. Many mulches are also full of nutrients which, as the mulch breaks down, become available to your plants. I’d estimate that a mulched garden bed uses at least 50% less water than an un-mulched one.
Mulch also encourages mychorrizae, those white threads that you see under older mulch and compost, these are absolutely fantastic for your garden and are the living, active part of fungi. They are how the fungi and plants communicate and they break both inorganic and organic materials down for plants to uptake. They’re so good that I’ll be writing a whole page about them later.
Things to look for in a mulch
You will be looking for different things depending on your purpose for your mulch.
Big bark chips are great for aesthetic purposes, as is grape marc (the leftovers from grape processing at wineries).
For more practical applications, you want something that’s not too green, too moist or too small. That rules out masses of grass cuttings.
Everyone’s had that experience of making a pile of grass cuttings and promptly forgetting about them or just putting them out of their mind for a bit…then forgetting about them. When you return and dig into the pile, most of the time only to find that the base of the pile has gone soggy, black and smelly.
This is caused by the pile of clippings breaking down anaerobically. Anaerobically means ‘without oxygen’ and is a very slow method of breaking down organic material. It takes a special kind of bacteria to live and work in this kind of environment. We’ll deal with this more on our ‘composting’ page.
The cause, though, interest us here. The clippings are two small and moist and too densely packed to allow either air or water to pass through. They’re even dense enough to stop some insects from getting through.
Most straw mulches are coarser and the pieces harder than grass clippings, so are ideal for mulches. If you buy your bales fresh from the farm though, look out for insects and weed seeds and you might have to reduce the size of the pieces a little, This is easy to do with a whipper snipper or lawn mower. At Ligaya Garden, we take a slightly different approach and all fresh organic material arriving on the block goes into the chicken pen for a while. The girls love picking through it and eating the bugs and seeds while breaking it into smaller pieces ready for the garden. They also poo in it and mix that in so that the mulch has a special kick to it.
Compost vs mulch
Size is the main difference between compost and mulch. Mulch has much larger pieces in it. It also hasn’t been broken down as much as compost and isn’t as readily available to your plants. It doesn’t mix into the soil as well as compost , being used more on the surface.
You can use compost as mulch but mulch doesn’t make good compost.
Many mulches, composts and potting mixes contain a lot of fine dust and even bacteria, Inhaling any of this can be bad for your health. Try to remember to wear a mask when handling any of these. If a mask isn’t available you can go ‘cowboy’ and tie something around your face.
I always dampen down first anything like this that I use. It reduces the amount of dust flying around.
Some brands of packaged mulch, such as sugarcane mulch from the hardware store or garden centre have had the dust extracted as they were packed. It still pays to err on the side of caution though.
Take the following steps and your mulching efforts will surely pay off –
- clear away any weeds and generally tidy up the area to be mulched
- wet the soil to be mulched well
- lay out any weed mat that you might be using
- if you are using cardboard or newspaper as a weed mat, make sure it is thoroughly wet before applying the mulch
- spread out a little nitrogen rich fertiliser (urine, aged manure, Blood and Bone)
- lay the mulch to a depth of 10 cm and wet it well
- don’t mulch right up to the base of your plants, leave a 3 -5 cm gap around them
There you go. Mulched.
The nitrogen rich fertiliser ins’t essential but some mulches will draw nitrogen out of the soil (or rather, the bacteria decomposing it will) which means that it will be unavailable for the plants until the mulch has broken down a bit or you fertilise again. This is the reason that sometimes, after they mulch, gardeners sometimes find the plants that they’ve mulched turning yellow. In most cases, that’s simply a lack of nitrogen,
Sheet mulching is when you cover a large area with some fairly large pieces of material. Newspaper and cardboard are the most popular materials to use. It can make a very effective weed barrier and save a lot of money on mulch.
When you sheet mulch with paper products, try to use un-coloured ones and don’t use glossy paper. The colours often contain toxic dyes and the glossy ones take forever to break down (if they ever do). Remember to take the sticky tape off of things too. Its annoying to find it in your garden a year later.
It’s OK to leave staples in the paper. They’re pretty small and usually made from steel which will rust. Just think of them as a little iron supplement for your garden.
As with other styles of mulching, wet the soil first and make sure you wet the mulching material first. Usually other mulching materials are applied on top of the paper products
If your mulch becomes water repellent
If you apply bone dry mulch or let mulch you already have on your garden dry too much, it will become hygrophobic (or hydrophobic…they’re two words for the same thing). This means that they will repel water.
New bales of dry straw are notorious for this. The only way to remedy it is with frequent applications of water. Don’t just pour water on it for an hour though, most of it will simply run off. Keep your sprinkler or water gun putting out a fine spray and work it over the dry mulch then come back in half an hour or so and do it again. Repeat this until the water can be seen soaking into the mulch and not running off.
If you have very thick mulch, the solution is not as drastic. Thick mulch rarely dries out completely in a garden situation. It is often still damp at the bottom. If the top has dried out and become water repellent, turn the mulch over with a fork and mix the repellent with the absorbent. Then water well. The absorbent will hold the water and let it soak slowly into the dry material.
A third solution is to wet the mulch, then cover it with plastic or a tarpaulin. Leave it overnight then come back in the morning, remove the cover and give it a bit more water. The humidity and moisture trapped in the mulch by the covering will have worked its way into the dry material.
Mulch can be used to cool your garden and home in Summer. It keeps the soil cool and the moisture in it evaporates, cooling breezes that pass over it.
Also, it absorbs some of the heat and reduces heat being reflected from the ground onto your plants or home.