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Get the Most from Your Mulch

June 11, 2018

Written by Dave G.

New gardeners quickly hear about this thing called ‘mulch’, but often all that hear is, ‘mulch regularly’, or ‘use mulch for weed control’ – without getting any real idea of what it is, what is so good about it, or the best ways to use it. Mulch, properly chosen and applied, is a substantial benefit to your garden, and can really deliver top results. So here is the A-Z of mulch, to help you make smart choices, and see your plants and garden thrive.

Top Tips on using mulch

Why should I mulch my plants?

There are two main reasons given for mulching – weed control and water conservation. Some – but not all – weed seeds need light to germinate, so covering the soil and excluding light will reduce germination. Result? Fewer weeds. However, many of the worst weeds grow from pieces of root, and they will quickly push through most mulching materials. Mulch is not going to save you the job of removing roots of perennials weeds when preparing your beds. That doesn’t mean that mulch won’t reduce weeds, it certainly will, just don’t expect it to work miracles.

Water is lost from soil by two routes. First it evaporates from leaves as part of the process of transport within the plant. Secondly it evaporates directly from the soil surface, rising up from deeper levels as the top layer dries out. Which route loses the most water depends on your planting density – more plants equals more water lost through leaves, but a significant part will still be lost from the soil, and this is where mulch helps a lot. In a sunny spot the direct sunlight warms the soil and speeds up water loss. So mulching protects the soil from the direct rays of the sun, keeping it cooler and reducing evaporation. Plus, the air trapped in the mulch acts like a duvet and stays moist, slowing down the rate of water loss too. It’s a win-win.

There is a downside though. Since the mulch dries out, some of the water from rain or irrigation goes into the mulch and never reaches the ground. This is particularly so with light showers, quick thunderstorms, or a casual watering. It takes steady rain to work through to the soil, so don’t be fooled by a shower into thinking you don’t need to water your plants. This is especially true with fine textured mulches like peat-moss, which tend to repel water, and absorb a lot too. When you do water, make sure you do it thoroughly, letting the water soak the mulch and then work its way down into the soil to a good depth.

Some plants grow best in cooler soil, and mulch will keep it cool. This is an important benefit if you are growing in hot areas plants that naturally grow in cooler regions.

What materials make the best mulch?

There are two broad categories of mulch – those that decompose and those that don’t. Gravel, stones, coarse bark and wood-chips are examples of mulch that doesn’t decompose. These last a long time – although over time they can become mixed with soil – and they need little or no replacement. The disadvantage is that they do nothing for the health of your soil – and healthy soil grows the best plants. In fact, if bark and wood become mixed with the soil they will rob it of nutrients as they very slowly break down, so never dig those kinds of mulches into your soil, or our plants will suffer.

The best mulches are materials that simultaneously work as a mulch and feed your soil. These are ‘organic’ mulches, and they can be anything from garden compost or rotted leaves to rotted animal manures (cow, sheep or horse) or food byproducts like cocoa mulch (which smells terrific too, at least for a while!). As these materials rot down they release nutrients, and also add binding materials to the soil, which improve things like drainage and nutrient retention. Taking care of your soil in this way is the big secret to having the best garden, so it’s important. The only disadvantage is that you need to add more mulch from time to time, to replace what rots down. How often depends on what you use, but once a year or every two or three years is normally about right. This can be done just by spreading new material on top of what is left – no need to either remove it or dig it in, although turning it into the top few inches is worthwhile if you have the time to spare. Spring or fall are the best seasons to do it, but if you live in cold areas it is better to wait until the ground warms before putting mulch down – because it stops the soil warming up quickly, especially if you put down a thick layer

What about using landscape fabric?

It’s common to see people putting landscape fabric under mulch, especially gravels and barks. They do stop these materials mixing into the ground, so it keeps them looking attractive longer. But if you get weed growth – and it does happen eventually – then it can be impossible to remove them through the fabric, making things worse, not better. Covering the ground also reduces water penetration and air movement – both necessary for good soil health. If you use an organic mulch over fabric then it won’t rot down into the soil, reducing its benefits as well. All in all, while the idea of putting landscape fabric down seems like good one, the benefits are minimal, and the long-term problems can be substantial, so invest the money in a richer, better-quality mulch instead.