What is mulch?

gardener holding a handful of bark mulch

Mulching is a technique that many gardeners are familiar with. It can help to control weeds, retain moisture in soil, protect the roots of plants from extreme temperatures and deter certain pests. It’s also a quick and easy way to improve the appearance of flower beds, vegetable patches, planters and other areas of the garden. But what exactly is mulch? In this guide, we explain the key features of this landscaping staple, including what it’s made of and the different types of mulch that are available.

What is mulch made of?

Mulch can be made from a wide range of different materials. Broadly speaking, it comes in two categories: organic and inorganic. The organic varieties feature materials that used to be alive, such as bark, leaves, wood chips, pine needles, straw, grass clippings and compost, while inorganic mulches are made from materials such as plastic, geotextiles, slate and stone chippings. While the organic varieties break down over time and enrich the soil underneath with nutrients, inorganic mulches are non-biodegradable and so do not break down in this way.

Is mulch compost?

Mulch and compost are related, but they are not the same thing. Compost, which is organic matter that has decomposed and is extremely rich in nutrients, can be used as mulch by simply spreading it in a layer on top of garden beds or other areas. However, it also has other applications. For example, gardeners often mix it with soil to improve its quality and enhance plant growth. It can also be spread onto aerated lawns.  

As mentioned previously, mulch is a much broader term. As well as organic materials such as compost, bark and wood chippings, it can be made from plastic, stone and a range of other inorganic materials.

Types of mulch

Here are some of the most commonly used types of mulch:

Bark and wood chippingsBark and wood chippings are popular choices of mulch for areas such as flower beds, garden paths and shrub borders. Helping to enhance soil fertility, they are easy to lay and have an attractive look that blends in naturally with plants, flowers and trees.

CompostCompost can be highly enriching to the soil underneath. However, it’s important to bear in mind that using it as mulch can mean it dries out and therefore is less biologically active than it is when it’s moist. For this reason, many gardeners spread compost around plants and then top this with another mulch, such as bark, to help keep the compost and soil underneath moist.

Straw – Straw or hay can be a good option for certain outdoor areas, including vegetable patches. Like bark, wood chippings and compost, it adds organic matter to soil as it breaks down. It’s also quick and easy to use. When you’re choosing these products, just make sure that you go for a seed and weed-free version.

Grass clippings – If you have piles of grass clippings left over after mowing your lawn, you could use some of this as mulch for your flower beds or vegetable patch. However, it’s a good idea to keep at least some of these clippings as a natural fertiliser for your lawn.

Plastic – In place of biodegradable organic mulch, there is the option of using sheets of plastic film, which can be spread tightly over soil. By using black plastic which absorbs lots of the sun’s heat, you can create a warmer microclimate at the soil surface. You should avoid using plastic under shrubs however since it can harm their roots. Also, bear in mind that plastic will not enrich the soil with nutrients.

Geotextiles – Also called landscape fabrics, these materials are designed to let water and air through them while preventing weeds from growing. Usually made from plastic, these synthetic water and air permeable membranes are spun or woven.   

Stone – Crushed stone or gravel mulch consists of small pieces of stone that can be spread over soil. It creates a long-lasting solution and can look attractive, but as with other non-biodegradable products, it doesn’t enrich the soil beneath it.

What types of mulch are acidic?

If you use an organic mulch that breaks down, it will release either alkaline or acidic substances into the soil. If you’re growing plants that like more acidic environments, such as rhododendrons and azaleas, you might be searching for a mulch that will help to create soil with a suitable pH. Pine bark and pine straw can be good options for this. In contrast, mulches made from hardwood tend to make soil more alkaline.

Using the right kind of mulch can enhance the look of your garden, make it lower maintenance and help your plants to thrive, so it really is worth knowing your stuff when it comes to these products.