Using Mulches: Depth of Mulch and Mulch Types

Mulches are an invaluable gardening aid, saving time and effort by keeping weeds down and water in. There’s nothing glamorous about this job. This is the time of year to build a mulch layer for perennials to prevent frost from penetrating or to add a layer of manure to the fruit garden. So get down and get dirty! We show you how it’s done.

Mulches reduce the amount of water lost by evaporation from the soil by forming a barrier. They are made from material such as gravel, bark or coco shell that are applied to the surface of the soil, whether this is around the base of trees and shrubs or on the surface of pots and containers.

Apply in spring or early summer

The best time to apply a mulch is in spring or early summer, before temperatures start to rise. This ensures that as much moisture as possible is kept in the ground, just as plants are actively growing. A mulch should be between 5cm (2in) and 7.5cm (3in) thick for it to be the most effective.

Reduces weeds

A thick mulch reduces weed growth by smothering them before they can break through the surface. Most mulches break down into the soil during the winter months to improve its structure and to provide nutrients or better drainage for next years plants. It is therefore important that mulches should be re-applied every year to ensure that an effective surface barrier is always maintained.

Reduce frost damage

Certain perennial plants, such as hostas and crocosmias, can be rather tender and prone to frost damage. By building up a mulch layer over the plant crown in autumn the frost will not penetrate to the roots of the plant to harm it. Mulches will also keep plant roots cool in hotter weather and are one sure way to protect from wilting during dry spells. Mulches can also be used in a decorative capacity.

Which mulch to use?

There are mulches that are specific in their usage but generally this only relates to manure. This should only be used on flowerbeds or the fruit garden. Never apply it to rock gardens as it will be too strong for the plants and can lead to lots of leafy growth at the expense of flowers; and on the surface of pots and containers it can encourage flies so avoid at all costs.

Follow this practical guide to find out which mulching materials are best suited to different areas of your garden and how they should be used.

Depth of mulch

The mulch layer should be from 2.5cm (1in) to 7.5cm (3in) deep, depending on the material.

Mulch types

Flowerbed mulch

This is made from coco shells and is ideal for mulching flowerbeds. It is pleasant to handle and has a strong smell of chocolate. In a few weeks, the shells bind together to make a loosely knitted, porous mat over the soil surface which provides excellent ground cover in larger borders where weeding can be difficult and time consuming. This binding effect means that a 5cm (2in) mulch layer will be adequate wherever it is used.

Fruit garden

Well-rotted manure can be bought from garden centers. It is useful in the fruit or vegetable garden as it provides a rich base of nutrients as it decomposes. When applying it to borders in spring, always use well-rotted manure and lay it 7.5cm (3in) thick. Unrotted manure should only be applied in autumn, in a layer 5-7.5cm (2-3in) thick. Always wear gloves when handling manure.

Rock garden

Alpine plants are small and slow-growing and must have a well-drained soil to grow well in a rock garden. By applying a thin layer of coarse gravel or grit around them, not only are competitive weed numbers kept under control, but the soil drainage is improved and rotting avoided because harmful winter rains quickly run through it.


Large pebbles create strong visual impact on the surface of containers while reducing water loss. They also ensure that the dusty surface of the compost will remain undisturbed, if pots are sited by open windows.


An excellent mulch available in the summer is grass clippings. These can be spread around plants or used to cover the odd gap on the soil surface and can be collected later and incorporated into the compost heap. It is less ornamental than coco shells, so use it at the back of borders where it won’t be seen. It is great value as it is free, whether from your own lawn or a neighbor’s. If weedkiller has been used on the lawn, then do not use the clippings for four cuts.

Island bed

Bark is especially useful for preventing competitive weed and grass growth in island beds; any young weeds that do appear through it are easily removed with a hoe. It is sold in three grades; coarse, medium and large. Each is equally effective as a mulch but differs in ornamental appearance and the speed at which it breaks down in the soil.

Path with gravel

Lay a fiber membrane where the path is to be sited and cover it with a 5cm (2in) gravel layer. The membrane will act as an underground mulch, stopping weeds from growing, so a hardcore layer under the gravel can be omitted, thus reducing costs.