What is mulch?

Mulches are loose coverings or sheets of material placed on the surface of soil. Mulches can be applied to bare soil or to cover the surface of compost in containers.

Depending on the type of mulch used, there are many benefits of mulching including:

• Help soils retain moisture
• Reduce watering
• Suppress weeds
• Improve soil organic matter
• Provide nutrients
• Deter some pests
• Warm up soil in spring
• Protect plant roots from extreme hot and cold temperatures
• Encourage beneficial soil organisms
• Provide a barrier for edible crops coming into contact with soil
• Give a decorative finish

Mulches can be split into two main groups; biodegradable and non-biodegradable. Both types suppress weeds by blocking sunlight which is needed to germinate and grow weed seeds, and conserve moisture by reducing evaporation from the soil surface.  

Biodegradable mulches

These break down gradually to release nutrients into the soil and help improve its structure. Layers will need replacing when the material has fully rotted down. Among the best materials are garden compost, wood chippings, processed conifer bark, leaf mould, well rotted manure, straw (for strawberries), spent hops (poisonous if eaten by dogs) and seaweed.

Non-biodegradable

Non-biodegradable mulches do not boost the fertility or structure of the soil, but they do suppress weeds, conserve moisture and some have the added advantage of looking decorative. Slate, shingle, pebbles, gravel, stone chippings and other decorative aggregates are often used as a mulch across beds. Sea shells, tumbled glass and similar materials can be used on the surface of containers, but it is best not to use any materials made from plastic. Dark coloured material will warm the soil in the sun whereas light coloured mulch such as white gravel will reflect sunlight and keep roots cooler in strong sunlight.

Sheet mulches or woven landscape fabric can be used for new beds or borders. After laying, slits can be made in the fabric, allowing direct planting through it. The downside is these mulches do not look very attractive, but they can be camouflaged with gravel, bark or others materials. To allow rain and irrigation water to reach the roots always choose a permeable sheet as a waterproof layer may cause surface runoff and drainage problems elsewhere. 

When to apply mulch

Mulches are best applied from mid- to late spring, when annual weeds have not yet germinated and herbaceous plants are dormant, and autumn, as plants are dying back. They can be applied around new plantings or to established beds and specimen plants. New plants that need to establish can be mulched at any time of the year when they will benefit from weed suppression and moisture retention in the soil. 

How to apply mulch

Beds and borders can be mulched entirely, taking care not to smother low growing plants or to pile mulches up against the stems of woody plants.

• To be effective, biodegradable mulches need to be between at least 5cm (2in) and ideally 7.5cm (3in) thick
• Lay mulches over moist soil, after removing weeds, when the soil is not frozen
• When creating new beds, planting through mulch sheets is effective
• Single trees and specimen shrubs are best mulched to the radius of the canopy
• As it decomposes over time, a biodegradable mulch will feed your plants and microorganisms in the soil reducing the need for additional soluble feed
• Ground covering mediterranean plants such as Thymus species may be best left unmulched as it can hold too much moisture around the stems and foliage

Problems

If laid correctly there is generally no problem with mulches. However, if they are in direct contact with the stems of trees or specimen shrubs they can cause the stem to soften, making it vulnerable to diseases.

To save water and suppress weeds, the type of organic material you choose is less important than putting on a thick enough layer. Thicker layers will block sunlight from weeds, insulate the soil better and reduce the amount of water evaporating.

Depending on the quality of the material there is a possibility of introducing weeds, pests and diseases to the garden and, with woodchips there is a slight risk of introducing honey fungus.

Once you have added a mulch to the soil you may need to apply extra water to reach the roots of the plants beneath, but mulch will also help rain to soak into the soil, and less water will evaporate, so you should find you need to water less frequently.

Using freshly chipped material such as woody prunings or grass clippings can encourage the microorganisms in the soil to grow but they may use up reserves of nitrogen, leaving less available for plant growth. If you have freshly chipped material, keep it stored for a few weeks before using.

There is no need to remove mulches to apply fertilisers. Fertilisers are spread over mulches in late winter and are washed down to plant roots by rain.

Avoid damaging roots of plants by hoeing weeds growing in mulches around permanent plants. Remove weeds by hand and add a further layer of fresh mulch.

Organic mulches can be easier to maintain as they can be replaced by adding another layer when it has completely rotted away. Gravel can sometimes mix with the underlying soil if not applied thickly, encouraging surface weeds. 

It is not uncommon for the white fungal mycelium of harmless saprophytic fungi to be found in soil that has been covered or enriched with an organic mulch. This is nothing to worry about and there is no need to dig out the mulch or white fungal growth.

How much water could be saved?

Organic mulch breaks down into the soil into materials that help with water retention. The RHS and Cranfield University have developed an on-line tool to help gardeners get an idea of how much water might be saved with techniques such as using mulch and help them make the switch from mains to rains.