Mulches and mulching

Mulching is generally used to improve the soil around plants, but it also gives your garden a neat, tidy appearance and can reduce the amount of time spent on tasks such as watering and weeding. Mulches help soil retain moisture in summer, prevent weeds from growing and protect the roots of plants in winter.

Using bark as a mulch to reduce water loss around a rose. Image: RHS/Tim Sandall

Quick facts

Suitable for The surface of bare soil and tops of containers
Timing Mid to late spring, autumn
Difficulty Easy

What is mulch?

Mulches are loose coverings or sheets of material placed on the surface of cultivated 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 in summer
  • Suppress weeds
  • Improve soil texture
  • Deter some pests
  • Protect plant roots from extreme 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.

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 leaf mould, garden compost, spent mushroom compost, wood chippings, processed conifer bark, 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. Crushed CDs, sea shells, tumbled glass and similar materials can be used on the surface of containers.

Sheet mulches or woven landscape fabric are ideal 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 it’s best to choose a permeable sheet.

When to apply mulch

Mulches are best applied from mid- to late spring and autumn, when the soil is moist and warm. It is best to avoid applying mulches in winter and early spring as the soil is too cold, and in summer, when it will be dry. They can be applied around new plantings or to established beds and specimen plants.

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, including their roots, 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

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.

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.

Bear in mind that once you have added a mulch to the soil you will need to apply extra water to reach the roots of the plants beneath.

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. 

A build up of mulch can produce a hard layer, which is difficult for water to penetrate. Avoid this by only replacing a mulch when it has completely rotted away.

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.

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