Research

Mulches

A mulch is an organic soil covering. The aim of a mulch is to reduce evaporation from the soil, so conserving moisture, and to suppress the apprearance of weeds by blocking light from reaching the soil surface. They also help insulate roots from extremes of temperatures and, as they break down, add organic matter to the soil.

Mulching materials should be applied in late spring over moist soil to a depth of about 7.5cm (3in). Do not heap the mulch around the stems of plants as this can encourage stem rot. The mulch should be added every one or two years to maintain a useful depth.

A range of materials can be used for mulching.

Garden compost: Well made garden compost is an excellent all-round mulch material, but many gardeners have insufficient volume of raw materials to meet their requirements.

Farm manures: Animal droppings mixed with bedding material such as straw or wood shavings provide some plant nutrients. However the release of ammonia may damage foliage if it is used fresh. This is avoided by composting for several months before use.

Wood chippings: Prunings or hedge chippings can be used as mulch if shredded and composted well. Fresh conifer clippings can contain plants toxins, and must be composted first for at least 6 weeks to avoid leaching of chemicals such as tannins out of the woody material which can harm plants.

Bark: This is readily available to buy and a popular choice.

Leafmould: Leaf mulches are satisfactory if composted for one or two years before use. Oak and beech leaves are particularly suitable.

Grass clippings: These are fine provided the mulch is not too deep because this leads to heating of the soil surface.

Cocoa shell: This material is useful but has a tendency to mat. Care must be taken where it is used because it is poisonous to dogs.

Pine needles: These can be used around acid-loving plants such as rhododendrons and camellias. Litter accumulated under conifer trees and hedges can be made use of in this way.

Straw: Straw is often used with strawberries for weed suppression and to lift the fruit off the soil. It is also used with Dahlia and Gladiolus. It may deplete nitrogen levels if dug in after cropping.

Artificial mulches: These include landscaping fabrics, polythene or gravel. They can be used as an alternative to organic mulches.

Studies at Wisley

This topic has been studied by the Soil Science group at RHS Garden Wisley.

For more information on the evaluation and characterisation of a range of garden mulching materials, click here.

Further reading

This is just one topic covered by the Society's monthly journal for members, The Garden, and a recent article of interest is listed below.

Pickering, J. 2000, February. Waste not, want not. The Garden (RHS) 125 (2): 106-109.

RHS Wisley Staff. 1996, May. Problem Profiles: mulching. The Garden (RHS) 121(5): 298-299.

See also: Composting

< Back to popular topics menu