Woody waste: using as a mulch

Wood and bark from chipped or shredded tree, shrub and hedge prunings makes a useful mulching material in the garden.

Fresh mulch laid around a recently planted tree

Quick facts

Suitable for All woody plants
Timing All year round
Difficulty Easy to moderate

Suitable for...

Felled trees or shrubs and prunings from hedges generate a large amount of woody waste in the garden. Whilst small amounts of the softer, greener material may be added to the general compost bin, larger branches and trunks cannot be disposed of easily in this way. Instead of burning it, chipping or shredding it can turn this waste into a valuable mulching material.

Bark and chipped woody waste can also be bought in bags from garden centres or in bulk from compost suppliers.

Why use woody mulches around the garden?

Chipped or shredded materials from both broadleaf (oak, beech, lime etc.) and conifers (conifers e.g. pine, yew, larch etc.) sources make an excellent mulch. These materials decompose slowly making them long-lived, reducing the need for re-application.

They also make an excellent protective layer, moderating soil conditions by absorbing moisture and slowly releasing it back.

As with other mulches they help suppress weeds and protect the soil from erosion, capping and compaction  by wind and rain.

How to use woody mulches around the garden

Mulches from broadleaf trees and shrubs

Hedge/shrub prunings and stems from younger trees or tree suckers can be shredded and applied directly as a fresh mulch on unplanted or planted areas.
 
Wood, including bark from larger, mature trees can be chipped and applied as a fresh mulch on both unplanted areas and around plants on well-established beds. However, be aware that woody materials may contain compounds that harm plants (phytotoxic) which are produced by the plants to deter potential herbivores and/or help supress the germination and growth of competitor plants (allelopathy). Whilst the concentration of such chemicals is unlikely to affect mature, well-established plants, it is better to avoid using fresh broadleaf chippings on newly planted beds.

Alternatively, this issue can be resolved by maturing or ageing woody materials before use. Ageing is the process of stacking the material into a pile and leaving it to weather for three to four months. This process will allow any phytotoxic compounds to be broken down, rendering the material safe even on newly planted beds.

Mulches from conifer trees and shrubs

Fresh conifer materials are more likely than broadleaf woody waste to contain phytotoxic compounds e.g. tannins, which can reduce germination and harm young plants. Again, this is unlikely to be an issue for mature plants, but chipped and shredded conifer mulches should be aged before use around establishing plants.

Mulches from mixed woody materials

If the material you wish to apply is an even mixture of a range of tree species, it can be applied fresh around established plants. When in doubt, it is better to age it or apply to non-planted areas of the garden.

Application of woody mulches

  • As with all mulches , take care not to  cover herbaceous perennials or small bulbs
  • Do not mulch directly up against the base of tree trunks or shrubs as this can create a moist, low oxygen environment which may encourage disease. Leave a 8-10cm (3-4in) gap
  • It is best  to apply mulch to beds in early spring, before annual weeds become established. Mulches are most effective at suppressing weeds if applied before they get a foot-hold
  • Woody mulches can be applied any time of year, but try to avoid applying mulch to dry soils
  • Get the right depth: wood chippings due to their larger particle are less likely to compact and can be applied at a greater depth, about 8-12cm (3-5in), compared to traditional composted organic mulches such as garden compost or well-rotted manure that are best applied at depth 5-7.5cm (2-3in)

Frequently asked questions

Will woody mulches acidify the soil? If so should I apply lime to counteract this?
While there may be transient changes in pH within the mulch layer itself, there is no evidence that this has any impact on the underlying soil. No lime required!

Will woody mulches ‘rob’ the soil of nutrients such as nitrogen causing plant deficiencies?
There is little evidence that woody mulches create any issues with nutrient deficiency. While there may be a small zone of nutrient uptake at the interface between the mulch and soil layers, this will have little impact on well-established plants rooted below this zone. As a precaution it is prudent to avoid woody mulches on annual beds, and around soft fruit or on vegetable beds where plants may be shallow rooted. There is no need to apply more fertiliser than usual  to the soil before mulching - in fact studies have shown that as woody mulches break down they release nutrients thus increasing long-term soil fertility.

In contrast to mulching, the incorporation or ‘digging-in’ of woody materials into the soil can create temporary nutrient deficiency issues. This is because the material is being mixed into deeper layers of the soil where plants feed. Here the material stimulates the growth of soil microbes, which then take up or ‘lock-up’ nutrients (typically nitrogen) leading to a reduced availability for plants for up to two years.

Can mulches made from diseased wood cause infection in healthy plants?
Studies indicate that wood and bark chip mulch made from diseased trees is unlikely to transmit pathogens to the roots of healthy trees, growing under good soil conditions. It is important that such mulches are not worked into the soil, as this will increase the risk. The RHS do, however, recommend that basal parts of trees infected with honey fungus, Phytophthora root rot, or parts of trees infected with stem diseases such as verticillium wilt or coral spot, are not used, to reduce the risk to plants.

Are there risks when shredding and using mulch from poisonous plants such as laurel and yew?
When shredded, leaves from cherry laurel (Prunus laurocerasus) and Portuguese laurel (P. lusitanica) will release miniscule amounts of hydrogen cyanide. Fortunately, gases diffuse very fast (10,000 times the speed of diffusion in water) so it is highly unlikely that a harmful dose of cyanide will be encountered in normal gardening operations.

It is possible, although improbable, that large scale shredding of cherry or Portuguese  laurel in a confined building might lead to enhanced concentrations so it is sensible to always undertake shredding outside or in an open fronted shed.

Can I use sawdust as a mulch?
Sawdust can be used in a similar way as wood chips, but apply in a thin layer only as it tends form a ‘crust’ and is prone to compaction. Same as with woodchips, it should be ideally stacked/aged before use. It can be composted, although best mixed with nitrogenous (green) garden waste. Due to the small particle size it is likely to compost faster than wood chipping. 

Large amount of sawdust may be produced during stump grinding.

Can shredded treated timber be used as mulch?
This would not be recommended. All old treated timber should go to landfill as older wood can contain arsenic or chromium, which are potentially harmful.

Can waste fibreboard, chipboard or other wood products be used as mulch?
No, these contain synthetic resins whose effect on plants, soil and the environment is unknown. They should be consigned to the council recycling centre.

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