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.