What a woodchip mulch does
- Smothers annual weeds and makes perennial ones easy to pull.
- Stops the soil drying out in summer and capping after heavy rain.
- Evens out soil temperature, which roots like.
- Feeds worms and fungi as it breaks down, building soil from the top.
How deep to spread it
Aim for 5 to 10 cm. Less than 5 cm lets weeds through; more than 10 cm on a border is wasted chip. Under trees and big shrubs, and on bare ground you're trying to clear, you can go deeper.
Tip: Pull it back a hand's width from trunks and stems. Chip piled against bark keeps it wet.
When to mulch
Late winter to mid spring is best, when the soil is moist and before weeds get going. Autumn works too. Avoid spreading over dry soil in summer: the mulch will keep the dry in as well as it keeps wet in.
Water the ground first if it's dry, pull any big weeds, then spread.
Where to use it, and where not to
Good: shrub borders, under hedges, around trees and fruit bushes, perennial beds, and ground you're clearing for planting next year.
Less good: seed beds and rows of small vegetables, where the chip gets in the way and can rob seedlings of nitrogen at the surface. Use compost there instead, or wait until the chip has rotted down.
Does woodchip rob the soil of nitrogen?
Only where wood meets soil, and only while it's fresh. On top of the ground, the effect stays in a thin layer, and established plants rooted below don't notice. Dug in, it's a different story, so keep fresh chip on the surface.