Notes on Wood Chip Mulch

As a follow-up to last week’s post about winter fruit tree spraying, I thought it might be helpful to elaborate on what I wrote about mulch in the final paragraph. You might recall that I expressed a love for using woodchip mulch on mature trees. This may have raised the hackles of some gardeners. Wood chip has received some negative press in recent years, mostly because of a process known as the “nitrogen draw-down effect”. In essence, nitrogen draw down occurs when a woody mulch (which is naturally high in carbon but low in nitrogen) leaches available nitrogen from the soil in order to decompose.

Studies conducted by US master gardener Dr Linda Chalker-Scott have shown this draw-down effect to mostly be a myth. In fact the opposite may be true. A wood chip mulch can actually increase nutrient levels in the soil, and any draw-down occurs only at the interface between soil and mulch. For shallow rooted plants such as vegies, wood chip may inhibit plant growth to an extent. But for deep rooted plants such as fruit trees, it’s a positive long term mulching ingredient. Don’t forget that wood chip is a recycled product. My local council offers “forest mulch” for free at a nearby tip. It’s made from all the chipped prunings that people dump, which means that quality can vary, but as a whole it’s a bargain. For slightly more quality control, I sometimes buy forest mulch from an arborist or landscape supplier. At anywhere between eight and $25 a cubic metre, it represents excellent value.

Wood chip mulches might be cheap, but they’re also cheerful. All natural mulches are beneficial for enhancing life within the soil, but the great thing about wood chip is that it mimics the kind of ecosystem from which most fruiting plants originate. The wild ancestors of most domestic fruit trees, particularly those within the rosaceae family (deciduous pome and stonefruit), grow naturally in forest margins and often form their own woodland habitats. Imagine the what’s laying around on the ground in a deciduous woodland. There will be fallen leaves from the previous autumn, dropped branches, rotting vegetation from dead sub-story plants – in other words, a soil organism’s paradise.

A “woodsy” mulch, as apple whisperer Michael Phillips describes forest mulch in his book The Apple Grower, encourages beneficial mycorrhizal fungi. In turn this beneficial fungi forms a symbiotic relationship with tree roots and enhances the take up of nutrients and moisture. Woodsy mulches should still be used with discretion, but for established fruit trees, they’re the ideal, soil building companion.