No-dig mulch – with bonus mushrooms

Brought up on a steady diet of old-school gardening books, as we get into autumn I would once have been convinced that now was time to prepare for the Victorian practice of double-digging. Excavating the soil from garden beds out to approximately 50cm, before mixing it with copious amounts of manure or compost before backfilling it, this was proper back-breaking work but with allegedly enormous benefits.

Fortunately, in subsequent decades a range of scientific trials compiled by Washington State University have shown that this practice is not only unnecessary, but in fact creates worse results by a whole number of measures. Following on the work of Washington State University’s associate professor Linda Chalker-Scott, I have become fascinated with easier, cheaper ways of improving the soil where I garden, and have seen results that have radically changed my way of thinking. As a total fanboy, I have even put my own spin on her ideas. As now is the perfect time to get started, here is a quick run-through of the process.

Chalker-Scott’s advice, in line with most modern scientific consensus, is to apply an annual mulch of organic matter to the surface of soil, and let worms naturally incorporate this. However, the really surprising departure she then makes from the horticultural mainstream is what she uses: wood chippings. Available either at very low cost or, if you make friends with a few arborists, for free, wood chippings are still usually regarded as a horticulture no-no. It is often thought these can transmit plant disease, potentially poison soils with toxic plant compounds, even suck up the nitrogen from soils as they decay. To my great surprise, the reviews of scientific trials involving wood chip mulch by Chalker-Scott have demonstrated that for anything but the most delicate seedlings these claims are either overblown, or just not based on any good evidence.

Get chippy: still regarded as a no-no, in fact wood chippings are usually fine.

Get chippy: still regarded as a no-no, in fact wood chippings are usually fine. Photograph: Don Nichols/Getty Images/iStockphoto

With I think a healthy level of scepticism, I decided to try this out on my mum’s garden, starting almost a decade ago after a neighbour, having some pruning work done, ended up with several barrowloads of chippings – and I was genuinely astonished. I applied a 10-15cm layer over the surface of beds containing both mature shrubs and smaller herbaceous plants in the autumn. Then, when I scraped the surface of the mulch the next spring I found that the compacted London clay I had struggled with for years was, as if by magic, a friable, dark crumb. This mulch may not be suitable for very small, newly planted specimens such as veg, but then the large particle size of each chipping would make that impractical anyway.

What’s my spin on her take? Around the same time, I also became interested in growing edible mushrooms at home, and the substrate they feed on often happens to be wood chips. I decided to scatter wine cap mushroom (Stropharia rugosoannulata) spawn over the surface of the mulch in the shadier spots and lightly mix in it. Kept well watered this not only speeds the decomposition of the mulch, giving you a quicker turnaround time to healthier soil, but offers up an unusual edible crop every year. Just around the time I feed it another layer of mulch. So thank you, Linda.

Follow James on Twitter @Botanygeek

The Guardian

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