Eco-Answers from the Pros: Using Wood Chips as Mulch

I had a pile of wood chips delivered to me at the end of last summer and want to use it as mulch in my flower and shrub garden this spring. Problem is it’s been sitting for only about five months and may have not decomposed enough to use as mulch. Can I put down a layer of some kind of fertilizer first, then put the wood chips over that?

wood chips up closelThere are two basic issues associated with fresh, unaged chips. The first issue is that the fungi breaking down the fresh wood will require nitrogen in order to complete the breakdown reactions. This may produce a layer of depleted soil at the interface between the soil and the mulch, which could inhibit seed germination but shouldn’t affect established plants. The larger issue is that the initial stages of decomposition can generate a lot of heat and can actually scorch plants. Both of these problems are issues with very fresh chips and decrease as time goes on. Five months will not produce finished mulch, but the material should be past the initial stages of breakdown. This isn’t to say that it’s great stuff to use immediately as these processes are still ongoing, but it’s far enough along that it can be used with a little extra care.

Fertilizer is not recommended as it is specifically designed to be fast acting; fertilizer will quickly disappear and you want a longer-lived nutrient source. I would recommend spreading a layer of leaves (or compost) down and then topdressing with the wood chips. As those leaves break down, they will release both nitrogen and phosphorus into their surroundings and should feed the soil nicely.

Dan Jaffe, Propagator and Stock Bed Grower at Garden in the Woods, Framingham, MA

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