Fresh Wood Chips

(Question)

Hello –
Are there any risks or concerns to applying wood chips to the soil in a very shady back garden with only evergreen shrubs, vines and a cherry blossom tree?
I’d like to turn the garden space into a play area for my child, but I don’t want to cause damage to the existing trees/shrubs.
Does it matter if the wood chips are fresh?
Thank you.

 

(Answer)

Thank you for contacting the Toronto Master Gardeners with your inquiry.

Gardeners know the benefits of a generous layer of mulch: reduced weeding, cooler soil in the summer, water conservation and the slow addition of organic matter as organic mulches break down, all of which leads to healthier plant growth.

Woodchips can be used as mulch as long as they are not obtained from pressure treated wood, such as MDF, which contain chemicals.  That said, wood is an organic product high in carbon and cellulose, so they need nitrogen and time in order to decompose. If fresh wood chips are added directly into the soil, the materials will bind up much of the soil’s nitrogen and render the spot useless for gardening for a season or two,which is why one should only use fresh chips as a surface mulch. In this case, nitrogen depletion would only be right at the soil surface, which may be one-reason wood chip mulches are efficient at suppressing seed germination. Several research studies have shown there is no nitrogen depletion problem for established trees & shrubs using fresh wood chips.

It is important to spread fresh wood chips out to let them dry, turning regularly to let the air get at them. Otherwise, they may produce substances such as methanol or ammonia which can harm plants. This “aging” process helps the chips to break down.

It is important to note not to pile the wood chips up against tree trunks – that can lead to potential problems with insects and fungal diseases due to the constant moisture on the tree trunks. Instead, spread them like a donut so they are not in direct contact with the trunks of trees.