What are the disadvantages of using wood mulch as a cover?

Question

I see more and more wood mulch along the roads, around houses, etc.

At first thought it seemed a good thing to me, because it allows lots of life to grow around, while not releasing a lot of CO₂ into the air -- apart from burning fumes which can be horrible to neighbours.

But are there disadvantages with this technique?

• First thing which I think might be wrong, is if a large pile produces methane while decomposing. The greenhouse impact would then be a concern.
• Second, if I spread wood mulch around my salad greens in the garden, is it an issue for the edible properties of the little ones? Could it add too much sulphur to the soil for example? And would this make my vegetables inedible or toxic?
• I heard it could also be harmful to germinating seeds. Is this remotely true?
• Are there any other disadvantages you might think of?

Answer 1

This paper recommends the use of ramial chipped wood ( RCW ) as a way of soil regeneration. The wood is taken from branches less then 7 cm in diameter, with dead leaves and chipped, and then incorporated into the soil. You should avoid the use of conifers, and use hardwood branches instead. Humus made from hardwood RCW can last over 1000 years, whereas humus made from herbaceous plants is short lived.

If using as a mulch, instead of mixing it in with the first 5 cm of soil, then you don't have to worry so much about nitrogen theft since the decomposition of wood is mainly by fungi and not bacteria. And since it's lying on the surface, it can't steal nitrogen from inside the soil. It may end up mixed anyway so it may have the same soil amendment effect.

Since it's used to suppress growth of plants, you need to plant established seedlings if using it as a mulch. However, I'm not aware of any draw backs using it as a mulch apart from providing a place for insects to live.

Answer 2

It rather depends what you mean by 'wood mulch'. Bark chips are not the same thing as wood mulch, for one thing, and they are more commonly used as mulch. If its fresh wood mulch, which wood it came from is important - whether its been through a chipper and was once a tree, what variety of tree it was, whether fresh wood chippings have been composted for a few months first before use, and which plants they're going to be spread around, all these things make a difference. Fresh wood chippings (which are essentially cellulose) from a felled tree, if spread on planted ground, will, at the very least, deprive the plants growing in that area of nitrogen for a while, since the bacteria responsible for breaking down the wood require nitrogen to function, so they rob it from the soil, leaving a shortage for plants. This can be addressed by applying nitrogen before applying the wood chips though, and over time, the nitrogen deficiency corrects itself without intervention, as the wood chippings decompose, so on unplanted areas, it doesn't cause a problem. Wood chippings from treated woods should not be used for horticultural purposes.

Regarding whether wood chips are harmful to seeds, again, its down to the wood used, and the size of the chippings - smaller seeds may find it difficult to grow through a 3 inch thick layer of chunky, raw wood chips for one thing, but I wouldn't recommend wood chips being used where seeds have been planted.

On the subject of methane gas, a large pile of wood chips may well produce methane gas because it'll become anaerobic if its not turned regularly, and that leads to methane production. Regularly turning or stirring the pile will prevent that happening.

There's quite a lot of information in the link below - much of it applies to trees, and the information is American based, but the general principles are outlined, with some detail regarding various compositions of wood mulch and their possible negative/positive effects on plants. With regard to your salad garden, I wouldn't recommend wood mulch for an area like that - its best use is in areas planted with trees or shrubs, where regular digging and replanting doesn't take place.

http://grounds-mag.com/mag/grounds_maintenance_mulching_basics_covered/