Our tree crews and I get asked the title question quite often. The short answer is that it’s safe to spread them on the surface but not if you rake them into the soil.
Some years back, an arboriculture professor at Clemson University in South Carolina conducted an experiment to answer the question. He planted three groups of trees. Around the base of one group, he spread fresh wood chips. He mixed fresh wood chips into the backfill soil for another group. He didn’t apply any mulch to the third group, which was a “control” group.
The professor tested the soil periodically to see if there was any change in the amount of nitrogen in the soil from the pre-planting and mulching. He also monitored the trees’ health and vitality. He took on this project to determine the truth to the old theory that fresh wood chips draw nitrogen from the soil. After a year, he found no difference in the soil nitrogen between the surface mulched area and the control trees and only a slight decline in the amount of nitrogen where he had mixed the fresh chips into the soil. They were perfectly safe.
If you are concerned about using fresh chips in your own landscape, just mix in some fertilizer before spreading them. Better yet buy wood chips in bulk. We double grind and age them until they’re a dark black in color.
I don’t recommend colored mulch. Mulch sold for its color has dye in it and some dyes are harmful to plants. Our mulch is black, its natural color. As tan wood chips age, they take on a gray color which then darkens to black. Naturally black mulch will do the most good in your landscape.
Ground wood chips are better for the environment and your landscape than inorganic mulches like stones. Inorganic mulches are purely decorative. Organic mulches decompose and return organic matter to the soil. Also, it’s made from chipping and grinding debris from tree pruning operations. Mulching saves this material from already stressed landfills.
The professor’s conclusion was in an article that appeared in two trade magazines several years ago. This professor was one of three prominent research arborists the author spoke to when researching the article. All three of them said that fresh chips would not deplete soil nutrients, and that fertilizer could be mixed in if you doubted these conclusions. I know the author and the three research arborists and respect their opinion.
It’ll cost less to buy ground wood chip mulch in bulk rather than by the bag. We’ll deliver it and dump it in the driveway for you to spread. Or our landscape professionals can spread it for you.
Thank you very much for your post! By spreading woodchips in the garden, the soil will not be exposed to sunlight. Also, seeds blown in by the wind cannot land on the soil and sprout. For these reasons, woodchips are an effective weed control measure. I agree that woodchips are safe to spread.