Fresh Wood Chips as Mulch
I often get the question, “Can I use fresh wood chips as a mulch?” There are three primary concerns about the effect that using fresh wood chips as mulch will have on plants.
First is that diseases on the wood could be transferred to ornamental plants. However, according to George Hudler (Department of Plant Pathology, Cornell University), it is very unlikely that the right combination of factors (pathogen, susceptible host, and environmental conditions conducive to infection) will come together to spread diseases in this way.
Second, is that naturally occurring allelopathic chemicals will be transferred directly to susceptible plants. However, in research conducted in the 1950s, scientists were unable to induce wilting in susceptible solanaceous plants when planting them directly into fresh, black walnut wood chips. Many do not consider this a problem, but there has been no follow up on this research. The bark and fruit of the black walnut contain the concentration of juglone, but very little is in the wood itself and there is likely to be very little black walnut in municipal mulch. However, to avoid confusion, remember that planting tomatoes in the root zone of a black walnut is a different situation and should be avoided.
Third, there is the concern about the fresh wood chips tying up nitrogen during their decomposition. An article by Frank Gouin (University of Maryland) states that wood chips (as opposed to pine bark) have a high carbon to nitrogen ratio and that they can cause significant nutrient deficiencies, particularly for herbaceous plants, or if they are incorporated into the soil.
Another concern is how mulch may affect pH. Research on the pH of wood leachate indicates that leachate is quite acidic and could significantly lower soil pH depending on the volume used. Also, if fresh wood chips are applied in too thick of a layer, the heat generated as they begin to compost can also injure herbaceous plants.
Therefore it is recommended that you stockpile the chips for three to six months before using them. If your pile is large, be sure to turn it several times during this period to ensure good aeration to aid decomposition. This should allow acidic compounds to dissipate as well as partially decompose the chips, so that heat and acidity will not harm your plants.