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Upper-Level Mulching

What’s good and bad for what

Never use colored mulches near annuals, shallow-rooted trees and shrubs or herbaceous perennials. These mulches are made using raw wood that serve as a source of food for microorganisms once it comes in contact with the ground. Microorganisms are better able to absorb nutrients in wood than are the roots of plants. As a result of the competition, plants — including weeds — starve and die. 
    Use colored mulches only around well-established deep-rooted trees and shrubs, for making pathways, sitting areas and playgrounds.
    Use hardwood bark mulches with caution.
    Unlike pine mulches, hardwood bark mulches contain up to 60 percent cellulose, which means they will decompose and rob nutrients from plants. They will also raise the pH of soils, making them less acidic. Repeated applications of hardwood bark can also result in the accumulation of manganese. When this occurs, the roots of the plants lose their ability to absorb iron and plant growth declines. Over the years I have seen numerous instances where the manganese and pH levels in the soil were so high that the only solution was total replacement of the soil.
    As you shop for pine bark mulch, be aware that not all bark mulches contain 100 percent bark. Some are made by blending one part pine bark and two parts wood chips. These blends are kept moist and turned periodically until the entire mass turns brown like bark.
    The truth is revealed if a piece of its wood reveals a yellow to light-brown center when broken. Once applied, fake bark mulches are more easily identified: After they have weathered a few weeks, the tannin-treated raw wood begins to lose its dark brown color.
    If that’s what you’ve got, the brown-colored raw wood will feed microorganisms, not plants.
    I was once called to investigate problems resulting from a mulch sale sponsored by a grocery chain. A large trailer load of double-shredded hardwood bark mulch had been trucked in and sold at cost. Buyers were immediately returning the mulch, complaining that it was killing their plants instantly.  Inspecting the load of mulch remaining in the trailer, I found it contained wood alcohol. I proved the presence of alcohol by cutting open a bag and throwing in a lighted match. The mulch immediately caught on fire. The mulch had been bagged while it was composting under anaerobic conditions, resulting in the formation of wood alcohol.
    Marble chips should not be used around plants that require acid soils. Marble chips are essentially chunks of limestone rich in calcium oxide, which will result in making the soil less acidic and eventually alkaline. That will be the death of acid-loving plants such as azaleas, rhododendrons, mountain laurels, andromeda, skimmia and Japanese hollies.
    Marble chips are safe around alkaline-preferring plants such as junipers, yews, pines, spruce and cherry laurel.
    Avoid using bluestone. I have seen numerous cases where plants have been killed after bluestone mulching. Like marble chips, bluestone contains high levels of calcium oxide. It may also contain metal contaminants, including nickel. The symptoms often go undetected for several years, by which time the damage is irreversible.


Ask Dr. Gouin your questions at DR.FRGouin@gmail.com. Please include your name and address.