News / Life / Clark County Life

Gardening with Allen: Mulch effectively stymies weeds

By Allen Wilson

Published: July 2, 2019, 6:07am

Bark mulch can reduce weed sprouting by 90 percent. Contributed by Allen Wilson

I have a lot more problems with weeds than my neighbor. He says that bark dust mulch is the main reason he has fewer weeds. Does bark mulch make that much difference?

Yes, a 1 1/2 – to 2-inch layer of mulch will reduce weed seed germination by about 90 percent. Weed seeds can live in the soil for many years. As soon as they are turned up to light it stimulates germination. Placing any kind of mulch on the soil will have the effect of preventing light from reaching the weed seeds. Perennial weeds which sprout from underground stems or roots such as Canada thistle are only controlled by a physical barrier such as weed barrier fabric.

Volcanic rock, coarse gravel and other kinds of decorative rock can also be used to reduce weed growth. A new artificial mulch made from chopped-up tires is now available. It is chopped into uniformly small pieces and dyed brown to look just like coarse bark mulch.

Weed barrier fabrics will prevent all light from reaching the soil and are often used in new shrub plantings. They are usually covered with bark or some other material to give a more natural appearance.

Weed barrier fabric is porous so both water and air can flow through it freely. Do not use polyethylene as a mulch because it does not allow water or air to freely move into the soil.

Bark is the most popular mulch because it not only reduces weed sprouting but gives a natural appearance to shrub and tree beds and reduces water loss. As bark is broken down in the soil it improves the physical characteristics of the soil. It also adds fertility to the soil. Bark is available in a variety of sizes, from fine dust to chunks. The most common bark is from Douglas fir trees. It is reddish brown in color, but ages to a darker brown. Hemlock bark is a darker brown color and ages to almost black. The least expensive way to purchase bark is in bulk by the cubic yard. If you don’t have access to a pickup, it can be delivered.

I use grass clippings as a mulch in my vegetable garden. I scatter them between the rows and plants about 2 or 3 inches deep. Clippings quickly turn to a straw color and soil is ready for another layer in three or four weeks. Grass clippings can be used around trees, shrubs and flowers, but I prefer the dark brown color of bark.

Corn gluten is another natural processed material which makes a good mulch but is more expensive than bark.

I generally avoid using chemical weed preventers because they can sometimes damage other plants, especially if used repeatedly. The best weed preventing chemical for use around vegetables and flowers is trifluralin, often abbreviated as treflan. The most common brand is Preen, but several others are available. Casoron is a longer lasting weed preventer which can be used around deeper-rooted trees and shrubs. Weed preventers do not kill existing weeds but prevent about 95 percent of new weed growth. Do not cultivate the soil after applying them since this brings new weed seeds to the surface.

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