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Black Covers Can Put Weeds to Bed… For Good

Have you experienced one or more of these garden scenarios?

I can’t bear to look… I’m going back inside
This stirrup hoe is great at removing large weeds, but brings lots of weed seeds up to the top two inches of soil where they have a good chance of germinating.

Un-controlled weeds compete with garden plants for water and nutrients, are hosts for insect pests and diseases, and can demoralize the toughest gardeners. Tilling, pulling, chopping, and hoeing are all fine weed control techniques under the right conditions, but they also disturb soil allowing even more weed seeds to germinate and flourish.

There’s another way: occultation. The common dictionary definition is “an event that occurs when one object is hidden by another object that passes between it and the observer.” For gardeners and farmers, it’s covering the soil to create a dark, warm, moist environment. In 2-4 weeks, this no-till technique can:

  1. smother and kill young weeds
  2. smother larger weeds and grasses
  3. accelerate the decomposition of mowed cover crops and weeds
  4. promote the germination of weed seeds and then smother the seedlings
Weed barrier pinned down over a bed filled with winter annual weeds. It was ready to plant in two weeks. Tough perennial broad-leaf weeds and grasses may be weakened but not killed.

When the cover is removed you are left with a mat of decaying organic mulch. New weeds will be fewer because the soil was not disturbed. Spread a 1-inch layer of compost and you’re ready to plant seeds or transplants. Of course, you could plant transplants through holes cut into the weed barrier.

Weed barrier used to cover mowed cover crop in spring. The weed barrier strip on the left was removed three weeks later and tomatoes were planted in the decomposing residues.

Gardeners can use black tarps or weed barrier fabric. Some farmers use black silage covers (new or used). Weed barriers allow water and air to penetrate while tarps and silage covers are water-resistant. I’m not aware of research that has compared the types of covers used in occultation. I’ve had good luck with heavy-duty woven polypropylene weed barrier fabrics (they last for many years). Four-foot widths work well for most gardens and strips of the fabric can be overlapped for wide beds. It’s important to completely cover the target area and pin down the edges of the cover with landscape staples, bricks, or soil. If possible, weed-whack or mow the weeds first. DO NOT cover the fabric with any organic material. Weed seeds will germinate on top and grow roots down through the weed barrier.

By Jon Traunfeld, Director, Home and Garden Information Center 

Additional information: this technique was explored in a research project by Jerry Brust, Ph.D., IPM Vegetable Specialist.

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