Black Covers Can Put Weeds to Bed… For Good

Have you experienced one or more of these garden scenarios?

  • It’s early April and chickweed, henbit and other winter annual weeds are growing so thickly in a vegetable or flower bed that the soil can’t be seen.
  • The winter rye cover crop you mowed last week is growing back.
  • You tilled and raked a bed that you couldn’t plant right away and now weeds are coming up everywhere.
  • The rainy summer weather is favoring weeds over crops so that the weeds are taking over walkways and dominating beds that you want to plant with fall crops.
  • Neighboring plots in your community garden have been abandoned and weeds are growing wild and reproducing!
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.

5 thoughts on “Black Covers Can Put Weeds to Bed… For Good

  1. Ann Coren March 16, 2018 / 4:27 pm

    I am appalled by this article. In an age when we all should be aware of our impact on this earth and the need to use sustainable technologies, you have written an article with promotes unsustainable materials and practices. 1. Weed barrier fabric and tarps are made from petroleum products, The manufacture of these materials poisons our world and the decomposition byproducts are poisoning us. 2. If you must create a weed barrier, at least use a non-petroleum product, like recycled cardboard or rags, lasagne layered with autumn leaves. If this is done in the fall the soil is perfect for planting in the spring without removing the cover. 3. The best way to keep healthy soil is to use a cover crop. If the winter cover that is established is native plants, such as the firld chickweed, Cerasteum arvense, then it can be kept in place and the vegetables can be inter planted. The native plants used support both pollinators and beneficial insects, eliminating the need for herbicides and pesticides.
    A recipe for living sustainable in this earth would be better than the article as written which perpetuates antiquated practices that are unsustainable and damaging to us and our earth.

  2. tonytomeo March 18, 2018 / 12:49 am

    Oh, I so hate working with saran; but it has been so effective, and it is such a tough material that can get moved from one site to the one next to it and so on. There are some situations where we can not use herbicides, and exotic (non native) cover crops have the potential to become invasive.

  3. Maryland Grows March 18, 2018 / 11:18 am

    Cardboard is an excellent tool for smothering turf and weeds to establish new gardening beds. I have found the weed barrier to be a nimble and flexible aid in managing cover crops, reducing soil disturbance and the need for mowing and weed-eating, and preventing the spread of undesirable weeds. Heavy-duty tarps and weed material materials can be folded, stored, and re-used for many years.

    Fossil fuels are involved in the production, distribution, and use of most farming and gardening inputs, including organic seeds and fertilizers. It’s critically important that we evaluate our products and practices in the context of sustainability and climate change mitigation. In my view, the materials offered in the blog post can be an appropriate choice.

  4. Mara March 18, 2018 / 11:57 am

    I wonder what makes the bigger ecological footprint for weed management. Plastic or soil disturbance. I suppose maybe it depends on the space involved. Personally, I use cardboard, cover crops (which sometimes includes plants labeled as weeds) and minimal soil disturbance. Not that I know it all.

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