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killing weed seeds with black plastic

Yes, the tarp brings with it moisture as it sweats, feeding the weeds and allowing them to germinate. But without the sun, those pesky weeds die off. Earthworms pop up to the surface of the plot and, finding nowhere to go, start moving horizontally, and in effect, naturally till your soil. While cooking the soil, fungi, bacteria, weed seeds and insects also are killed off in the dark, moist environment.

A good tarp de-weeding takes from six weeks to six months, the longer the better, if your weed garden is prolific. Leaving the tarp down from fall to spring is the longest you should leave it. A hot, dry summer damages the soil, and the tarp reinvigorates it while working to suppress the weeds. Organize your planting schedule around the end of your flowering season. Summer sun makes the tarp work more efficiently, while spring, with its clouds and rain, delays the effectiveness of the tarp.

The Tarp and Moisture

Occultation isn’t some kind of hocus-pocus; it simply means spreading something over the earth that excludes light and inhibits growth. The Canadian farmer John-Martin Fortier popularized the occultation method, and the technique soon spread to both large- and small-scale farmers and gardeners. If a patch of grass in your front lawn has died, ruining its green, green grass of home appearance, spread a black tarp over it for several months, and the illness should be gone along with the weeds. Smothering the weeds with blackness prevents light from encouraging growth, while at the same time moisturizing the soil.

Using cardboard to suppress weeds has been effective in low-wind areas; however, don’t lay the ink side onto the soil. Black plastic sheeting and black trash bags also are not recommended as they aren’t heavy enough or thick enough to prevent high winds from blowing them away. You’ll also have to watch for birds pecking through the plastic, creating holes for the weeds to grow.

Much discussion and disagreement have occurred as to which is better: a black tarp that blocks out all sunlight or a clear tarp, which lets the sun penetrate. Researchers at the University of New Hampshire found that the black tarps had a success rate of 100 percent weed suppression, while clear tarps, although they killed the weeds and cover crops, didn’t completely suppress all the weeds.

There is lots of work involved since soil solarization entails getting to the root of the problem, underground.   And we will not be taking the shortcut of using herbicides, so that means a bit more work. But if you do not mind getting your hands dirty, then let’s roll up our sleeves and begin stopping our weedy foes in their tracks.​

Now you truly have a “clean slate” with which to work. Remove the plastic and lay down landscape fabric. You should try to use one of the stronger types of landscape fabric if possible, just in case—in spite of your best efforts—any sharp objects remain in the ground (which would puncture the landscape fabric).

Preparation

Cover the raked, moistened area with a clear polyethylene sheet. The edges of the sheet can be held down by cinder blocks to keep the plastic from blowing away. If the raking mentioned above was done diligently enough, there will be no sharp objects sticking up to puncture the plastic. The sheet of clear plastic can be anything from 1 to 6 mil. in thickness. In the Northern hemisphere, the best time for soil solarization is June and July, when the sun is at its peak. UIE recommends keeping the sheet of clear plastic tightly stretched out over the area for about 2 months. During that time, the sun will be killing weeds for you—”cooking” them before they have a chance to sprout. Plant pathogens will be killed, to boot.

Of course, if you use an organic mulch (such as a bark mulch), it will eventually decompose anyhow, becoming fertile ground for weeds. What can you do? Well, you had better keep new weeds pulled, faithfully. Vigorous roots pushing downwards can stress landscape fabric and breakthrough. On the bright side, these weeds should be relatively easy to pull, since mulch is a lot looser than dirt, and weed roots will not become impossibly entrenched.

If there are shrubs and trees present, cut them down with an ax or chainsaw. The ground needs to be smooth before you begin soil solarization (since you will be spreading plastic over it), so you will also have to remove the stumps left behind. If you are looking for a cheap way, use a tool called a “mattock.” Dig and chop your way with the mattock under the root-ball to access and remove the taproot. Warning: this is hard work and may be feasible only for smaller stumps.