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killing weed seeds with black plastic

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.

Now use a steel rake on the area that you have just tilled, wielding it like a fine-toothed comb to remove the majority of the uprooted weeds. Next, rake the area again, this time with the object of evening out the soil as best you can and removing stones, twigs, etc. The final preparation for soil solarization will require the use of a garden hose. According to the University of Idaho Extension (UIE), you should moisten the area that you have just raked to “conduct and hold heat, to stimulate weed seed germination, and to prevent dormancy of below-ground vegetative plant parts.”

Run a mower over the land to reduce the weeds’ height further. Now that all the weeds are as short as possible and the stumps have been removed, rent a large tiller to uproot all the weeds. Since this plot of ground is uncultivated soil, you will need a tiller that has some power: Do not undertake this task with a small garden cultivator! Allow the tiller’s tines to dig deep enough into the ground to loosen the weeds, so they can be removed—roots and all, if possible.

Preparation

When you cut slits in the landscape fabric and install new plants, be careful that you don’t get dirt all over the landscape fabric. After all, why prepare a home for airborne seeds? Sure, you will be applying mulch. But airborne weed seeds can wend their way through mulch particles. If they find dirt, then they are “weeds waiting to happen.”

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.​

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.

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.

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.

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.

The Tarp and Moisture

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.

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.

Once the flowering or growing season is over, clean out your patch. Remove all previous growth visible to the eye, and if you’re covering grass, mow it as low as you can. Spread the tarp over your target, weigh it down heavily on all sides. The heavier the tarp, the less susceptible it is to heavy winds; then, let it go to work until it’s time to remove it. Shake the dirt and weed bits out of the tarp if it’s not too heavy, clean it off and store it for another time.

Occultation typically requires at least four weeks to be effective. The longer you keep the covering in place, the more effective it will be, up to about six weeks, at which point efficacy begins to level off.

Black plastic actually absorbs light, whereas clear plastic allows light and heat to pass through. So occultation takes longer.

Why choose occultation over solarization?

Tarps blow away easily and need to be held down.

While occultation takes longer, there are a few benefits.

Tilling prior to solarizing or using occultation will speed up the process and can have the added benefit of aerating soil that has been compacted.