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killing weed seeds with black plastic

If you want to turn a section of lawn into a flower bed or vegetable garden, soil solarization is an excellent way to kill the grass, weeds, some insects, and nematodes without using chemicals.

When the lawn is dead, you have a few options. Clear away dead grass where you plan to put walkways or otherwise need an even surface. Where the lawn is to become planting bed, just treat the dead stuff as compost. Plant into or mound soil atop it. Eventually, it will disintegrate either way.

How to solarize soil

Summer is the best time of year to solarize. The air is warm, but more importantly, this is when the sun has its greatest heating potential. After six weeks or so, your lawn will be gone with a minimum investment of time, energy, money, and, best of all – no herbicides!

The process, which is called solarization, uses the heat of the sun’s rays to literally cook plants, weed seeds, nematodes, insects, and soil pathogens (the “bad guy” fungi, bacteria, etc. that bring diseases to plants) in the uppermost layers of soil.It also makes nutrients more available to plants later grown in solarized soil.

Since solarization works best in the upper foot or so of soil, don’t rototill the soil after you are done. (Actually, it’s best not to rototill ever. Rototilling, or turning the soil deeply, brings seeds and pathogens to the surface where they will proliferate.)

Soil solarization is a preventive, organic method of killing weeds before weed seeds even sprout.   But the advice below is also meant for homeowners wishing to start a garden with a clean slate, reclaiming a patch of land where weeds have taken over, in such a way as to reduce to a minimum the hassle of future ​weed control. Want to transform a piece of land that has “gone to pot” into usable space? Then the method explained below may be the solution to your problems.

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.”

Preparation

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.

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.​

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.

Occultation is similar to solarization, but opaque coverings are used instead of clear. While it may sound counterintuitive, fields covered in clear plastic become hotter than fields covered in black plastic.

Solarization is the process of placing a clear plastic tarp over a field, garden bed or lawn to heat up the soil underneath. The intention of solarization is to kill weeds or grass, though it can have added benefits of reducing pathogen populations in the soil.

What is occultation?

Heating the soil can have significant impacts on soil biology, but these changes are not necessarily negative. There are few studies about the long-term impacts of soil solarization on microbial communities, but it is an emerging area of research.

Housam Kanaan, H., S. Frenk, M. Raviv, S. Medina, D. Minz. Long and short term effects of solarization on soil microbiome and agricultural production. 2018. Applied Soil Ecology, 124: 54-61.

Moisture is a key ingredient for weed seed germination, so watering before installing your tarps will improve performance. Water until the soil is moist down to about 12 inches before adding tarps.