Plastic sheeting is a valuable tool for managing garden beds without tillage. There are two different types that we can use depending on the need. One type is clear plastic that is used in a process called solarization. The other type is black plastic that is used as a movable mulch for smothering existing vegetation.
Small-scale gardeners can find sheet plastic that will do the job at practically any hardware or building supply store. I recommend finding plastic that’s at least 4 mil thick; it’s difficult to keep lighter-weight products from puncturing. The plastic sheeting carried by these sources is designed for short-term jobs and typically will only last two or three years.
Larger-scale growers will want to invest in silage tarps and greenhouse-grade clear plastic. Silage tarps and greenhouse plastic are both stabilized against ultraviolet deterioration and should last five years or more, depending on use. They are usually six mil and easier to use without puncturing.
When plastic sheeting is not in use, it should be stored off the ground. Hardware-store grade should be protected from sunlight. Rodents can quickly ruin stored plastic sheeting. Think about how you might protect the plastic from them.
Plastic sheets can be purchased in many different sizes. It’s easy to cut smaller pieces from larger sheets to fit certain areas. Be aware that large pieces of commercial-grade silage tarp and greenhouse covering are quite heavy. For larger scale growers, this becomes the limiting factor for how large an area can be practically covered with one sheet. Although larger pieces are available, few growers are using anything more than 25 x 100 feet.
Clear plastic and solarization are used to effectively kill living organisms in soil. This includes weed seeds, living roots, bacteria, fungi, nematodes, and many other species of soil organisms. Keep in mind that solarization is a useful tool, but it does destroy the good along with the bad. It should be used for certain purposes, but not as an every-year, on-going method of preparing no-till beds.
To solarize an area of ground where you will be harvesting topsoil for permanent-bed fill, first till the soil to a depth of 2.5-3 inches (6.5-7.5 cm). Try to be precise with the depth of your tillage since that is about how deep solarization is completely functional in moderate climates. When tillage is complete, water thoroughly and then cover with clear plastic, burying or weighting down the edges.
Leave the plastic in place for about two weeks of sunshine in a warm time of the year, longer in cloudy weather or if daytime temperatures are not at least in the 70s F. (21°+ C). Three or four days would be sufficient if it is super-hot. The idea is to get soil temperature above 130°F (54° C) three to four times for at least a few hours at a time to kill off all weed seeds and living roots. It is a good idea to insert a probe thermometer to the depth of your tillage and monitor this temperature.
Once solarization is complete, remove the plastic and allow the soil to dry out a bit before handling. If you are harvesting topsoil deeper than the original tillage, just remove the prepared soil and repeat the process.
Solarized soil should be completely free of viable seed and living roots, making it useful for filling permanent beds where we wish to have a clean start. If taken up soon after the solarization is complete, it will have very little living fungi and bacteria, so it is a good idea to mix in some good-quality compost when building beds to re-inoculate life into the soil. Obviously, we want to make sure the compost used is also free of unwanted seeds.
Another useful place for solarization is just ahead of starting fine-seeded, slow-germinating crops such as carrots. Solarization will give us a greater degree of weed control and more time for the crop to establish, than will the dark-tarping techniques that I describe next.
Dark-tarping with black plastic or silage tarps (which are typically black on one side and white on the other) is the primary way no-till gardeners control vegetation without disturbing the soil. This allows us to terminate harvested crops, cover crops, and/or weeds. For vegetation termination, dark plastic needs to stay in place three to six weeks. The warmer it is, the less time it takes. When vegetation is semi-dormant from cool temperatures, it takes longer to kill it with dark tarps.
When preparing to tarp, tall vegetation should be mowed first. Commercial-scale growers prefer flail-type mowers to chop all vegetation relatively finely and drop it straight down in place. At the home-garden scale, a small mulching-type walk-behind mower can do the same job, or, with a little more work, even a string trimmer. Without a flail mower, some of the largest toughest crops such as corn stalks or tomato vines might have to be manually removed. It is not necessary for the tarp to touch the ground everywhere to effectively kill the vegetation. It is important to have the tarp tight on the ground around the edges to shade out all light. The light deprivation is what kills the vegetation, although in warm seasons, heat buildup under the plastic can accelerate this process.
The plastic should be weighted down carefully around the edges. My preferred weights are sandbags since they are easy to handle and do not damage the plastic. Stones, strips of lumber, or other solid items can also be used for weight. In windy areas, some weight should be distributed across wide pieces of plastic. It is also more important to mow the vegetation short if wind is expected. Billowing plastic, even when well-weighted around the edge, is likely to tear loose and end up in undesired places. While plastic does not have to touch the soil in the center to effectively kill vegetation, in the early spring there is benefit from having dark plastic against the soil to transfer the sunshine’s warmth into the ground.
Multiple beds could be covered with one sheet of plastic, but, in the case of permanent raised beds, it’s often most convenient to have plastic sheets sized to cover each bed individually. Just be sure to cut the plastic pieces with enough overhang that they can be securely weighted down along the sides of the beds.
Dark tarping is a very useful tool for vegetation termination, and, unlike solarization, it typically does not harm soil life. But it does not prevent the germination of weed seeds or the regrowth of perennial weeds from roots that can survive long periods of no light. I do not recommend leaving tarps in place longer than necessary to kill vegetation. This is a tool for controlling vegetation, but, for the health of our gardens, we need to maximize the capture of sunshine and the growth of vegetation for as many days of the year as possible. In a future article, we will explore additional methods of vegetation control and termination without tillage.
