Black plastic works best on annual beds, because the plastic will tear and require replacement every year. Red plastic is sometimes used for annual vegetable crops such as tomatoes (Solanum lycopersicum), which may benefit from the red reflected light. The plastic blocks both sunlight and oxygen from reaching weeds, so they can’t germinate or grow. Holes in the plastic allow your desired plants to grow and receive sunlight. Plastic doesn’t allow moisture penetration, however. Installing a drip irrigation system over the top of the bed before laying the plastic is preferable; otherwise, you must water each plant by hand where the soil is exposed at the base of the plants.
Clear plastic can stop weeds before you put in your new garden bed. Sunlight passes through the plastic, and the heat becomes trapped, effectively cooking the weeds and seeds near the soil surface. You must till in any fertilizer or amendments before you solarize, because tilling afterward can turn up deeply buried but still viable weed seeds. After moistening the soil, cover the bed with 2- to 4-ml plastic. It takes four to six weeks of intense sunlight to kill the weeds, so use this method during the hottest, sunniest part of summer.
Thin plastic sheets can kill weeds and prevent future weed growth in your vegetable or ornamental garden beds. Solarization methods use plastic to prepare new beds, while mulching with plastic stops further weed growth after you plant. Plastic also retains soil moisture so you can use less water, while warming the soil in cool weather.
Water sometimes pools on the surface of black plastic mulch. Poking a few holes allows the water to drain into the soil, but too many holes makes it easier for weeds to penetrate through the plastic and invade the garden bed. Plastic also warms up the soil. Although this is beneficial in cool weather or early in the season, it can stress plants during hot weather. Covering the plastic with a 1-inch layer of straw helps cool down the bed. If you use plastic in an ornamental garden, a thin layer of bark mulch can camouflage the unattractive plastic. Dispose of plastic at the end of the growing season, because it won’t decompose in the garden or in a compost pile.
Any amendments or fertilizer should be added to the garden before you install the plastic. After installation, soluble fertilizers are the only amendment you can add easily. After the soil is prepared and any drip systems are installed, lay the plastic over the soil surface. Overlap the edges by 6 inches so weeds won’t grow between the joints. The outer edges require anchoring so they don’t blow up. You can place rocks or soil on the edges, or anchor them down with garden staples. Cut a hole in the plastic for each plant, making it large enough so you can plant the roots without the plastic resting against the stems.
One key principle of solarization and occultation is preventing water from reaching the soil after the initial irrigation. So these methods will be less effective in low spots that receive significant drainage, and where water can enter underneath the tarp from the edges.
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.
Tarps blow away easily and need to be held down.
To prepare the seedbed:
Clear plastic tarps from your local hardware store (2-6 mil) are sufficient for polarization. The process typically takes around 2 to 3 weeks during hot summer months. The process is complete when the vegetation underneath the tarp is dead.
Black plastic actually absorbs light, whereas clear plastic allows light and heat to pass through. So occultation takes longer.
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.
New fields and lawns
In order for weed weeds to germinate, they need warmth, good seed to soil contact, moisture and, in some cases, light. By preparing a seedbed prior to solarizing or using occultation, you increase the likelihood that weed seeds will germinate under your tarps compared to a field that has not been prepared.
Smith, Grace, Sonja Birthisel, Eric R. Gallandt. Comparing Solarization & Occultation. 2017. University of Maine.