Soil tarping is a non-chemical approach using plastic sheeting or tarps to prepare a garden area for planting. This technique employs clear or opaque materials to manipulate the environment beneath, killing vegetation and stimulating weed seeds to germinate and then perish. The time required is highly variable, depending on the specific method chosen and the prevailing environmental conditions.
Solarization vs. Occlusion: Understanding the Mechanism
The two primary methods of soil tarping—solarization and occlusion—rely on different principles to eliminate weeds. Soil solarization uses a clear plastic cover to create a greenhouse effect, which is highly dependent on intense sunlight and high ambient temperatures. The clear plastic allows solar radiation to pass through and become trapped, causing the top layer of soil to heat up significantly.
Temperatures under a solarization tarp can reach between 100 and 140 degrees Fahrenheit, effectively pasteurizing the soil. This extreme heat kills active weeds, dormant seeds, and certain soil-borne pathogens. Because this method operates through thermal killing, it is the faster technique, but it is only effective during the warmest months of the year.
Occlusion, sometimes called occultation, uses an opaque material, typically a thick black silage tarp, to block all light from reaching the soil surface. This method kills weeds by starving them of the light necessary for photosynthesis. The darkness forces weed seeds to germinate (a process called fatal germination), and the resulting seedlings die from exhaustion without light.
While the opaque tarp warms the soil and retains moisture, it does not achieve the high pasteurization temperatures of solarization. Occlusion is a slower process, but it is less dependent on peak summer heat and can be used effectively across multiple seasons. The goal is to deprive the plants of light long enough to deplete their energy reserves.
Factors Determining Tarping Duration
The time a tarp must remain in place varies widely, dictated primarily by the method used and the type of weed present. For solarization, the duration is typically four to eight weeks, but this timeframe must coincide with the hottest, sunniest part of the summer season for maximum soil heating. Effective thermal control is achieved when soil temperatures remain consistently high for several weeks.
For the occlusion method, the required time is generally longer, especially for established weeds. Annual weeds, which reproduce only by seed, can often be eliminated within six to twelve weeks of continuous darkness.
Perennial weeds, which have extensive root systems and energy reserves (like Bermuda grass or bindweed), require a much longer tarping period. These persistent weeds may need the tarp to remain in place for six months or more, sometimes through a full growing season, to fully exhaust their underground energy stores. In some cases, particularly difficult perennial species may require the tarp to be left on for over a year for complete control. Thicker, UV-resistant tarps are the preferred material for these long-term occlusion projects to ensure durability.
The season also plays a role in the speed of occlusion, as warmer soil encourages faster weed seed germination, leading to quicker exhaustion. While occlusion is effective year-round, the process slows considerably during colder months when biological activity in the soil decreases. Even a few weeks of occultation can be beneficial for quickly preparing a stale seedbed before planting.
Preparing the Area and Post-Removal Soil Care
Successful tarping begins with proper preparation of the area before the plastic is laid down. Any tall weeds or existing vegetation should be mowed or cut down close to the soil surface to ensure the tarp makes good contact with the ground. Removing bulky material allows the tarp to lie flat, which is important for solarization to maximize heat transfer to the soil.
Before securing the tarp, the area should be watered thoroughly, as moist soil conducts heat more efficiently for solarization and stimulates weed seeds to germinate under both methods. If soil amendments like compost or fertilizer are to be incorporated, they should be applied and worked into the soil before the tarp is placed.
After the required period has passed and the tarp is removed, the soil should be handled with care to preserve the weed-free condition. The most important post-removal step is to avoid deep tilling or heavy cultivation, which can bring dormant weed seeds from lower soil layers up to the surface. These newly exposed seeds will then germinate and negate the effort of the tarping process.
The treated area should be planted immediately or covered with mulch to prevent new weed seeds from establishing. Restoring organic matter through surface application of compost can help to revitalize the soil biology after either tarping method.