Soil solarization is a non-chemical method that uses the sun’s energy to treat soil, suppressing unwanted weeds, seeds, disease-causing organisms, and certain soil-dwelling pests. This technique traps solar radiation beneath a clear plastic cover, creating a greenhouse effect that significantly raises soil temperature. The resulting heat and moisture combination sterilizes the upper soil layers, making it a viable way to prepare a lawn area for new planting without the use of herbicides or harsh chemicals. It is a process that requires careful preparation and execution during the warmest part of the year.
Preparing the Area for Solarization
Solarization is most effective when performed during the hottest, sunniest time of the year, typically during the peak summer months of July and August. The goal is to maximize the temperature increase beneath the plastic to reach lethal levels for soil-borne pests and weed seeds. Temperatures in the top six inches of soil should ideally reach between 110° and 140°F for a sustained period.
Begin preparation by clearing the area of any large debris, rocks, or sharp objects that could puncture the plastic sheeting. Existing turf or weeds must be mowed down as low as possible, a process often called “scalping,” to remove bulk plant material. This ensures better contact between the plastic and the soil surface, as air pockets significantly reduce the heat transfer efficiency.
Next, the soil must be thoroughly irrigated to a depth of at least 12 inches, as wet soil conducts heat much more efficiently than dry soil. The moisture also stimulates dormant weed seeds to germinate, making the resulting seedlings more vulnerable to the intense heat and steam trapped beneath the plastic.
Before applying the cover, the soil surface should be smoothed and leveled to eliminate clods and air gaps, ensuring the plastic lies snugly against the ground. Light tilling to a shallow depth (no more than 6 inches) can help break up compacted soil. Deep tilling should be avoided as it brings up new weed seeds from deeper, untreated layers.
Executing the Solarization Process
The actual solarization begins with the selection of the correct material: clear, UV-resistant polyethylene plastic sheeting is required, typically between 1 and 4 mil thick. Clear plastic allows solar radiation to pass through and heat the soil directly, trapping the thermal energy in a “greenhouse effect.” Opaque or black plastic is generally less effective for solarization.
To begin covering the area, unroll the plastic sheet, making sure it completely covers the prepared area with an extra margin around the perimeter. If multiple sheets are necessary, they must be overlapped by several inches to prevent heat and moisture from escaping. Creating an airtight seal around the entire edge of the plastic is critical.
To achieve this seal, dig a trench approximately 4 to 6 inches deep around the perimeter of the treatment area. Lay the edges of the plastic into this trench and then backfill it completely with soil, ensuring the soil is packed down tightly. This trenching process secures the plastic against wind and traps the heat and moisture necessary to elevate the soil temperature to lethal levels.
The duration of the treatment is dependent on the climate and the intensity of the sun, but the plastic must remain in place for a minimum of four to six weeks. In cooler regions, the process may need to be extended to eight weeks to ensure adequate sterilization. The sustained high temperatures kill weed seeds, fungi, bacteria, and nematodes in the upper 6 to 8 inches of the soil profile.
Post-Treatment Soil Management
Once the required solarization period has been completed, the plastic sheeting can be carefully removed from the area. The soil will be extremely hot and saturated immediately after removal, so it is beneficial to allow the soil to cool and aerate slightly for a few days before any further work is done. This brief cooling period helps the soil environment stabilize before planting.
When preparing the area for the new lawn or planting, avoid any deep soil disturbance, such as tilling or digging, that penetrates deeper than the top two inches. Deep disturbance can bring viable weed seeds from the lower, untreated soil layers to the surface, where they can germinate and re-infest the area.
If soil amendments like compost or fertilizer are desired, they should be worked gently into only the surface layer of the soil. Solarization, while effective at controlling pests, can temporarily reduce populations of beneficial soil microorganisms, and incorporating organic material helps to restore the soil biota.
Immediate re-seeding is a preventative measure against rapid re-infestation, as bare soil is highly susceptible to colonization by airborne weed seeds. By quickly establishing the desired turfgrass, the new growth acts as a physical barrier and competition against subsequent weed germination.