Soil solarization is an eco-friendly weeding technique that uses the sun’s heat to eradicate weeds from your garden beds or lawn. It involves covering moist soil with plastic sheeting during the hot summer months to increase soil temperatures to levels that kill weed seeds and roots. When done correctly, soil solarization can eliminate up to 99% of unwanted vegetation without the use of chemical herbicides.
What is Soil Solarization?
Soil solarization, also known as solar heating, is a hydrothermal weed control method that harnesses the power of the sun to generate heat under plastic sheets placed on top of lawn or garden beds. The increased heat from solar energy kills weed seeds, seedlings, and mature plants in the top few inches of soil. It is an especially effective organic pre-emergent weed control.
This non-chemical process uses solar radiation to trap heat under transparent plastic (typically polyethylene), which raises the soil temperature to levels lethal to many common garden weeds. Temperatures of 115-140°F (depending on climate) can be reached when using clear plastic sheeting. The hot environment created under the plastic tarp essentially sterilizes the soil and stops future weed growth.
Soil solarization is most effective for controlling annual weeds and the propagation systems (seeds, rhizomes, etc.) of perennial weeds. It also helps eliminate soil-borne plant pathogens and nematodes. The transparent covering allows sunlight to penetrate and heat up the soil while preventing evaporation. It pasteurizes the soil while conserving beneficial soil microbes that aid plant growth.
Benefits of Using Soil Solarization
- Provides weed control without chemicals
- Non-toxic and safe for people, pets, and the environment
- Improves soil health by fostering beneficial microbial activity
- Heats deeper compared to shallow tilling/cultivation
- Low cost and accessible to home gardeners
- Minimal labor required
- Can treat large areas efficiently
- Allows planting shortly after removing the plastic tarp
- Effective for controlling annual weeds, perennial weed seed/root/stolon systems, and reducing nematode and disease pressure
When to Use Soil Solarization
The ideal time for solarizing soil is during the hottest, sunniest weeks of summer. Maximum solar radiation and high ambient temperatures are needed to sufficiently heat the soil to lethal levels for weeds.
Solarization is usually done over 4-6 weeks during peak summer conditions. In northern climates, the best window is from early July through August. In southern regions, late May through June is optimal.
For all climates, soil should be moist before covering it with plastic. Pre-irrigate beds 24-48 hours prior to laying the tarp. The soil moisture conducts heat, allowing the ground to reach higher temperatures. Avoid solarizing bone-dry soil.
Also time soil prep to kill existing weeds before solarization. Eliminate weeds through shallow cultivation or using a non-residual herbicide 2-3 weeks prior. This prevents weeds from reflowering and setting new seeds under the tarp.
How to Solarize Soil
Follow these steps for successful DIY soil solarization:
Gather Supplies
- Plastic sheeting – Use 1-2 mil clear polyethylene plastic tarps. Thicker plastic blocks too much light.
- Anchors – Weight edges with boards, landscape staples, sandbags, bricks, etc. to keep the plastic secured to the ground.
- Gloves & kneepads – For working on the ground and handling the plastic sheeting.
- String – Helps measure and layout sheeting.
- Landscaping rake – Useful for smoothing out soil prior to laying the tarp.
- Shovel – To bury the edges of the plastic for complete seal.
Prepare the Area
- Eliminate existing weeds through cultivation, mowing, or herbicide treatment.
- Irrigate the area deeply 1-2 days before laying the plastic. Soil should be damp but not saturated.
- Grade and rake the soil surface smooth. Remove rocks, sticks, roots that could puncture the plastic.
Install Solarization Plastic
- Measure dimensions of the area and cut UV-stabilized polyethylene to size. Add extra for overlap.
- Begin on one edge of the bed and unroll the plastic over the prepared soil. Allow 6-12″ overlap between sheet edges.
- Cover perimeter edges with additional 6-12″ plastic that goes horizontally across the ground.
- Bury all sheet edges in a trench around the bed to seal and anchor the polyethylene.
- Weigh down the surface and overlaps every 3-4 feet using bricks, sandbags, landscape staples, planks, etc. Eliminate gaps.
- Check periodically for rips, tears, breeches and re-cover immediately to keep the area sealed.
- Allow soil to heat consistently under the plastic for 4-6 weeks through the hottest summer weather.
Post Solarization
- After solarization period, remove all covers and anchors. Pull back the tarps when the evening air starts to cool.
- Rake the surface lightly to eliminate any remaining weed seedlings or roots.
- Allow the solarized area to aerate for 1-2 days before seeding or transplanting.
- Sow new crops as desired. Avoid disturbing the soil deeply as this can bring untreated seeds up from lower layers.
Tips for Maximizing Effectiveness
- Wet soil before and after applying plastic tarps
- Remove any weeds, roots, stones prior to covering for best heat transfer
- Overlap multiple sheets by 12 inches and seal all edges
- Bury edges in trenches 6 inches deep and weigh down tarps for total contact
- Use clear plastic for maximum light penetration and heat generation
- Raise plastic on beds/ berms to increase heat on the soil surface
- Ventilate tarps on extremely hot days > 100°F to avoid overheating
- Supplement with mulches before & after for added weed prevention
- Solarize repeatedly for 2 years to deplete the weed seed bank
- Rotate and alternate crops to prevent weeds from adapting
Frequently Asked Questions
Does soil solarization really work for killing weeds?
Yes, research shows that 4-6 weeks of midsummer solarization can eliminate 70-99% of weed species as deep as 4 inches in the soil. The ultra-hot temperatures kill seeds and plant parts located in the top layers.
When is the best time to solarize soil?
For most regions, the optimal window is July-August when sunlight hours are at peak intensity. Time solarization to align with the hottest daytime temperatures in your climate.
What plastic is best for solarization?
Use clear polyethylene (1-2 mil) sheeting with high greenhouse light transmission. This creates a closed greenhouse effect under the plastic to maximize soil heating. Avoid opaque, colored, or thick plastic tarps.
Can I solarize different areas separately?
Yes, you can solarize distinct areas on a staggered schedule based on timing that works for each section. Just be sure to prep and seal each section properly when it’s time to cover with plastic.
How soon can I plant after removing the plastic?
It’s best to wait 1-2 days before seeding or transplanting. This allows the soil to aerate and settle. Avoid digging too deep initially as this can undo the solarization by bringing up untreated weed seeds from lower layers.
Conclusion
Solarization is an eco-friendly alternative to chemical-intensive weed control that takes advantage of free solar energy. With proper timing and setup, a few weeks of hot summer temperatures under plastic tarps can effectively sterilize garden and lawn areas for a fresh start. Use soil solarization as a sustainable technique to eliminate weeds without herbicides. Combine it with moisture-retaining mulches afterward for continued weed prevention. With this full-season weed management approach, you can grow in-ground or raised beds productively with very low maintenance.