Soil solarization is a simple, chemical-free way to clean the top layer of garden soil: cover moist soil with clear plastic in the heat of summer, and the sun heats the soil enough to kill many weed seeds, soil pests and disease organisms. It costs little, uses no chemicals, and can rescue a bed that is full of weeds or carrying a persistent soil disease. It also has real limits — it only heats the top 15 cm or so, it kills beneficial organisms as well as harmful ones, and it needs weeks of hot sun — so it is a tool for specific situations, not a routine treatment. This guide explains how it works, when to use it, and how to do it properly.
Solarization works because clear plastic lets sunlight through and traps the heat underneath, like a greenhouse. Moist soil conducts heat better than dry soil, so the bed is watered first, and the plastic is sealed at the edges so the heat cannot escape. On a hot summer day, the top 5 cm of soil can reach 50–60°C under the plastic, and temperatures stay high enough, for long enough, to kill many weed seeds, nematodes, and the fungi that cause wilts and root rots.
When solarization is worth doing
Solarization is worth considering when a bed is overrun with weeds and their seeds, when a soil-borne disease such as fusarium or verticillium wilt keeps killing the same crop, when root-knot nematodes are a problem, or when you are starting a new bed on weedy ground and want a clean start. It is also useful before planting a high-value crop that is sensitive to soil disease, such as tomatoes in a bed where they have failed before. It works best in hot climates and in the hottest part of summer — in cool or cloudy regions it may not get hot enough to be effective, and in that case other methods are better.
It is not the right fix for every problem. It does not control deep-rooted perennial weeds such as bindweed or couch grass, whose roots survive below the heated layer. It does not fix poor soil structure, compaction or nutrient deficiency — those need compost and the treatments in our soil compaction and clay soil improvement guides. It does not kill all disease organisms, and some weed seeds survive. And it should not be used routinely every year, because it also kills beneficial soil organisms and disrupts the soil life that keeps a bed healthy. Use it as a targeted treatment for a specific bed and a specific problem, then rebuild the soil life afterwards with compost.
Step-by-step: how to solarize soil
1. Pick the right time. Choose the hottest 4–6 weeks of summer, when days are long and sunny. In most temperate regions that means late June to August. The bed needs to be in full sun for most of the day — shaded beds will not get hot enough.
2. Prepare the bed. Clear the bed of plants, weeds and debris. Break up the surface with a fork to a depth of about 15 cm, and remove large stones and clods so the plastic lies flat against the soil. Level the surface roughly.
3. Water thoroughly. Soak the bed deeply so the top 30 cm is moist. Moist soil heats far better than dry soil, because water conducts heat down into the ground. This step is essential — solarizing dry soil does not work well.
4. Cover with clear plastic. Use clear, not black, polyethylene sheeting, 25–50 microns thick (roughly 1–2 mil). Clear plastic lets the sun's heat through and traps it; black plastic shades the soil and heats it far less. Lay the sheet over the bed, pull it tight so it touches the soil everywhere, and bury the edges in a shallow trench 15 cm deep all the way around, so no heat escapes and no wind gets under it. A tight seal matters more than anything else.
5. Leave it for 4–6 weeks. Leave the plastic in place for at least four weeks, and six weeks is better. Do not lift it to check — every lift lets the heat out. The soil needs sustained high temperatures, not a single hot day, to kill weed seeds and disease organisms.
6. Remove the plastic and rebuild. Take the plastic off, let the bed cool, and immediately add 5–8 cm of compost to rebuild the soil life that the heat reduced. Fork the compost lightly into the top 15 cm, mulch the surface, and plant. The bed is now clean and ready, but its biology is depleted, so compost is not optional after solarization. See our composting basics guide.
What solarization controls — and what it does not
Solarization reliably reduces many annual weed seeds in the top 15 cm, root-knot nematodes, and the fungi that cause fusarium wilt, verticillium wilt and some root rots. It also kills many soil insects and their eggs in the heated layer. It is less effective against deep-rooted perennial weeds, against weed seeds below 15 cm, and against some heat-tolerant disease organisms. It does not control pests above ground, and it does not improve soil fertility — in fact, it can temporarily increase soluble nutrients as organisms die and break down, followed by a dip as the soil life rebuilds. That is another reason to add compost afterwards.
If solarization does not get hot enough in your climate, consider alternatives: a thick mulch and hand-weeding for weeds, crop rotation for soil diseases — do not plant tomatoes, potatoes or other members of the same family in the same bed year after year — and resistant varieties where they are available. Check your local extension service for advice on the soil diseases in your area and whether solarization is effective there.
Common mistakes
The most common mistake is using black plastic, which shades the soil instead of heating it — always use clear plastic. The second is solarizing dry soil, which does not conduct heat well enough. The third is leaving the plastic on for too short a time: two weeks is not enough, and four to six weeks is the minimum. The fourth is a poor seal at the edges, which lets heat escape and wind lift the sheet. The fifth is solarizing a shaded bed, which will not get hot enough. The sixth is skipping the compost afterwards, which leaves the bed biologically empty and vulnerable to re-infection. And the seventh is solarizing every year as a routine, which degrades soil life over time — use it for a specific problem, then rely on compost, mulch and rotation to keep the bed healthy.
Safety notes
Wear gloves when handling plastic sheeting and soil, and wash your hands afterwards. Work in the early morning or evening when laying plastic in hot weather, and drink water — heat exhaustion is a real risk on a hot day. Dispose of the plastic responsibly after use; do not burn it, and do not leave it to break down in the garden. If you are unsure whether solarization will work in your climate, or what disease you are dealing with, check your local extension service for advice before taking a bed out of use for six weeks.
Frequently asked questions
How long does soil solarization take?
At least four weeks, and six weeks is better. The soil needs sustained high temperatures over weeks, not a single hot day. Choose the hottest part of summer and do not lift the plastic to check, or the heat escapes.
Should I use clear or black plastic for solarization?
Clear plastic. It lets sunlight through and traps the heat underneath, heating the soil to 50–60°C in the top layer. Black plastic shades the soil and heats it far less, so it is useful as a weed-suppressing mulch but not for solarization.
Does solarization kill beneficial soil organisms?
Yes, in the heated layer — it kills many beneficial bacteria, fungi and worms as well as pests and weed seeds. That is why adding compost afterwards is essential: it reintroduces the soil life the bed needs. Do not solarize routinely every year.
What temperature does solarization reach?
On a hot summer day, the top 5 cm of soil under clear plastic can reach 50–60°C, and the top 15 cm can reach 40–50°C. Temperatures fall with depth, which is why solarization only treats the top layer and does not control deep-rooted perennial weeds.
Can I solarize soil in a cool climate?
It is less effective in cool or cloudy regions, because the soil may not stay hot enough for long enough. If your summers are mild, you may get better results from mulching, hand-weeding, crop rotation and resistant varieties. Check your local extension service for advice on whether solarization works in your area.