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Irrigation, Soil & Fertility TimingSoil Solarization Timing
Solarization uses sun-heated soil under clear plastic to reduce weeds and some soil pests, and it only works if you run it during the sunniest, hottest weeks your area offers.
Key points
- Solarize during your sunniest, hottest weeks, not by calendar date.
- Allow weeks, and extend if soil temperatures stay low.
- Wet the soil deeply and keep the plastic tight to the surface.
- Clear plastic generally outperforms black.
- Expect limits with deep perennials, nematodes and some pathogens.
When is the best time to solarize?
The goal is to hold soil under clear plastic during a stretch of sunny, calm weather. University of California integrated pest management guidance describes the hottest and sunniest weeks as best, with clear days heating soil far more than cloudy or windy ones. Climate decides the window, not the calendar.
Choose the part of your year with the longest run of strong sun and little cloud. If you are in a cooler or foggier region, expect to need a longer treatment or a different tool.
Weather forecasts help you plan. A solarization run that starts just before a cloudy, cool stretch wastes time. Check the long-range outlook from the National Weather Service and aim to start at the beginning of a settled sunny period.
How long should the plastic stay down?
The UC guidance suggests plans in the range of four to six weeks in many situations, longer in cool, windy or cloudy places or for stubborn pests, and shorter in very hot summers. Treat that as a range, not a rule, and let soil temperature tell you how it is going.
A soil thermometer at a few inches deep shows whether the top layer is reaching the temperatures the guidance describes. If it is not, extend the treatment.
Moisture in the soil does two jobs: it carries heat deeper and it makes soil organisms more susceptible to heat. That is why wetting the bed before covering is stressed in extension guidance, and why dry soil under plastic does far less.
How do you prepare the soil?
Remove debris and break up clods, then smooth the surface so the plastic lies flat against it with few air pockets. Form raised beds before covering, if you use them. The soil should be wet to a good depth before or right after laying plastic, because moist soil conducts heat far better than dry soil.
Cover soon after watering to keep the moisture in, and avoid re-irrigating during treatment unless the soil dries out, since added water cools it.
Keep records: start date, finish date, soil temperatures, weather and results the next season. Those notes tell you whether your climate supports the method.
What plastic and edges work best?
Clear film transmits sunlight and works better than black, which is mostly a weed-control tool in cooler places. Film thickness is a trade-off between heating and tear resistance, and windy sites favor sturdier film. Cloudy or milky films block light.
Bury the edges in a shallow trench and pack soil over them to keep the plastic tight and keep heat in. Check for tears every week or so and patch them with tape made for the purpose.
What are the limits and what comes after?
Heat is strongest near the surface and fades with depth, so deep-rooted perennial weeds, some nematodes and certain heat-tolerant pathogens can survive. Shade reduces effectiveness, and results vary from year to year.
After removing the plastic, cultivate shallowly to avoid bringing untreated soil and weed seeds to the surface. Then plant promptly or cover the bed, since bare solarized soil can be recolonized. Plastic disposal and recycling options differ by area, so check locally.
How do you use a soil thermometer well?
Place the probe several inches deep in the treated area, in the sunniest and shadiest spots, and read it in the afternoon when temperatures peak. Record the readings, because a trend over several days says more than a single number.
If the readings never reach the temperatures described in extension guidance, the treatment is probably not working as hoped. Extend the time, repair gaps in the plastic or accept that solarization may not suit your climate and look at other tools.
Is solarization a fit for your goals?
It suits small beds and high tunnels where you can control moisture and cover, and it works best where summers are long, hot and sunny. It can reduce certain weeds and soilborne problems at the surface, but it ties up the ground for weeks, which is a real cost in a short season.
For example, if you need the bed for a late-summer planting, solarization may not fit your calendar. Compare it with options such as crop rotation, sanitation and cover crops, and ask extension which problems solarization has been shown to help in your region.
Timing at a glance
| Item | Window | Caveat |
|---|---|---|
| Soil solarization, inland California | Roughly June through August, with July most reliable, per UC IPM guidance | Climate drives this, not frost dates; coastal and desert areas differ, so check local extension guidance. |
| Soil solarization, treatment length | Typically four to six weeks, longer in cool, cloudy or windy sites | Confirm with a soil thermometer and your local extension office; results vary by site. |
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.