Annuals, seedlings
Indiscriminate
High (>$5000/gross)
Low/no risk
Low–moderate risk
Low/no risk
Low (sensitive plants, ground-dwelling animals, plastics)
Overview
Soil solarization is a hydrothermal method (combining moisture and heat) for controlling weeds by using plastic sheeting to capture radiant energy from the sun. Solarization is a simple method appropriate for open sites where the plastic can be undisturbed for the 4- to 6-week solarization period. Solarization is most effective in warm, sunny locations such as the Central Valley, desert valleys, and other inland areas of California. It has also been used successfully in the cooler coastal areas of California during periods of high temperature and no fog. Solarization differs from tarping in that the goal of the treatment is to heat the soil to a temperature that is lethal to the target pest(s) whereas the mechanism of control using tarping is light exclusion and physical suppression of the emerging seeds or sprouts.
Soil moisture is important in this process since wet soil conducts heat better than dry soil. Moisture also stimulates germination making seedlings more vulnerable to the heat.
Because an area must be fully covered by plastic sheets, it is best suited to areas of less than 1 acre, and optimally less than 0.1 acre, in relatively even areas. Additionally, arrangements must be made for the removal and disposal of the plastic.
The effect of solarization is greatest at the soil surface and decreases at deeper soil depths. The maximum temperature of soil solarized in the field is usually from 108° to 140°F at a depth of 2 inches and from 90° to 99°F at 18 inches. Control of weeds is usually best for those found in the upper 6 inches of soil. Solarization generally does not control perennial weeds since perennials often have deeply buried underground vegetative structures such as roots, corms, tubers, and rhizomes that may resprout.
How to Use
The procedure must be conducted during the hottest time of the year to ensure that very high soil temperatures are achieved. For most of the state, that is in late summer but in parts of desert and Central Valley regions, solarization can be done from late spring to early fall.
In general, transparent or clear plastic is most effective for solarization, as the heating rays from the sun will pass through the sheet and be trapped to heat the soil below. Sunlight will also stimulate germination of many weed seeds that are at or near the soil surface. Usually black plastic is less effective because it absorbs and deflects part of the heat, rather than trapping as clear plastic does. However, in cooler or coastal areas, black plastic is sometimes better than clear because weeds will not grow beneath it, as they will under clear plastic when temperatures under the plastic are too low to kill them. In this case, the black plastic should be left in place for several weeks during the hottest part of the year. In this case, the process is serving more as tarping than solarizing.
Several thicknesses of plastic material are available. Thickness is measured in mils; 1 mil = 0.001 inch (0.025 mm). Consider the following factors when deciding on the appropriate thickness to use:
- Thinner gauges of plastic (3 mil or less) convey heat more effectively than thicker gauges. Very thin plastic (1 mil) provides greater heating but is also more susceptible to tearing from wind or animals walking on it.
- Slightly thicker plastic (1.5 to 2 mil) is better in windy areas.
- Thicker plastic (4 mil or more) can be used if the treated area is small.
Plastics designed for large-scale solarization are usually treated with an ultraviolet (UV) inhibitor so they will not break down quickly in sunlight. These are usually supplied in 4 to 12 ft wide rolls, often 1000 ft long.
Plastic sheets without UV protection should be watched closely so they can be removed before deteriorating to the point where removal and disposal are difficult. If a longer solarization period is desired, small areas can be covered again with fresh plastic. Any holes or tears should be patched with durable patching tape.
Double layering plastic in cooler or smaller locations may be helpful as it could gain an additional 2° to 10°F over that obtained with a single layer. In this case, a small layer of air between the layers of plastic is needed. This can be achieved by placing a polyvinyl chloride (PVC) pipe between the two layers.
There are generally five steps to solarizing soil:
1. Make the area to be covered as smooth and flat as possible. Raking or cultivation before installation of solarizing plastic may increase seed germination, which will increase the effectiveness of the treatment, and a smooth bed will allow the plastic to lie snugly against the soil, producing fewer air pockets. Air pockets between the plastic reduce soil heating and are also more likely to result in the plastic flapping and tearing in the wind.
2. Irrigate the soil. Because solarization is a hydrothermal process, if the soil is not moist, the effect of solarization will be reduced considerably. In all but high clay soils, best results are obtained if the soil is wet to saturation or at least wetted down to a couple of inches past where the majority of the weed seeds or vegetative propagules are. This is because the area must stay as moist as possible for the entire treatment period. Soils with a high amount of clay should be moist but not overly wet.
3. Install plastic tarp. Install the plastic tarps to cover the site as soon as possible after the water has been applied to reduce evaporation. Unless the soil gets dry during the course of soil solarization, do not irrigate again, as this will lower the soil temperature and lengthen the time required for successful solarization. The plastic must be held as tightly as possible against the soil. One way to hold it down is to dig a trench 4 to 6 inches deep around the perimeter of the area that is going to be solarized. Lay the plastic out over the area with one edge in the trench. Cover that edge with soil to hold it down. Pull the plastic tight from the other side and bury that edge in the corresponding trench. Do the same with the other sides and then walk around the perimeter of the trenched area to pack the soil down around the edges of the plastic. It may also be helpful to add bricks or other weights around the edges to hold the plastic in place. The closer to the soil surface the plastic is, the better the heating.
4. Maintain solarizing temperature. The cooler the soil temperatures, the longer the plastic needs to remain in place to raise the temperature to desired levels. The goal is to maintain daily maximum temperatures in the top 6 inches of soil to 110° to 125°F. You can use a soil thermometer or temperature probe to check. Check the plastic tarps frequently and repair any tears that develop. About six weeks of soil heating during the warmest time of the year is usually sufficient to control most susceptible weeds. In cool, windy, or cloudy locations, it may be necessary to leave plastic in place for up to 8 weeks. Conversely, during very hot weather in the warmer parts of the state, the plastic may only need to be in place for 4 weeks.
5. Remove plastic. After solarization, the plastic must be removed. When doing so, take care not to disturb the underlying soil, which can bring up viable weed seeds from untreated edges and furrows or from deep layers that did not reach lethal temperatures. The area should be planted immediately with seeds or transplants. Delaying revegetation may invite weed colonization from outside sources.
Non-Chemical Weed Control: Solarization, Robert Freese from Cal-IPC on Vimeo.
Optimal Conditions for Use
Optimal conditions include: hot temperatures, no shade or canopy cover, a level or south-facing site, well-moistened soil, and a mowed, bare, or cultivated soil surface. Soil should be flat with few clods so that the plastic is right on the soil surface. This can be done by dragging a chain-link fence piece or thick piece of lumber across the area prior to soil wetting.
Soil should be thoroughly wetted. If natural rainfall is not sufficient, supplemental irrigation is required. Sufficient rainfall may not overlap suitable high temperatures in all areas of the state.
Because solarization is most effective during the hottest time of the year (late summer for most of California), seeds that would be germinating in the fall and winter are controlled best. Because seeds that are at in the top 4 inches are the primary target of this method, it is imperative that the soil is not disturbed so that seeds deeper in the soil profile are not brought to the surface where they can germinate.
Caveats
- Not effective on dry soil. A water truck or other water supply source is needed to pre-irrigate. May need a long hose and pump for sites that are farther from the road. In desert areas where the temperature is expected to be high even in the spring but water is limited, plastic can be applied soon after later season rains but the plastic may need to remain in place for a longer period of time.
- Can be effective on all but north-facing slopes but is most effective where solar angle is maximized (e.g., flat or south-facing slopes).
- Due to the weight of the plastic rolls and the need for pre-irrigation, the site should be relatively accessible by vehicles (no more than 300 ft from a roadside is recommended).
- Plastic must be intact. Any holes from animals walking across or edges being lifted by wind must be repaired immediately.
- Trenching the site perimeter to secure the plastic material is typically the most difficult and time-consuming step.
- Soil texture is important. In rocky sites, it will be difficult to place the plastic close to the soil surface. Additionally, trenching for burying the edges can be difficult. In those cases, the edges can be lined with bricks for small areas.
- If the desirable plants are perennial, there is a better opportunity for solarization to control the weeds and have less impact on wanted populations. If the desirable plants are annuals, they would likely be impacted. It is also easier to apply the plastic if there are larger patches of weeds and fewer desirable plants to work around.
Potential Hazards to Humans, Environment, and Cultural Resources
Human safety. Low risk. Because of the high heat generated, the operator should be aware of a chance of minor burns while patching plastic tarps. Rolls of plastic are very heavy and may need equipment or multiple people to carry or install.
Cultural resources. Low-moderate risk. Trenching to install the plastic may disturb some sites. Be aware that permits may be needed if cultural resources are near trenching areas.
Habitat. Low risk. This technique is typically used on highly degraded sites as this is a non-selective method for suppressing or killing plants. Plastic should be removed and disposed of after use.
Sensitive species. Low-moderate risk. Immobile species such as plants and some soil dwellers will be impacted. Species trapped under the plastic such as snails, lizards, and small burrowing mammals will be impacted and may be killed.
Erosion. Low risk. Plastic sheets will protect sites from erosion during solarization, however bare exposed ground will be vulnerable to increased erosion after sheets are removed. If the plastic is on a slope and rainfall occurs, areas that are immediately downslope will receive extra runoff and could be susceptible to extra erosion.
Consider Combining with the Following Non-Chemical Methods
Cultivation and smoothing the soil surface prior to applying plastic will allow the plastic to be placed with little air between it and the soil surface. It will also bring buried weed seeds to the surface where they will be exposed to higher heat. Low mowing will suffice if cultivation is not possible. Biomass should be removed after mowing to ensure the plastic can be installed close to the soil. Competitive planting can be used to revegetate bare ground once tarps are removed.
Don’t Use This Technique When/For
This technique is not effective at sites that are shady or otherwise do not get full sun, sites that are rocky such that plastic does not have direct soil contact, and sites with burrowing animals (e.g., pocket gophers) that can tear plastic. This technique is not effective for controlling perennial plants, hard-seeded plants such as clovers, or Malva species which appear to tolerate solarization.
Other methods should be used when restoration overlaps with time needed to solarize (generally 4–6 weeks in summer). Planting should be done as soon as possible after the plastic is removed. Do not use solarization in a relatively intact native habitat since it is not selective and will kill desirable plants.
Supplementary Information
Black plastic is also a good tool for deactivating seed and vegetative propagules at remote sites. Removed seed heads and vegetative material accumulated after pulling is placed in black plastic bags and left in the sun. Plant debris can get above 140°F in the bag. See “Invasive weeds in remote locations can be killed with solar tents”.
References
Elmore, C.L., J.J. Stapleton, C.E. Bell, and J.E. DeVay. 1997. Soil Solarization: ANonpesticidal Method for Controlling, Diseases, Nematodes, and Weeds. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 21377. https://vric.ucdavis.edu/pdf/soil_solarization.pdf
Stapleton, J.J., UC Statewide IPM Program, Kearney Agricultural Center, Parlier, CA Solarization https://ucanr.edu/sites/Solarization/
Stapleton J.J., C.A. Wilen, and R.H. Molinar. 2019. UC IPM Pest Notes: Soil Solarization for Gardens and Landscapes. UC ANR Publication 74145. Oakland, CA.
Authors and Credit
Lead Author: Cheryl Wilen, University of California Integrated Pest Management Specialist, UC ANR
Co-authors:
Robert Freese, Senior Project Manager, Irvine Ranch Conservancy
Suggested Citation:
Wilen, C. Solarizing. Weed Control User Tool (WeedCUT) – Methods for Managing Weeds in Wildlands. weedcut.ipm.ucanr.edu. California Invasive Plant Council (Cal-IPC) and University of California Statewide Integrated Pest Management Program (UC IPM). Accessed [date].
Efficacies
| Rating | |||
|---|---|---|---|
| Plant | Flowering Period | ||
| Winter | Excellent | ||
| Spring | Excellent | ||
| Summer | Excellent | ||
| Fall | Excellent | ||
| Multiple Seasons | Fair | ||
| None | Poor | ||
| Plant | Germination | ||
| Winter | Excellent | ||
| Winter / Spring | Good | ||
| Spring / Summer | Good | ||
| Opportunistic | Fair | ||
| Plant | Palatability | ||
| Yes | No Information | ||
| No | No Information | ||
| Partial | No Information | ||
| Plant | Plant Growth Form | ||
| Grass | Excellent | ||
| Forb | Good | ||
| Shrub | Ineffective | ||
| Tree | Ineffective | ||
| Vine | Poor | ||
| Plant | Plant Type | ||
| Annual | Excellent | ||
| Biennial | Poor | ||
| Perennial | Poor | ||
| Plant | Propagule Production | ||
| Low (<1000/square meter) | Excellent | ||
| Moderate (1000–10,000/square meter) | Excellent | ||
| High (>10,000/square meter) | Excellent | ||
| Plant | Rate of Spread | ||
| High (doubling in <10 year) | Good | ||
| Moderate (50–75% increase in 10 years) | Good | ||
| Slow Rate (25% increase in 10 years) | Good | ||
| Plant | Resprouting / Regenerative Capacity | ||
| Low | Poor | ||
| Moderate | Poor | ||
| High | Poor | ||
| None | Excellent | ||
| Plant | Seed Life | ||
| Short (≤3 years) | Excellent | ||
| Moderate (4–10 years) | Fair | ||
| Long (>10 years) | Poor | ||
| Plant | Type of Reproduction | ||
| Seed | Excellent | ||
| Vegetative | Poor | ||
| Seed & Vegetative | Poor | ||
| Plant | Type of Vegetative Reproduction | ||
| Rhizome / Stolon / Stem | Good | ||
| Bulb / Corm / Tuber | Ineffective | ||
| Root sprout / Sucker / Crown sprout | Ineffective | ||
| Site | Existing Desirable Plant Cover | ||
| <10% | Excellent | ||
| 10–25% | Poor | ||
| 26–50% | Poor | ||
| 51–75% | Poor | ||
| >75% | Poor | ||
| Site | Ground Condition | ||
| Muddy | Excellent | ||
| Smooth | Excellent | ||
| Cobbly | Poor | ||
| Rocky | Poor | ||
| Site | Habitat | ||
| Marsh / Wetland | Poor | ||
| Riparian | Poor | ||
| Grassland | Excellent | ||
| Shrubland | Poor | ||
| Woodland / Forest | Poor | ||
| Site | Level of Tolerable Disturbance | ||
| Low | Fair | ||
| Medium | Excellent | ||
| High | Excellent | ||
| Site | Slope | ||
| Flat | Excellent | ||
| Moderate (10–40%) | Fair | ||
| Steep (>40%) | Fair | ||
| Site | Target Area | ||
| <40 square feet | Excellent | ||
| 0.001–0.01 acre | Excellent | ||
| 0.02–0.1 acre | Excellent | ||
| 0.2–1 acre | Good | ||
| 2–10 acres | Good | ||
| 11–50 acres | Ineffective | ||
| 51–100 acres | Ineffective | ||
| >100 acres | Ineffective | ||
| Site | Targeted Invasive Plant Cover | ||
| <1% | Excellent | ||
| 1–10% | Excellent | ||
| 11–25% | Excellent | ||
| 26–50% | Excellent | ||
| 51–75% | Excellent | ||
| >75% | Excellent | ||
| Site | Vehicle Accessibility | ||
| Roadside | Excellent | ||
| <100 feet from road | Excellent | ||
| 100–1000 feet from road | Fair | ||
| >1000 feet from road | Ineffective | ||
