Home & Garden

Step-by-step instructions on solarizing your soil

Triangular raised planter beds seen at Nancy Mack and Bill Geissert’s home garden.
Triangular raised planter beds seen at Nancy Mack and Bill Geissert’s home garden. Fresno Bee file

Last week’s column was meant to encourage home gardeners to try soil solarization as a non-chemical cure-all for soil-borne fungal problems, bacterial disease pathogens, weeds and weed seeds, pest insects and their eggs, as well as partial control for nematodes.

Elinor Teague

This week’s column provides detailed (but easy to follow) instructions for solarizing the soil in your planting beds, raised beds and containers. These instructions are taken from the University of California’s Integrated Pest Management website, ipm.ucanr.edu; search for “Soil Solarization for Gardens and Landscapes.”

The hotter the soil temperature gets during solarization, the more effective the process will be. There are four steps in the process.

Step 1 – Cultivate and clear the area to be solarized. Use a wiggle hoe to cut down weeds or hand pull them and remove all rocks and debris in the area so that the plastic tarp (step 4) can be pulled taut to lay tightly against the soil without air pockets. Consistent, even heating of the soil is important for good results.

Step 2 – Level and smooth the soil with a rake or hoe.

Step 3 – Irrigate to a depth of at least 12 inches. Wet soil conducts heat better than dry soil. The top 18 inches of soil can heat to temperatures between 108 to 131 degrees at 2 inches deep and 90 to 99 degrees at 18 inches. Most soil pests, fungal spores and weed seeds in the top 6 inches of hot damp soil will be eradicated or controlled. Plant roots usually extend within the top 12 inches of soil.

Step 4 – Lay down a clear translucent plastic tarp on the soil surface. The best way to keep the tarp taut is to bury the edges in a shallow trench one side at a time and fill the trenches with dirt, stones or bricks to hold the tarp tight.

Clear plastic is best because weeds are less likely to grow underneath it. Thin plastic tarps transmit the heat from the sun better than thicker plastic, but the thinner plastic tears easily. Tarps, including painters’ tarps, of 1.5 to 2 millimeters thick are more durable. Our intense summer heat really shortens the solarization process to three to four weeks. Thinner tarps may work just fine for shorter solarization times in non-windy areas.

Soil used for container plants should always be a sterilized potting mix not garden soil, but sometimes the previously sterilized container soil can become contaminated with root-borne pathogens (for instance,verticillium wilt), weed seeds (such as oxalis from bird droppings) or pest insects (for instance, fungus gnats). Replacing container soil with fresh sterilized potting soil after scrubbing out the pot or container is the first choice, but soil from containers can also be solarized by placing the soil in a black plastic bag and setting the bag on some sort of platform to allow for air circulation and heating around the bag. Temperatures inside the bag can reach 160 degrees within an hour, hot enough to kill all soil-borne pests and diseases. The pot will still need to be scrubbed with a wire brush before replacing the soil.

Elinor Teague: etgrow@comcast.net