Gardening: Two ways to do soil solarization in your yard

In last week’s column, I discussed the effectiveness of soil solarization in killing weed seeds and underground root systems (Bermuda grass) and its ability to seriously reduce populations of underground pest insects (nematodes) and bacterial and fungal pathogens (fusillarium and verticillium wilts).

Solarization uses the sun’s rays in summer to heat the top 12 inches of soil to at least 130 degrees, which is high enough to roast most pest insects and their eggs and fry weed seeds, bacteria and fungal spores. When summer temperatures and daily sunlight hours are at their peak in June, July and August, it only takes 4 to 6 weeks maximum for the solarization process to be complete.

This week’s column provides instructions for soil solarization — both the proper way and the lazy way.

The proper method of soil solarization (highly recommended and most effective) begins with clearing the soil surface of mulches, rocks and pebbles, weeds and debris. The soil is then leveled as much as possible, raked smooth, and soaked to a depth of 1 foot (wet soil conducts heat more efficiently than dry). A 1.5 mil of clear, not black, plastic is placed over the soil bed. UV-protected commercial grade clear plastic sheeting is preferable since it will degrade more slowly than other plastic sheeting.

The edges of the plastic sheeting are secured and sealed by shoveling dirt over the edges with a few rocks or bricks added to prevent wind from lifting the plastic.

Four to six weeks later, depending on how high temperatures have been, the plastic can be removed. It used to be recommended that the newly sterilized soil should then be amended with compost or other amendments containing beneficial organisms, but research has shown that populations of beneficial organisms and fungi quickly rebound after fertilization. Earthworms, which help aerate the soil and add their own beneficial excrement to it, seem to disappear (perhaps into deeper, cooler soil levels) during solarization, but they reappear quickly.

Newly solarized soil can be planted or left fallow until temperatures drop in fall or until we get sufficient rain. Birds, wind, foot traffic, and lawn and garden tools that aren’t cleaned between uses can bring in weed seeds. If not planting, cover bare solarized soil with a three-to-four-inch layer of compost or humus that can be turned in later.

Now for the lazy method of soil solarization: (not highly recommended, not particularly effective, but sometimes necessary).

Nutgrass sprouted heavily from long dormant rootlets around the drip irrigation stanchion in our vegetable garden one year. The stanchion had begun to leak (nutgrass thrives in wet soil) and the nutgrass quickly filled in all the spaces between the mature squash plants and the tomatoes. It was both too late to use an herbicide, which targets nutgrass sedges when they have only two to three blades, and the nutgrass was too close to the vegetables to spray. So, a beloved family member fixed the leaky water pipes, laid down a couple of large black plastic bags over the top of the nutgrass and secured the edges (mostly) with bricks. It worked! Fairly well. Most of the nutgrass was killed and the next spring’s crop of nutgrass was weak, slow to grow and easily treated with an herbicide.

This story was originally published June 3, 2015 at 3:12 AM with the headline "Gardening: Two ways to do soil solarization in your yard."