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How to solarize the soil to get rid of weed seeds

Even after you rid your garden of weeds, they can sprout again – weed seeds remain in the soil.
Even after you rid your garden of weeds, they can sprout again – weed seeds remain in the soil.

A reader recently asked how to prevent weed seeds from sprouting on the completely bare area that used to be her lawn. She’d ripped out the grass in order to plant a drought-tolerant landscape, found hardpan just underneath the surface, missed the spring planting window, then had a yard full of weeds. The unwatered dirt is now bare, but she knows that weed seeds will germinate with the first rains.

She’s in luck. There’s still time for her to solarize the soil in her yard. Solarization should kill about three-quarters of the viable weed seeds as well as disease pathogens, nematodes and pest insect eggs. In July, when temperatures are highest and days are longer, it only takes three to four weeks for solarization to be effective. Do it now, and soil should be ready for tilling, amending and replanting in four to six weeks, or about the middle of September.

Full instructions for soil solarization can be found on the UC IPM (University of California Integrated Pest Management) website, ipm.ucanr.edu/PMG/PESTNOTES. The process entails removing all debris and weeds, soaking the soil to a depth of 1 foot, then laying a 1-millimeter thick layer of clear plastic over the area to be solarized. The edges of the plastic can be sealed with dirt, stones or bricks. The sun’s rays will heat the top 8 to 12 inches of soil to 140 degrees. After four to six weeks, the soil will be sterilized and ready for planting.

In this reader’s case, though, she has another couple of steps ahead of her since her soil is a baked heavy clay or hardpan. Drought-tolerant plants need good drainage to keep their roots fairly dry. Clay soils hold water when soaked and hardpan layers can completely block drainage. Early farmers in the central San Joaquin Valley used dynamite to blast through the hardpan which is now illegal. We have to dig, poke or drill through hardpan layers with backhoes, pick axes or augurs to break it up. Very hard work.

Hardpan layers vary in thickness and location, even within the same yard. The reader will need to determine just how thick her hardpan is and make decisions. In spots where the clay is more than 3 or 4 inches inches thick, she may be limited to digging planting holes only or bringing in sandy loam to top the hardpan. These would be the least laborious solutions, but since she’ll need to top the area with at least 1 foot of sandy loam, it can get expensive. In areas where the hardpan is fairly thin she can break it up with augurs or pick axes, remove it, amend the remaining soil with gypsum, sand and compost or humus, then plant.

If she chooses to leave the hardpan in place, she can lay down a 3- to 4-inch layer of mulch as well as weed cloth barriers to prevent the weed seeds from germinating. Applying pre-emergent herbicides in fall will also prevent germination.

Send Elinor Teague plant questions at etgrow@comcast.net.

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