Gardening: Consider taking a year off from planting to sterilize soil

Soil solarization is one of the most effective, cheapest and environmentally sound methods of reducing populations of soil-born pathogens, fungi, weed seeds and pest insects and their eggs. Soil solarization uses the heat of the sun’s rays to basically cook the soil, raising the temperature in the top twelve inches to 130 or even 140 degrees, which is hot enough to kill most living organisms. (Strangely enough, populations of beneficial micro-organisms are much less affected by the intense heat and will quickly expand into the newly sterilized soil, increasing the soil’s healthy bacteria and fungi levels).

This fourth drought year with mandatory water restrictions already in place and water rationing a possibility, take a year off from planting in your garden and sterilize your soil in anticipation of future planting.

Nematodes, verticillium and fusillarium wilts, and hard-to-kill underground weed rhizomes (bermdua grass) and tubers (nutgrasses), are common problems in our Central Valley gardens. All can be controlled if not eliminated completely by solarizing the soil.

▪ Nematodes are microscopic, parasitic roundworms worms that invade root systems, sucking out the nutrients, and thereby weakening the plants’ vigor. Signs of nematode damage include knots or galls on puny-looking root systems and greatly reduced green growth and flower and fruit production. Some branches may die, but not necessarily all. After solarization, the nematodes in upper soil levels will have been killed by the heat but nematodes in lower soil levels, below 12 inches, will survive to re-infest the upper levels. Soil infested with nematodes will need to be solarized every two to three years to control nematode populations.

▪ Verticillium and fusillarium wilts are caused by soil-born fungi that enter into and plug up plants‘ vascular systems. Affected plants can’t draw up sufficient water and nutrients from the roots; leaves and stems consistently look wilted or droopy. Plants with wilts often show a dark stain just under the outer stem or bark surface. Wilts can remain in the soil for many years, repeatedly affecting new plantings. Soil solarization has proven effective in controlling wilt fungi.

▪ If you’re planning to remove your bermuda grass lawn this summer in order to replace it with water permeable hardscape and drought-tolerant plants (always a good idea), try scalping the grass, then solarizing the lawn bed before applying herbicides. Solarization, when done correctly, can kill off most of the bermuda grass rhizomes and greatly reduce the number of herbicide applications needed to eliminate every last pesky rhizome (it often takes at least three to four herbicide applications over a period of months to eliminate bermuda grass).

▪ Nutgrass is not a grass (single-sided blade); it is a sedge (three-sided blade) which grows from underground tubers that can store enough water to survive up to four years in drought conditions. Herbicides that control for nutgrass are most effective if applied in spring when new nutgrass blades are just one or two inches high. Nutgrass thrives in over watered areas, and is nearly impossible to get out of lawns. Soil solarization will kill most nutgrass tubers.

Next week: instructions for soil solarization, both the lazy way and the proper way.

This story was originally published May 27, 2015 at 5:00 AM with the headline "Gardening: Consider taking a year off from planting to sterilize soil."