Gardening : Solarization Isn’t Something New Under Sun
Want healthier, more vigorous plants? Want to spend less time weeding? Would you like to depend less on dangerous, expensive garden chemicals?
Well, all this can be yours, and the best part is that the basic ingredient is free.
The name of the game is soil solarization, and all you need is some clear polyethylene sheeting and a lot of good old southern California sunshine. But don’t delay--summer is the time to get busy. Hucksterisms aside, this simple method can yield exciting results in situations varying from big agriculture to back yard vegetable patches and hillside wildflower gardens.
Soil solarization uses the sun’s heat trapped under a plastic tarp to kill weed seeds and diseases in the soil before planting. It is most useful for vegetable gardens that are replanted seasonally or new plantings such as lawns, perennial borders or wildflower meadows.
The idea is hardly high-tech and actually may owe its origin to farmers in India who found that plowing their fields just before the hottest part of the dry season lessened the number of weeds that came up with the monsoon rains. Apparently, several weeks of intense solar radiation and 100-plus degree temperatures killed ungerminated weed seeds in the ground.
Researchers found that the sun’s seed-killing effect could be increased by covering the ground with a plastic tarp to trap the heat. In one test conducted in India, the temperature two inches below the surface of a plot of tarped soil never dipped below 113 for 48 days. This prolonged heating destroys most weed seeds and injurious funguses. A welcome but poorly understood bonus is that plants seem to grow faster and more luxuriantly in solarized soil, due to an increase in nitrogen, the most important nutrient for plant growth.
In the San Joaquin Valley, solarization has proven itself to farmers who contend with verticillium, a virulently destructive fungus that attacks tomatoes, cotton, strawberries and melons. Where any of these crops have been grown for a few years, the soil is likely to be infected. Fumigating the soil with methyl bromide, a highly lethal chemical, used to be the preferred way to destroy the fungus, but now solarization provides a safer alternative.
On a smaller scale, the Theodore Payne Foundation, which operates a native plant nursery in Sun Valley, is using solarization to establish new wildflower plantings. For many years, a west-facing hill overlooking the nursery was completely dominated by mustard, wild oats, red bromegrass and other weeds introduced from the Mediterranean region of Europe.
Now each year a section of the hill is readied for wildflower seeding by tarping it in early or mid-summer to kill weed seeds. The tarp is removed in the fall and wildflowers seeds are scattered. When rain eventually brings up the wildflowers, there is hardly a weed among them to be found. Solarization is known to work especially well on the cool-season weeds that are the bane of wildflower growers.
Whether you are planning a flower bed, vegetable garden, lawn or wildflower meadow, the solarization process is the same, but timing is critical. Everything depends on how hot you get the soil, so solarization should be carried out during the summer months. In a more refined garden setting, you could use sheeting 1 or 2 mils thick, which lets more heat through.
Bury the edges of the tarp in shallow trenches to prevent the heat from escaping. If you use wide sheets (20 or 40 feet), you can cover a large area with minimum trenching, but you will need to have some friends on hand so a sudden gust doesn’t pull the tarp out of your hands. Eight- or ten-foot wide sheets are easier to handle and would do for small areas, but there is more edge to bury in relation to the size of the plot.
In cooler coastal areas it may help to use one tarp on top of another. That way it will take longer for the soil to heat up but the temperature will be maintained better than with a single sheet.
The longer you leave the tarp in place the better. Four weeks is probably the minimum in hot areas and perhaps six to eight weeks in cool areas. Check frequently for tears in the plastic and repair them with tape. After removing the tarp you are ready to plant. If possible, only cultivate the plot shallowly to avoid bringing up live weed seeds from the depths of the soil.
Solarization doesn’t work equally well on all weeds. Bermuda grass is notoriously difficult to kill by any means. If you want to try solarizing it out of existence, carry out the process described above, then cultivate the soil again and cover it with the tarp for a second period of four or more weeks.
At the Payne Foundation, one early soil tarping revealed that seeds of one common weed, sweet-clover, actually enjoyed the heat treatment. While all other weeds were destroyed, sweet-clover came up by the thousands.
Even if solarization isn’t perfect, it is a help for gardeners looking for ways to reduce the use of toxic chemicals that damage the environment.
If you want to see soil solarization in action, the tarped area illustrated in the photo will remain in place all summer. The Payne Foundation’s nursery is located at 10459 Tuxford St., Sun Valley, (818) 768-1802. Their summer hours are Wednesdays through Saturdays, 8:30 a.m. to 4:30 p.m..
Gardener’s Checklist
For dedicated gardeners, here are suggestions from the California Assn. of Nurserymen on what to do in the garden this week:
Remember to keep the mower blades a high 2 to 2 1/2 inches tall to give the lawn enough foliage to produce food.
Roses suffer in the heat of August. Water deeply, mulch around the root zone if you want roses into the fall months.
Plant garden mums now. There will be a good selection at your nursery.
Feed tuberous begonias and fuchsias with 0-10-10 fertilizer.
Divide and transplant bearded iris.
