By Val Whitmyre, U.C. Master Gardener

Soil Solarization

Unfortunately, whenever we water our plants, weeds are sure to grow. Each season brings different types of weeds. You think you've pulled every one, but it won't be long before more appear to take precious water away from your plants' roots.

Weeds disperse seeds by the thousands, so we can never really be rid of them. Weeds are a constant part of gardening, and some people, including me, enjoy pulling them. It's a no- brainer exercise that allows my mind to wander. Sometimes great (and not so great) ideas emerge unexpectedly.

But sometimes it's just too hot for that kind of work. One way to make this chore easier during hot summer months is to solarize your soil. You can let the sun do the work by trapping its heat underneath a clear polyethylene plastic tarp.

Soil solarization kills weed seeds and pathogens that live beneath the soil. These pathogens include viruses, bacteria, fungi and nematodes. Particularly sensitive plants include heirloom tomatoes and strawberries.

I have a 12-foot by 12-foot plot in the back yard where nothing has grown but weeds for the past 14 years, probably because I haven't thought about what I want there. I could have an arbor or more space for vegetables, but first I have to clear this year's weed crop and dump it in the waste recycling bin. I don't put weeds in my compost pile because I'm lazy about keeping the pile hot enough to kill their seeds.

If you have a similar weedy spot and want to solarize it, rake up all plant material, rocks and dirt clods to make the plot as level as possible. Prepare the soil for planting by digging in compost, then rake smooth. You want the tarp to cling evenly to the raked soil with no air pockets to hinder heat production.

After thoroughly watering the plot, it's time to roll out the clear plastic tarp. Black or other colored plastic will not produce as much heat build-up, and you want as much heat as possible. The tarp should be one to two millimeters thick for the most heat efficiency. Anything thicker is not as efficient. Smooth out all the air pockets and bury the edges in the soil using rocks as anchors.

For the highest possible heat, the best time to solarize is in July and August when the night soil temperatures remain warm. Keep the tarp in place for at least four to six weeks.

When I read about solarization, my immediate question was, "How long before weeds reappear?" A Napa friend who has been solarizing her raised beds for several years tells me that the procedure keeps weeds down for two to three seasons.

Not all weeds are killed by solarizing, but most are, and the process is well worth the effort. I will be doing it at the beginning of July next year. In the meantime, I will lay black and white newspaper covered by grass clippings and fallen leaves over the area for the winter to help retard weed growth. Next summer the sun will do my work for me.