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Can You Solarize Garden Woes Away?


Q: Mike: Late blight is our chief tomato headache. (Our tomato leaves and fruits develop little grey-brown patches that spread, especially in damp weather.) I saved two old double-pane glass sliding doors with the intention of making them into some sort of greenhouse contraption, but could I instead lay them on last summer's tomato plot for a few months to solarize the soil and kill any blight spores there? You don't have to use plastic, do you? And does solarization work only in the summer? Or could heating the soil from March thru May or June do the job?
    ---Kate, near Boone, NC
Mike: My cucumbers have had severe wilt problems for the past four years. The plants die off before they can produce more than one or two cukes; sometimes before they can produce any. I solarize the soil every year in late spring to try and kill off whatever disease may be in the dirt, but it keeps coming back. This year I thought for sure the soil was sufficiently heated due to a few hot days. Thanks for any help,
    ---Lorraine in Torrance, CA
A. Soil solarization is probably the best way to get rid of otherwise insurmountable problems, like persistent weeds, soil-borne disease and even destructive root-knot nematodes. (Not to be confused with the beneficial nematodes we're always recommending, 'bad' nematodes are pretty much only a problem in the South; which is good, because solarization is really only practical in very warm climates.)

Here's the basics of soil solarization: You take the plot of land you wish to purge, till it all up, remove any rocks or clumps of plant material, carefully level it out, soak it until the soil is saturated at least 70% two feet down, then stretch clear plastic tightly overtop, making sure the sides are well anchored. Several sources suggest burying the sides of the plastic in trenches dug next to the plot to insure a tight fit for the length of the project.

And sorry folks, but the length of time necessary for this project to succeed is an absolute drop-dead minimum of six to eight weeks in the hottest, deep-South areas of the country during the hottest months of the year, in a year with lots of sunny days. In the mid to upper South, we're talking all of June, July and August (and pray for sun). In more Northern parts of the country, its only going to work if you leave the plastic on ALL season in a full-sun site during a hot and sunny summer. And you should use two sheets of plastic, which heats the soil more by trapping warm air between the layers.

Sorry, but it really does take that long and require that much heat. That's why this is such an extreme procedure—you're agreeing up front that you won't have use of those plots that summer.

Studies show that the thinnest plastic (a mere one-mil thick) heats the soil best. But thin plastic also rips easily, and so a common recommendation is to meet somewhere in the middle of heat and strength with a two-mil sheet.

Kate in North Carolina MIGHT be able to use her old door panels to cook the bad news out of two small plots, but I'd worry about getting a tight enough fit, and about the glass breaking, laying flush to the ground all season like that. I would instead cobble those doors into super-groovy cold frame lids, which could extend her salad growing options to virtually twelve months of the year, and rely on plastic to cook her dastardly dirt.

Her disease is a tough one. Many gardeners misuse the word 'blight' as a synonym for other, less fatal, diseases; but (unfortunately) her description sounds like the same notorious organism that starved millions of my Irish ancestors when it leveled potato crops during the Great Famine. It is unusual for this bad actor to appear repeatedly in a home vegetable garden, and my first suggestion would be to do everything to try and avoid it up front. Here's my short list:
  • Make sure that ALL of last years' plant debris is removed from the area;
  • Don't grow any potatoes in your garden for awhile;
  • Cover the soil completely with high-quality compost;
  • Space the plants twice as far apart as last season;
  • And do everything you can to keep the leaves of your plants dry, with the exception of a weekly morning spray of compost tea.
And yes, maybe cook up a plot for use the season after that if all those tactics still fall short. Late blight is not among the diseases that solarization has been proven to defeat in scientific studies, but it can't hurt. And the heating process increases the numbers of many beneficial soil organisms that will help combat all kinds of disease in subsequent seasons. AND it makes several important nutrients (including Nitrogen, Calcium and Potassium) more available to plants. That all adds up to more vigorous, naturally resistant plants if dread diseases do return to play.

Solarization is a sure cure for the most common tomato disease problem, the nasty soil-borne wilts known as verticillium and fusarium. Tuskegee University researcher Dr Clauzell Stevens explained that it can make those bad boys (and Southern blight) go bye-bye for a full three seasons in a little piece he did for ORGANIC GARDENING magazine back in 1991. He also said it should banish bad nematodes for at least two years, and that the buried seeds of crabgrass, purslane, barnyard grass and pig weed should be reliably fried. In a hot enough summer, even the notorious yellow nut sedge may go down!

Lorraine in California seems to be 'solarizing' her soil at the wrong time of year and probably for MUCH too short a period of time. And I would not give up a bed for an entire season on a guess that something lurking deep in the soil was to blame for those cucumber woes. The vast majority of cuke diseases are spread by cucumber beetles while they feed, so I suggest she first try using row covers to keep those pests off the plants early in the season, and then have a variety of organic controls (soap sprays; oil sprays; spinosads…) on hand for when the covers have to come off. See this recent Question of the Week for more tips on controlling cuke beetles and the diseases they spread.

But if you got the room, the time, the heat, the patience, the inclination (and of course, the troubles) go ahead and try and a little solar assistance!

Here's a nice short course on the topic from Clemson.

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