Plant Rx: Wash well behind the leaves

For centuries, savvy gardeners used soap sprays to combat bugs.

Andrew Jackson Downing, a gardening celebrity of the 19th century (who would have designed New York City's Central Park if his life had not been cut short in a steamboat accident), wrote in 1845 that a "wash of soft soap is very good for many purposes ... penetrates all the crevices where insects may be lodged, destroying them."

Then, DDT and other harder-hitting, longer-lasting pesticides developed during World War II left soaps on the sidelines.

Yet here we are in the more environmentally conscious 21st century, and soap sprays are back in vogue — for the same reasons they fell out of favor. Soaps biodegrade quickly and are relatively nontoxic to most creatures (including us).

Pests on plants don't always warrant calling out the sprayer, but when spraying is needed, soap may do the trick.

You could just douse your rose bushes with leftover, soapy wash water, an aphid remedy once popular among British gardeners. Or you could use soap more deliberately, dissolving some tincture of green soap or Ivory soap shavings into water to make up your own mix.

Add 1 to several tablespoons of soap per gallon of water, or enough to make suds. Test a little of the solution to make sure it won't damage the plant as well as the bugs. Don't expect consistent results, though, because washing soaps vary in composition. (Note that soaps and detergents are not equivalent; soap is one kind of detergent, but all detergents are not soaps.)

These days, you can buy soaps specially formulated for garden use. Garden soaps, like washing soaps, are made by combining naturally occurring fats with an alkali such as sodium or potassium. Advantages of modern garden soaps come from choosing specific fats and alkalis.

Soaps act by disrupting cell membranes, and depending on the formulation, those membranes might be those of insects, weeds or disease-causing organisms. Insects most affected by soaps are soft-bodied, slow-moving ones such as aphids, mealybugs, scale and mites.

Now is when these insects start to build up on houseplants.

Caterpillars and beetles are not generally bothered by soap sprays.

Different soap formulations are used against weeds. Soaps toxic to weeds are more or less toxic to all plants, so have to be directed right at the weeds. That's easy enough with weeds poking up between brick pavers, but you're better off with a hoe for weeding a bed of flowers or vegetables.