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.
Disadvantages of soap sprays on paving are that they can leave a white residue and be slippery until they wash away.
Whether used against insects, weeds or diseases, soaps are contact poisons, effective only as long as target organisms are wet.
This is both good and bad. Sprayed perennial weeds often have enough energy stored in their roots to resprout, so need repeated treatment. Hand weeding might prove easier. Similarly, repeated treatments are needed to kill insects that hatch from eggs on treated plants to get each flush of hatchlings. Soaps have no effect on insect eggs.
On the plus side, beneficial ladybugs and lacewings hanging around houseplants and garden plants usually have enough time to up and fly away before being doused with a soap spray. Once the spray dries, all harm has passed and they can return.
For maximum effectiveness, spray either weeds or garden plants with soap when the weather is overcast or cool, and drying is slowed.
The best water for mixing up a soap solution is soft water, just as for bathing; rainwater is ideal. And once the soap is dissolved, no more shaking is needed — further shaking might cause too much foaming.
Avoid spraying a stressed or blooming plant.
Finally, thoroughly douse whatever plant you spray so that, to reiterate Mr. Downing's advice of 167 years ago, the soap "penetrates all the crevices."