Plus Ça Change . . .

In gardening as much as anything else “the more things change, the more they stay the same.” In 1845, Andrew Jackson Downing, famous horticulturalist (and native son to New York’s Hudson Valley) that a “wash of soft soap is very good for many purposes . . . penetrates all the crevices where insects may be lodged, destroying them.”

Savvy gardeners were using soap sprays even back in the 1700’s, and they were continuing to do so right through the beginning of the 20th century. Then DDT and other harder hitting, longer lasting pesticides that were  developed during World War II left soaps along the wayside.

So here we are in the 21st century, and soap sprays are back in vogue, ironically, for the same reasons that they fell out of favor. Soaps biodegrade quickly and are relatively nontoxic to most creatures — including you and me! Pests on plants don’t always warrant calling out the sprayer, but when spraying is needed, soap may do the trick in these (hopefully) environmentally conscious times.

You could just douse you rose bushes with leftover 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 you own mix. Add one to several tablespoon of soap per gallon of water, or enough to make suds. And test a little of the solution first to make sure it won’t damage the plant also.

Vintage houseplant sprayers

Don’t expect consistent results, though, because washing soaps are variable in composition. (Note that soaps and detergents are not equivalent; soap is one kind of detergent, but all detergents are not soaps.)

Different Soaps, Different Purposes

These days, soaps are available that have been specially formulated for garden use. Garden soaps for spraying, like washing soaps, are made by combining naturally occurring fats with alkali such as sodium or potassium. Advantages of these 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 are soft-bodied, slow moving ones such as aphids, mealybugs, scale, and mites.

Aphids on grape stem

Now is when these insects start to build up on houseplants, which is what started me thinking now about soap sprays in the first place. Caterpillars and beetles are not generally bothered by soap sprays. (“Safer” is one appropriately named soap for insect killing.)

Cottony cushion scale on jasmine

Different soap formulations are used against weeds than against insects. Soaps toxic to weeds are more or less toxic to all plants so have to be directed right at the weeds. My favorite weed killing soap is that containing ammonium nonanoate, found in such easier to speak products such as Mirimichi Green, Ortho® GroundClear® Weed & Grass Killer, and BioSafe Weed and Grass Killer.

Edged with organic weedkiller

As contact killers, these soaps are easy enough to use against weeds poking up between brick pavers, but you’re better off with a hoe for weeding a bed of flowers or vegetables. Disadvantages to soap sprays on paving, if used to excess, are that they can leave a white residue and be slippery until they wash away.

Make Contact

Whether used against insects, weeds, or diseases, soaps are contact poisons, so are 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. Soaps have no effect on insect eggs.

On the plus side, ladybugs and lacewings hanging around houseplants and garden plants usually have enough time to up and fly away before being thoroughly 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 like for bathing; rainwater is ideal. Once the soap is dissolved, no more shaking is needed — further shaking might cause too much foaming. Avoid spraying a stressed or blooming plant when you’re trying to kill insects or disease organisms.

Finally, thoroughly douse whatever plant you spray so that, to reiterate Mr. Downing’s advice of 150 years ago, the soap “penetrates all the crevices.”