Q: My windowsill herb and salad greens garden is infested with tiny bugs that lay white eggs on the undersides of the leaves. Insecticidal soap causes the leaves to turn brown and fall. Is there another herb-safe remedy? — Paul Girard, Baltimore, Md.
A: The perpetrator sounds like whitefly, a common pest of plants grown indoors; the “eggs” are actually nymphs, immature insects. You might also have winged aphids, which resemble gnats. Both flies and aphids suck the juices from plants, preferring soft, lush leaves like those of salad greens and basil rather than the tougher foliage of rosemary, thyme, or oregano. Although a soapy solution is one of the safest defenses against insect damage, it can cause leaf burn, the browning you describe. Unlike their counterparts outdoors, windowsill-grown plants are not hardened off by exposure to the elements, leaving them extra-sensitive to chemicals, no matter how safe. To avoid burning, use insecticidal soap or horticultural oil (also low in toxicity) at half the recommended strength — 2 1/2 tablespoons per gallon of water. Since a wet soap-spray coating can act as a lens on the leaf, concentrating light and heat, remove the plants from direct sun before spraying and wait until they are dry before putting them back. (Plants should also be watered in advance: A turgid leaf is stronger than a wilted one.) Spray both sides of the leaves, and repeat this treatment according to the label directions to hit all the new hatchlings. If the plants are small, dunk them in a bucket of the solution. Even with safe soaps and oils, wait a week after an application before harvesting foliage, and then wash it thoroughly.
Many herbs actually make good insect repellents; use them before an infestation gets out of hand or to prevent a recurrence. Either fresh or dried, or infused in a tea for spraying—chamomile, chives, coriander, dill, fennel, feverfew, garlic, lavender, pennyroyal, rosemary, and tansy all drive away pests . Try growing a few of these herbs among your other plants.
The harmless — to humans — sticky yellow cards (not pesticide-laced no-pest strips) often recommended for trapping whiteflies and aphids don’t really catch many, and in a small garden such as yours, you need to eradicate pests completely. These creatures multiply at amazing rates: Aphids reproduce even without a partner; one insect today might mean hundreds in a few days’ time. Both whiteflies and aphids thrive in hot, dry, poorly ventilated areas, so open a window or turn on a fan. Of course, your windowsill may just be too sunny for salad greens — generally cool-season crops, they need shading from hot afternoon sun; an east-facing window is ideal. By contrast, Mediterranean-native herbs like rosemary and thyme, which languish in cool shade, prefer to face south. Consider dividing your greens and herbs between windowsills with different exposures: The plants will be healthier and better equipped to fend off pests on their own—or to tolerate an occasional spritz with soap or oil when winged intruders arrive.