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At this time of year houseplants tend to look their worst. Why wouldn’t they? Most of our homes aren’t bright or humid enough for them. Many homes are overheated, although this winter some rooms may be too cold or drafty. The plants also are completely dependent on our sometimes imperfect care: We forget to water; we under water; we forget to dump the dish so they sit in water.

Some plants get severely pot bound, and we make them wait – their root systems so crowded they can hardly take up water or nutrients – because we’d prefer to do our repotting projects in warmer weather. All these conditions add up to the condition we call “plant stress.”

As with a physically stressed animal, a stressed plant is vulnerable to disease or to attack by insects. So the first rule in managing plant problems is that we must prevent them by meeting the plants’ needs and keeping them healthy. Plants that are healthy rarely develop diseases or pest infestations, or if they are attacked or infected they are more likely to survive.

Before reviewing some specific, predictable plant problems, I must stress two other key principles – part of any IPM (Integrated Pest Management) approach:

1. Know your plant’s name. I don’t mean Susie or Bruce or “the fuzzy purple one.” I mean identify the plant, or at least the larger plant family to which it belongs. If you know the name or grouping, you can learn its needs and possibly adjust its living conditions. (The Plectranthus or Begonia looked poorly until I discovered they like to be warmer than my cold country kitchen allowed. Ferns always get sick if they don’t have a humid atmosphere. Cacti require infrequent watering.) The identity also will let you research that plant’s common problems. And if you decide you must treat the plant, you need to know its name because pesticides are only good and legal for plants that are listed on their labels.

2. Decide your threshold of tolerance. I can live with a few fungus gnats or scale insects that leave sticky residue on a few plant leaves, but can you? I will patiently trim some leaves and hand-treat others. I know that warm weather will come and I can get the plants outside just in time for beneficial insects and sunshine to solve everything. But sometimes a plant is just too infected, infested or damaged to keep trying. Sometimes the plant just isn’t that important. Or sometimes one sick plant endangers your large collection – not worth it. Only you can make the decision.

The most likely problems

Let’s figure that you’re a good houseplant person. You provide great light and humidity, water properly, inspect plants regularly and wash the dust off their leaves sometimes. (I haul my huge lavender plant into the shower periodically.) You repot into sterile potting mix. You give them a winter rest period. You start fertilizing sometime in early spring, but you never overfertilize (the surest way to develop aphids). And you always inspect and isolate, temporarily, a new plant that enters your home in order to prevent introducing a problem. Right?

As good as we think we are, doing all those things right, it will still be true that most houseplant problems – well over 90 percent – are still for “cultural” reasons, all about the growing conditions.

When your plant does develop a pest problem, it is most likely to be from the following short but nasty little list:

• Aphids. You can see them: small, pear-shaped soft-bodied insects, easy to squish. They are attracted to new, succulent growth on many kinds of plants, and heavy fertilization just invites them. Wash them off repeatedly with a heavy stream of water. Rub them off with a cloth. Horticultural oil is my product of choice; see info below.

• Cyclamen mites. Invisible; sign is curling and distortion of new growth, on begonias, African violets and cyclamen. Inspect new plants carefully. Destroy infested plants.

• Fungus gnats. Small dark flies develop in damp soil or potting mix, or around standing water. Dry everything up; repot plants; let plants dry between waterings. Trap larvae by putting a piece of potato into the potting soil: If white larvae show up, that’s fungus gnats. No products are required.

• Mealybugs. Fuzzy, cottony, white, waxy insects found on the undersides of leaves. They suck sap and leave distorted, stunted, yellowing leaves. Typical hosts: croton, dracaena, ficus, gardenia, jade plants, palms, among others. Kill them by touching them with a Q-tip dipped in alcohol, or use horticultural oil.

• Scales. Brown bumps or shells (on leaves) cover little crawling insects that suck juices from your plant; sometimes leave sticky honeydew. Scratch them off with your fingernail; wash the plant; use a toothbrush; apply horticultural oil.

• Spider mites. Eight-legged tiny arthropods that thrive in hot, dry houses, affecting many kinds of plants. You’ll see dotted areas, yellowing leaves. Humidify. Wash them off (under leaves); try insecticidal soaps labeled for them.

• Thrips. Visible only with a magnifying glass; leave pale patches, dried out leaves; fecal matter evident. African violets, begonias often attacked. Thrip-specific pesticides exist but best advice: Discard the plants.

• Whiteflies. You’ll see a cloud of flying insects (small, yellowish) when you brush the plant; tiny eggs under the leaves; honeydew. Geraniums, fuchsias, herbs vulnerable. Serious problem in greenhouses; whiteflies also transmit disease. Try horticultural oil or labeled pesticides or destroy the plants.

Treatments of choice

Many pesticides are labeled for these pests, but my advice: Don’t use anything at home that could hurt you, your family or pets. It’s not worth it. Hand methods – hosing off, scraping, trimming infested leaves, Q-tips and alcohol – are quite enough for most problems. My favorite product is called Indoor Pharm, a gentle but effective horticultural oil for indoor use. (Similar products may also be found.)

Horticultural oils work by smothering insects, even scales. Even better, helpful spiders and “lady bugs” (correctly lady beetles) often gobble up houseplant pests if you’re tolerant and patient.

Somehow, in spite of us, most houseplants will live to see another summer on the deck or patio.

Sally Cunningham is a garden writer, lecturer and consultant.