Those Pest-y Houseplants

When the outdoor winter landscape turns drab in most areas of the country, it's nice to have lush, tropical houseplants growing indoors to brighten your mood. Keeping houseplants growing well in winter can be a challenge, especially in cold climates. Many of them have tropical bloodlines; most grow best in areas with warm temperatures and high humidity -- neither of which is in abundance indoors. When a houseplant is struggling in the wrong growing environment, other problems, such as pests, can take over. Insects can thrive on stressed plants, especially where there is a natural lack of predators. Not only are the pests harmful to your plants, the exudates from their feeding can cause damage to rugs, floors, and furniture.

Although there are numerous pesticides on the market to control houseplant pests, many gardeners are leery of spraying harmful chemicals indoors. Fortunately, there are ways to control the insects without resorting to harmful sprays. The first step is creating a "houseplant friendly" environment in your home.

Imitating the Humid Tropics

Most foliage houseplants prefer a warm, humid environment. While it's possible to heat your houseplant-filled room to the 70° to 80° F. range they like, the result will be a drier environment. As temperatures climb above 67° F, air humidity levels drop dramatically. Unless you have a humidifier to keep the humidity at the 40 to 50 percent range, it's better to keep the room on the cool side (60° to 70° F) in winter to keep the humidity higher. However, don't allow the temperature to go below 60° F since some houseplants, such as African violets and gardenias, may stop flowering and drop leaves.

Another way to increase humidity is to group houseplants together and place them on a tray filled with 1 to 2 inches of crushed stones. Keep water in the tray below the bottom of the pots. The water will evaporate and provide humidity.

Let There be Light

While many houseplants grow as understory plants in subtropical and tropical forests, they require more intense lighting in winter in our temperate climate. Plants not receiving enough light will produce tall, leggy growth and dull, pale leaves. Keep plants growing well by placing them near a sunny window or door, but away from cold drafts. Consider using full spectrum or halide lights to provide the light intensity and duration that plants need.

Watering and Washing

Periodically washing leaves to remove dust, and allowing some air (not cold air) movement around plants will improve plant growth and also reduce the number of insects that call your plants home. Tropicals grow more slowly during these darker months, so they need less water and can become stressed from overwatering, which causes brown leaf tips, leaf drop, and wilting. Plants also need less fertilizer because they are growing more slowly. Excess fertilizer causes succulent growth -- a magnet for aphids and other sucking insects.

Control Strategies

Even if you are meeting the cultural requirements of your houseplants, insects can still be a problem. If you placed your houseplants outdoors in summer, check them carefully for insects before moving them indoors in fall. After the first few weeks indoors, check again for any insect eggs that may have hatched.

Here are the six most common houseplant pests, and some organic controls to keep them at bay. When using a spray, always test it on a few leaves before spraying the whole plant because some houseplants (e.g., ferns) are sensitive to specific sprays. Always use the recommended dosage amounts.

Aphids

These small, soft-bodied insects may be pale green, pink, black, or yellow, depending on the species. They like to cluster on tips of new growth and leaf undersides, sucking out plant juices and causing leaves to become distorted and yellow. Aphids secrete a sugary fluid called "honeydew" that may spur the growth of a sooty, black fungus on leaves and cause stickiness on leaves, floors, and rugs. Aphids like fast-growing plants, such as hibiscus.

Controls: Place the plant in the shower and knock aphids off the leaves with water, or wash the leaves in a sink full of soapy water. Cut back on high-nitrogen fertilizers, which stimulate new growth. For widespread infestations, spray the foliage with insecticidal soap, neem oil, or horticultural oil.

Fungus Gnats

These small, dark-colored flies jump and fly across the soil surface and around the house. Although they generally don't harm plants, they're a nuisance. The adults lay eggs in potting soil. If numerous enough, the larvae can damage plant roots.

Controls: Fungus gnats feed on organic matter and fungi in the soil. They especially like moist, rich soil. Let the top layer of the soil dry out between waterings to discourage their egg laying. Avoid using fish emulsion fertilizers since they foster the fungus that gnats like to eat. Drench the soil with Bacillus thuringiensis 'israelensis' (B.t.i.) or neem oil to control the larvae.

Mealybugs

These soft-bodied insects group in white cottony masses and suck sap from plants. Their feeding weakens the plant and causes leaves to shed. Like aphids, they excrete large amounts of sticky honeydew. While adults tend not to move once settled on a leaf, flower, or stem, the young (crawlers) can move around the plant. Mealybugs favor cacti and jade plants.

Controls: For small infestations, wash the leaves in a shower to dislodge the insects, or dab individual mealybugs with a cotton swab doused in rubbing alcohol. The alcohol will desiccate the insects, killing them. For larger infestations, spray with insecticidal soap, neem oil, or horticultural oil.

Scale

These insects may be flattened and brown, or thick, white, and covered with a waxy or woolly substance. They appear like small bumps on leaves and stems. Leaves of infested plants turn yellow, and the overall vigor of the plant declines. Like mealybugs, the young are mobile, while the adults tend to settle in one area to feed. Ficus and citrus are often attacked by scale.

Controls: Flicking or rubbing the scale off the twigs by hand or dabbing them with rubbing alcohol often controls small infestations. For larger populations, spray horticultural oil or neem oil to cover the shells and suffocate the scale. Scale insects may hang on the twigs and leaves even after they are killed.

Spider Mites

These small insects aren't usually evident until their population soars and they form their characteristic webbing. Their feeding can cause stippling of the leaves and leaf drop. They thrive in hot, dry conditions and are particularly fond of cyclamen, Norfolk Island pine, and schefflera.

Controls: Keep plants misted because high humidity discourages the mites from getting established. Wash off infested plants in a shower or sink full of soapy water. Spray insecticidal soap or horticultural oil on severely infested plants. In houses with many plants, consider releasing predatory mites, such as the two-spotted mite (Phytoseiulus persimilis). These mites feed on spider mites and reduce the population without harming the plants, pets, or people. It will take a few weeks for them to control an infestation.

Whiteflies

These tiny, white insects feed in large numbers on leaf undersides, sucking out plant juices. They secrete honeydew that may cause the growth of a sooty, black fungus on leaves. Their feeding can cause leaves to turn yellow and drop. They are easily disturbed and fly around when you brush against an infested plant. They are often found on hibiscus and ivy.

Controls: Whiteflies are attracted to the color yellow, so you can trap them by hanging yellow cards coated with a sticky substance, such as Vaseline, around your plants. Or suck them up with a vacuum cleaner as you shake your plants. In serious cases, spray plants with insecticidal soap, neem oil, or horticultural oil. In houses with many plants, consider releasing the whitefly parasite (Encarcia formosa). This miniature wasp kills whitefly larvae but doesn't harm plants, pets, or people.

Article published on December 21, 2004.

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