Check houseplants for pests

Monitor houseplants for insect problems that often occur when plants become stressed. New houseplants or gift plants can also harbor pests. Carefully inspect them for pests or isolate these plants before adding them to your collection. Pests to look out for include the following:

Spider mites. Look for webbing in leaf axils, stippled foliage, and weak, off-color leaves. Mites are often difficult to see without a lens.

Scale. Characteristic sticky, clear honeydew is produced on leaves by these small, immobile, rounded insects usually found on stems and veins of leaves.

Mealybugs. Easy to spot, these insects resemble crowds of tiny cotton puffs.

Whitefly. Whiteflies can be a major problem in many greenhouses and homes because they can quickly move to neighboring plants. Look for tiny, white, moth-like insects often found on the undersides of leaves.

Fungus gnats. The adult black gnats fly around the plant but do no damage. The immature larvae in the moist soil can chew plant roots. They are often a problem in overwatered plants or overly moist flats of seedlings.

• During the winter, most houses are too dry for house plants. Humidity may be increased by placing plants on trays lined with pebbles and filled with water to within one half inch of the base of the pot. Group plants together as the water evaporating from the potting soil plus water lost through the plants foliage (transpiration) will increase the relative humidity in the vicinity of the plants. A humidifier can increase the humidity in a room or throughout the entire house. Misting would have to be done several times daily to appreciably raise the humidity level and is not practical.

• Clean crusty clay pots by adding one cup each of white vinegar and household bleach to a gallon of warm water and soaking the pots. If the pots are heavily crusted, then scrub with a steel wool pad after soaking them for 12 hours.

• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.