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How to keep houseplants pest-free

Most houseplants like these from Shotwell Floral and Greenhouse are native to tropical regions. David Samson / Forum News Service1 / 4
Systemic garnules are applied to soil to protect plants from the inside out while Mosquito Bits are used for controlling small black fungus gnat flies. David Samson / Forum News Service2 / 4
Don Kinzler, Growing Together gardening columnist for The ForumMichael Vosburg / Forum Photo Editor3 / 4
Combination systemic insecticide and fertilizer tablets are available suche as these from Shotwell Floral and Greenhouse in Fargo. David Samson / Forum News Service4 / 4

FARGO — Mother Nature didn't make houseplants. Even the Bible doesn't mention creation of indoor plants. God never said on the sixth day "Let there be houseplants." No, all houseplants originated outdoors, native somewhere in the world before humans decided they'd look nice indoors.

Many plants of tropical or desert origin have adapted well to sharing living space with humans. But just because they're nestled comfortably indoors doesn't mean houseplants are immune from attacks by insects and diseases.

Here are some guidelines for keeping houseplants pest-free:

• Houseplant diseases are rare, except for root-rot, which is almost always caused by soil staying consistently too soggy. Inquiries whether diseases are causing houseplant decline usually reveal improper care or growing conditions as the cause, instead of disease organisms.

• Most rotting-type diseases can be prevented by allowing the soil to dry between thorough waterings.

• Unlike diseases, insects are common invaders of indoor plants. They can be present on plants when they're purchased or hitchhike indoors if plants are summered outdoors. Existing insect-free plants can become infected when new plants are brought into the home.

• Insects aren't always visible on new plants, but insect eggs might be present in the soil, ready to hatch.

• Insect problems sometimes go unnoticed until the population balloons into visible colonies causing severe plant symptoms.

• Aphids are small, pinhead-sized oval-bodied insects that can be white, green or tan in color. When viewed with a magnifying glass, aphids have two tailpipe-looking projections at the rear. Their favorite spots to congregate while sucking sap are leaf undersides and in the outer tips of new plant growth. Aphids exude a clear sticky honeydew, which can accumulate on leaves and surrounding surfaces.

• Mealybugs are easily seen and recognized by the white cottony growth that covers their tan bodies. These nearly stationary insects congregate along stems and under leaves.

• Spider mites' tiny size makes them barely visible. Shake leaves on a white paper, and use a magnifying glass to locate slowly moving specks. Spider mites often go undetected until symptoms show, usually multiple tiny pale dots speckling the leaves plus webbing when populations balloon. Spider mites can be difficult to control because they're easily unnoticed until plants are severely infested.

• Aphids, mealybugs and spider mites can be controlled by applying systemic houseplant insecticide granules to the soil. The material is taken up by plant roots, where it's spread internally in the plant, protecting from the inside out as insects suck the plant's sap or chew leaves. Systemic granules work well and can be considered houseplants' first defense against insects.

• Other products to apply at the first sign of insects include insecticidal soap, neem oil or aerosol houseplant insect sprays. Gently washing plants can also reduce insect numbers.

• Fungus gnats are small, black flies that annoyingly hover around houseplants, often flitting around when leaves are disturbed. The adult flies cause little plant damage, but their larva stage feeds on plant roots. A product called Mosquito Bits is labeled for fungus gnat control. It's a biological product and a form of the Bacillus thuringiensis that controls green cabbage worms. Applied to houseplant soil, the product kills larvae that would hatch into adults, disrupting their circle of life.