Douglas A. Spilker, Ph.D., is a consulting ornamental plant pathologist and entomologist, garden writer and lecturer. Dr. Doug can be reached at

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Move the Plants, Not the Pests
by Douglas Spilker, Ph.D.       #Containers   #Insects   #Pests

A potted hibiscus is an ideal flowering container plant, but be on the lookout for harbored aphids.

Container gardening is one of the fastest growing sectors of the gardening world – and why not? Containers can be grown where traditional gardens cannot, such as apartment balconies, courtyards, decks and patios. Since most containers are portable, there is a strong temptation to bring this instant landscape and color into the home once autumn transitions into the cold of winter. However, in addition to the preparation of the plants’ horticultural needs, extra precautions need to be taken to ensure that no unwanted visitors hitchhike into your home on these container plants and jeopardize the health of your current houseplants or cause a nuisance in the home.

If you see spider mite webbing, consider composting the plant instead of inviting trouble into your home.

Check Your Plants at the Door
Several days before bringing the plants indoors, remove any dead or yellowing leaves, and prune if needed. Remove all dead and rotting plant material from the surface of the soil since it may harbor moisture-loving pests, such as slugs and snails or insect eggs. As you do this, carefully inspect the leaves, stems and soil surface for plant pests such as mealybugs, scale, mites, aphids and caterpillars. Do not be surprised to find other hitchhikers such as spiders, ants or wasps.

For the easily removed pests such as aphids, caterpillars and spiders, pick them off by hand or knock them off with a stream of water from a hose. Showering the plant is a good idea anyway to remove dust and pollen from the leaves, but be sure to get underneath the leaves too. If you see pests, or evidence of a pest, like chewed leaves, stippling (yellow dots), insect droppings, sticky leaves or mite webbing, a pesticide treatment may be warranted. Spray the plants while they are still outside with low-impact pesticides such as horticultural oil, insecticidal soap or pyrethrum.

Before bringing plants indoors, remove yellowing leaves and any rotting material on the soil surface. Slugs and snails conceal themselves in these moist areas.

How Do You Decide Which Plants to Keep?

You may want to keep them all, but be realistic about your space with reasonably adequate light, and away from winter drafts and heating vents.

Do not bring in plants with signs of pests or diseases. If you must keep it, be sure to treat before bringing them indoors.

Keep only healthy plants. If it has been struggling outside, it is not going to do better indoors under low humidity and low light.

Give priority to uniqueness – a stunning color, a Mother’s Day gift or a sentimental favorite. You can always throw it out later.

Try something new. I never thought gerbera daisies would make it indoors, but they can provide bursts of color during those bleak wintery days.

What is Horticultural Oil?

Oils are an important tool in managing certain pests, including aphids, mites and scales, but some oils can also control plant diseases, such as powdery mildew. Horticultural oils are highly refined petroleum oils with an emulsifying agent that allows them to be mixed with water for spraying. Oils commonly affect the insect pest by blocking their air holes (spiracles), causing them to suffocate. Oils pose few risks to people or desirable species, including many beneficial natural enemies of insect pests. Horticultural oils usually dissipate quickly, thus leaving little residue. Avoid spraying stressed plants or when conditions do not favor rapid evaporation (such as high humidity), which may result in leaf burn. Always read and follow label instructions.

Don’t Let Them Go to Pot
Next, look for growth on the pots and for unwanted inhabitants in the potting mix such as earthworms, snails or ants. To get rid of mold, lichens and mosses, scrub the outside of dirty pots with a solution of 10 percent household bleach and then hose them off. A good way to inspect for soil inhabitants in small or modestly sized pots is to soak the pot in a tub of lukewarm water for about 15 minutes. Any soil intruders can be removed as they float to the surface. For larger pots, consider a soil drench of a systemic insecticide. Consult your local extension office or garden center for available products

Depending on what comes out of the pot, you might want to consider repotting the plant, especially if you find an ant colony (look for white eggs.) Ants are now the number-one indoor pest, and are difficult to eradicate once in the home. If you do repot the plant, remove the potting medium from the root mass with a spray from a hose, and then scrub the interior of the pot with the bleach solution. If the roots have filled the pot, repot in a slightly larger pot with fresh potting soil.

Plant Quarantine and Care
If you have the space, keep the plants you are bringing indoors away from your other houseplants. Two to three weeks should be enough time for signs of pests to show up that you might have missed in your previous inspections (or were in the egg or larval stages in the soil). If any pests emerge, the plants should be treated, but do so in the garage or another out-of-the-way place.

Watering practices and household humidity affect pests in a couple of different ways once the plants are in the home. Some hitchhiker pests, such as fungus gnats and spider mites, may show up much later as the environment changes. Excessively moist soil favors the development of fungus gnat larvae, which may have come in with the soil. Fungus gnat populations can be reduced to levels that are not a serious nuisance by allowing the soil surface to dry between waterings. Since spider mite problems are worse when plants are placed in a very hot, dry environment, increasing the humidity around the plants may deter spider mite explosions. However, it is best that a plant with signs of spider mites be discarded rather than saved, since spider mites easily spread from one plant to another. If you must keep an infested plant, be sure to treat it before bringing indoors.

If all goes well, in just a few short months, the temperatures will again rise and these plants can be moved back outside. However, in the meantime, you should be able to bring the beauty of nature indoors – hopefully pest free!


A version of this article appeared in a September/October 2013 print edition of State-by-State Gardening.
Photography courtesy of Douglas Spilker, Ph.D.


Posted: 08/28/17   RSS | Print


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