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Index : Container Plants & Patio Gardening
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- 15. Soil Pests Can Cause Problems in Potted Plants - Top
- Potted plants, unlike those in the garden, have a limited amount of soil to grow in. If the soil in the pot "goes bad", for one reason or another, the plants growing in it usually dies. To keep plants healthy, it's important to know what types of soil problems can arise, and how to deal with them when they do.
Probably the most common cause of bad soil is a build-up of salts. They accumulate over- time from small amounts deposited by tap water and fertilizers. The white crust which develops on the soil surface and on pot rims and sides indicates a level of salts injurious to plants.
To get rid of excess salts, just run lots of slightly-warm water through the pot. Use three or four times the amount of water that you would apply in a normal watering. The excess salts will be dissolved and wash out the drainage hole. Do this every three or four months to keep salts from building back up.
Another common problem is soggy soil. This is caused by an excessive amount of poor-quality peat in the potting soil. Rather than draining, the peat traps the water, much like a sponge. When roots set in soggy soil, they eventually rot.
Unfortunately, it's impossible to wring out soggy soil! The only solution is to repot the plant in a soil that is lighter and better draining. The soil mix selected should container a high-grade horticultural or sphagnum peat moss. It should also have an adequate amount of perlite. Unfortunately, most potting soils have only a small amount of perlite (the white crusty particles).
However, you can buy perlite separately and mix it in to the potting soil. Use about 1 part of perlite to 3 parts standard potting soil and mix thoroughly. The perlite will keep the soil well drained and prevent water-logging.
Excess peat and wet soils can also attract insect pests. The two most common of these are springtails and fungus gnats. Both can be found in the soil of pots. Although more common on plants grown indoors, they can occasionally be found in pots outdoors on porches and patios.
Springtails are tiny wingless insects about 1/16 to 1/8 inch long. They get their name from their ability to catapult themselves through the air three or four inches, accomplished by a spring-like mechanisim in their tail. When soil containing springtails is disturbed, they can be seen flitting about. Although they most commonly feed on decaying organic matter (moist peat), they can also feed on plant roots.
Safer's Insecticidal Soap is registered for control of springtails in and around the home. A soil drench of this product is the most effective way to eliminate these pests. Follow label directions as to the amount to use and the method of application. Plants may also be repotted in new, well-drained potting soil. The old soil should be washed from the roots of the plant and discarded, to dispose of springtails and prevent their transfer to the new soil. If a pot is overrun with large numbers of springtails, it's best just to get rid of the plant, soil and all!
Fungus gnats are minute, mosquito-like insects about 1/8 to 1/10 inch long, grayish to black in color. Larvae or maggots of the fungus gnats live in the soil and are legless, thread-like, white with a black head and no more than 1/4 inch long. The adults will be seen flying about infected plants. Like springtails, the larvae of fungus gnats feed on decaying organic matter in the soil, but will also feed on plant roots. Plants infected with fungus gnat larvae may appear wilted, with a loss of vigor, poor growth and yellowing foliage.
Because fungus gnat larvae require plenty of soil moisture to survive and develop, on the the most effective means of control is to let the soil dry as much as possible, without injury to the plants. This practice will kill many of the maggots. Killing the adult gnats will also prevent the laying of additional eggs in the soil. Adults can be easily killed with any pyrethrin spray or aerosol labeled for "gnats" or "flying insects.
Finally, if the pot soil is heavily contaminated with salts, root-rot disease organisms, or insect pests, it's best to pitch the plant, soil and all, and start fresh using clean, quality potting soil.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
- Updated: January 26, 2003
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