Documents: Special Interest: Seasonal:

Houseplant Care During Winter...

or why I have fungus gnats!
by Marg Fleming
by Marg Fleming

1979 - BSc. Botany University of Toronto, 1981 - MSc. Forestry University of Toronto, 1982-1986 - Horticulture Teaching Master - Niagara College , St. Catherines Ontario., 1986 - 2000 - Owner/Operator of Cedar Valley Botanical Gardens - Brighton Ontario, 2000- Present - Manager of Horticulture Toronto Zoo

Public Speaking Topics - Perennials, house plants, garden design

December 26, 2010

With the holidays past and New Year’s resolutions in place the cold days of winter generate a renewed interest in our houseplants. Indoor plants are partial to warm temperatures and long days. We attempt to fool them with our furnace-driven warm weather and the luminescent lengthening of our typical winter days. But despite this, a plant’s internal sensors still detect the real conditions outdoors and they shut down for a “long winter’s nap”.
Make no attempt to waken plants from dormancy at this time. Their metabolic processes have slowed so they cannot accommodate regular waterings and fertilizing. As daylight increases in late January or early February, plants will respond by producing new shoots. Then we can slowly begin to increase watering and revive plans to re-engage a modest feeding schedule.
It is a houseplant enthusiast’s downfall to satisfy his or her latent gardening instincts by prodding dormant plants with periodic splashes from a watering can. A tropical plant’s natural dormancy involves a measure of drought, so resist the urge to keep the soil constantly moist. Allow the plant to consume existing soil water then rest in the dry soil for a few days before watering. Even allowing the plant to wilt would be a definite indication that the plant indeed required water. However this technique should be used only if the plant collection can be constantly monitored.
Houseplants slow their metabolism for the winter months to such an extent that most common houseplants can survive several weeks unattended. To prepare houseplants for an extended absence place several thick layers of newspaper in the bathtub and moisten with water to the point of saturation. Leave no standing water. Place plants in their pots on the newspaper to benefit from the humid air released by the damp paper. The engulfing humidity will slow the drying of the soil so, depending on the species, plants should remain damp enough for several weeks. Be aware that bromeliads and other tough leafed tropicals are more likely to survive this type of treatment. Delicate houseplants and those with non-retentive leaves can be left for only a modest period of time.
How can you tell if plants have been over-watered? One sure sign is the appearance of fungus gnats. Soil that has been watered too regularly provides ideal conditions for the decomposition of organic material in the soil. Rotting organic matter attracts fungus gnats – those fruit fly-like flying insects found on the soil surface. The flying gnat is the adult stage of this insect and virtually harmless to the plant. In fact a modest population can live uneventfully along with your indoor collection while the immature larvae consume the organic fraction of the potting soil. But if conditions prevail that favour the fungus gnat (such as constant watering), populations explode and food soon runs out. With organic matter at a premium in the soil, the fungus gnat larvae residing in the soil turn to the consumption of plant roots.
An infestation of fungus gnats can be treated by drenching the soil with a recommended insecticide. Spraying is only marginally effective since the plant is being compromised primarily beneath the soil surface by hungry larvae. Alternatively, modify/reduce your watering schedule allowing the plant to dry out for extended periods before re-soaking. Fungus gnats hate dry soil and with conditions turning unfavourable populations in time should drop to acceptable levels.
Do not consider repotting fungus gnat-affected plants to achieve a drier mix. Repotting will add to the stress of the plant’s current condition and possibly push its recovery past the point of no return. 
When the enjoyment of your holiday poinsettia has evaporated, cut plants back to about four inches and store in a cool basement. Monitor soil moisture regularly ensuring that the soil never becomes dry. In early April after its cool-induced dormancy, retrieve the plants and place in a bright window encouraging the plants to re-leaf. Gradually start a regular fertilizer program after the first new leaves have fully expanded. 
Plant outdoors in early June in part shade and monitor throughout the summer for moisture and fertilizer needs. Let the great outdoors renew your poinsettias (and your interest in them) until danger of fall frost prompts us to retrieve them indoors once again.


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