In winter, when we spend most of our time indoors, houseplants can add beauty, color, warmth, and contrast to living spaces. Several scientific studies indicate that they improve indoor air quality, too.
Successful houseplant horticulture doesn’t have to be difficult. You need to start with plants that are healthy and free of pests. And you need to understand how indoor environments affect plant growth. Even healthy plants may not survive (and certainly won’t thrive), unless they’re given the amounts of humidity, light, water, and fertilizer that they require.
Most houseplants will grow adequately at temperatures of 65°-75°F (18°-24°C). Cooler nighttime temperatures may further stimulate growth, thereby improving the health of many foliage plants and extending the beauty of certain flowering plants. This does not hold true for all plants, however. African violets, for example, may suffer if temperatures fall below 60°F for a prolonged period of time.
Be aware that plants grown on cool windowsills may be exposed to cold drafts or sudden freezing conditions. Some may suffer injury after just a few seconds of exposure. And since most plants will not tolerate hot air blowing on them, indoor gardeners should avoid placing plants next to or over heating ducts.
House plants generally do best with a southerly exposure, although plants that normally do not need direct sunlight usually do well in east-facing windows. Keep in mind that plants will not grow or bloom as they should unless the amount of light they receive is similar to that found in their native environment. In fact, availability of natural light may well be the indoor gardener’s most important consideration. Symptoms of insufficient light include small leaves, long, thin stems, and a lighter than normal color.
Unless you can provide supplementary light, it’s best to select plants that require medium to low light. Keep in mind too, that plants receiving light from a window may need to be turned from time to time and that keeping foliage clean will allow plants to take full advantage of the amount of light that’s available to them. By cleaning houseplants, you also improve their appearance, stimulate growth, and help control insects (e.g. mites, aphids). Always wash foliage with a soft cloth or sponge, using a very mild solution of (insecticidal) soap and room temperature to lukewarm water. Avoid washing hairy surfaced plants, like African violets. Instead, gently dust hairy-leaved plants with a soft, clean artist’s paintbrush.
Almost all plants native to temperate or subtropical environments require relative humidity levels of about 40% to 50%, which is also a preferred relative humidity level for human health. Many tropical plants will adapt to this humidity level, but there are those that demand higher. During winter months, relative humidity levels in homes can be as low as 10%. Almost all plants, other than desert plants, will wilt when exposed to such low humidity for prolonged periods, even when the soil they are growing in contains adequate moisture. Humidity levels in a home or room can be increased by using a humidifier or by setting a pan of water on a radiator or wood stove. High humidity areas such as bathrooms and kitchens are often ideal for indoor plants.
Although misting will temporarily raise humidity levels, the added moisture will evaporate quickly and minerals in tap water may cause unsightly water spots to appear on foliage. Many indoor gardeners contend that misting sets the stage for fungal and bacterial leaf spot diseases, as well. Keep in mind too, that softened water should never be used, that chlorinated water should be allowed to sit overnight before it’s used in misting or watering, and that some plants are quite sensitive to fluoride salts.
Houseplants need to be watered thoroughly. They need a drink. Not a bath. Water should be room-temperature and soils should be allowed to dry between waterings. Excess water should be able to drain and should be immediately removed. Plants must never be allowed to sit in standing water. And never assume that a wilting plant needs water. When soils become saturated roots can rot, which can cause wilting and eventual death.
Most gardeners agree that houseplants need very little or no fertilizer at all during winter months. Those that do feed their plants use dilute solutions of complete and balanced water-soluble fertilizers and they keep feeding to a minimum. The idea is just to maintain the plants until longer days and warmer conditions return.
Photo of Snake Plant courtesy NYS Horticulture Study Guide for Youth; Cornell University Dept. of Horticulture.