Helping Houseplants Feel at Home

Now that the growing season has drawn to a close, many avid gardeners will turn their attention to nurturing those plants they have growing in their homes. Fall is an ideal time of the year to start or add to one’s collection of houseplants because many retail outlets offer attractive prices on plants at this time of the year. A fact of life is, however, there are no houseplants in nature. All plants in nature grow out-of-doors and have basic needs for light, temperature, water, carbon dioxide and plant nutrients. The amounts of each needed for maximum growth vary according to species. Horticulturalists have selected certain species of plants that are able to withstand the austere, low-light conditions characteristic of indoor settings and named them "houseplants." Alluding to their ability to withstand adversity, someone once mused that houseplants are simply "plants that die more slowly than others." In spite of that pessimistic forecast, there are a number of environmental factors that can be manipulated to help prolong the life of a houseplant to the greatest extent possible.

Light
Light is life to green plants and is the single most important factor to consider when providing an optimum indoor plant environment. Plants use the energy from light and, along with carbon dioxide and water, manufacture food and release oxygen. The growth or longevity of a houseplant is usually directly proportional to the amount of light it receives. When considering light (natural or arti. cial) for the purpose of plant growth, three parameters must be taken into account: (1) quantity, (2) quality and (3) duration. Quantity refers to the amount of light available for plant growth. Most of us still use the old English system and refer to light intensity in term of foot-candles (f.c.). Natural light intensity in the home varies according to the number of windows a room contains, the exposure of those windows (north, south, east or west) and the presence of curtains or other materials that tend to block light. Light intensity in a typical living room can range from as little as 10 f.c. to as much as 1,000 f.c. Suf. ce it to say that in most cases light is the limiting factor for the growth of houseplants, and care must be taken to select plant species that are able to tolerate the amount of light present or to supplement natural light with arti. cial light. It is much easier and less expensive to match a plant that requires the same light conditions of your home than to alter the light to suit a certain species of plant.

Light quality refers to the wavelength or color of light. Photosynthesis is energized by light in the blue and red color wavelengths. Therefore, arti. cial light sources emitting those two colors of light would be most effective in promoting plant growth. Incandescent lamps emit an abundant amount of red light but very little blue. Some . uorescent lamps (e.g., plant lights) emit abundant amounts of blue and red, but they are not available in lamps with large wattages and high light output. Additionally, light from fluorescent lamps cannot be reflected effectively, and the fixtures must be placed relatively close to plants if they are to be effective. The latter takes away from enjoying the beauty provided by having plants indoors.

Light duration refers to the amount of time the plant is exposed to light. Basically, the amount of plant growth is directly proportional to the amount of light energy received by the plant. To a certain extent, increasing the duration of the light can compensate for inadequate light quantity. Since we cannot control sunrise and sunset, the duration of light exposure can only be manipulated though the use of artificial lighting.

Temperature
Since temperatures fluctuate in nature, most houseplants are able to tolerate modest fluctuations in temperature. Many of the species favored for indoor use grow best at nighttime temperatures in the range of 65-68 degrees F and daytime temperatures approximately 10-15 degrees warmer. Any effort on our part to reduce fuel costs by turning down our thermostat a few degrees (from 72 to 68) probably will result in healthier houseplants as well. Plants growing in above-optimal temperatures (especially at night) tend to become spindly in appearance due to increase rates of respiration. Care must be taken to make certain plants are not located in front of heating (cooling) ducts because the air discharged from these ducts is well above (below) the optimal temperature and injury or death of a plant is possible. Additionally, although locating a houseplant in a sunny, south-facing window might increase the amount of light it receives, such a location can become to hot for the plant, which has no means of cooling itself as do animals.

Relative humidity
Perhaps the biggest change houseplants must adapt to from the outside world is the extremely low relative humidity characteristic of most indoor settings. Many houseplants are native to tropical rainforests, where the relative humidity is high. In contrast, the relative humidity in the average home during the winter months of the year is actually lower than that of the Sahara. Relative humidity affects the rate at which plants lose water (transpire). The lower the relative humidity, the greater the rate of water loss by a plant. One way to increase the humidity around our houseplants is to install a humidifier in the home. This can be done by attaching one to the heating system of a house or by purchasing a free-standing humidifying unit. A second way to increase the relative humidity around houseplants is to place them on trays filled with pebbles and water.

Water
More houseplants are killed by overwatering than for any other reason. That said, a common question asked by houseplant owners is, "How often should I water my plants?" The answer to this question is difficult and varies from situation to situation. Light, temperature and relative humidity all affect the rate of water use by houseplants. Higher amounts of light, warm temperatures and low relative humidity will dictate a need for more frequent watering than the opposite. The frequency also will vary according to the size and age of the plant and the size and type of contain in which it is growing. Containers that "breathe" (e.g., clay) will require more frequent watering than those that do not. The roots of most potted plants are in the bottom twothirds of the pot, and it is this area that should feel dry to the touch before watering, not the surface of the growing medium. If water is needed, apply it until excess water drains from the bottom of the container. This will help to leach excess fertilizer residue out of the growing medium and give assurance the bottom two-thirds of the container has received water. Most people use some sort of saucer or "carpet saver" to collect the excess water that drains from the pot. This excess water should be discarded soon after it is collected to prevent the growing medium from which it drained from remaining overly moist by taking up the water by "wick" action. Any potable water is considered safe and satisfactory for houseplant use unless it has been soften by a contact process water softener. The latter imparts a high degree of salinity to irrigation water and can damage roots and harm growing medium structure.

Fertilization
Houseplants need mineral fertilizers to grow just as outdoor plants do. A water-soluble fertilizer containing nitrogen, phosphorus and potassium is preferred over slow-release types because the latter tends can be erratic in the release of its nutrients depending on temperature and state of hydration. Various companies market houseplant fertilizer; most have an analysis in the neighborhood of 20-20-20. Since growing conditions in a home are much less ideal than out-of-doors, houseplants need less fertilizer than do outdoor plants. As a general rule, fertilize houseplants only during the months of spring, summer and fall since the reduce light and temperatures of winter result in reduced growth. Always read and follow label directions when applying fertilizer. Some fertilizers are formulated to be applied at a very dilute concentration each time plants are watered; others are applied in a more concentrated solution at intervals of two weeks or longer. When applying any fertilizer, always apply sufficient solution so that water drains from the bottom of the pot. This will help to prevent the accumulation of salts in the pot and possible root damage.

Having plants in the home is a good way for gardeners to counter the doldrums of winter as they await another growing season. With a bit of effort and reasonable care, houseplants will remain welcome attractions to the home for many years.

David Trinklein, , Associate Professor, Horticulturist, UMC, (573) 882-9631