The end of the outdoor growing season does not necessarily mean an end to the enjoyment of working with plants. Many avid gardeners turn their attention to nurturing plants they have growing in their homes at this time of the year. The winter months are an ideal time to start or add to one's collection of house plants since many retail outlets offer attractive prices on them during the "off season" for outdoor gardening.
A fact of life, however, is there are no house plants in nature. All plants in nature grow out-of-doors and have basic needs for light, water, temperature, air and plant nutrients. Correct levels of each of these environmental factors needed for growth varies 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 "house plants". Alluding to their ability to withstand adversity, someone once mused that house plants are simply "plants that die more slowly than others". In spite of that pessimistic forecast, there are several environmental factors that can be manipulated to help prolong the life of a house plant 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. In most cases the growth or longevity of a house plant is directly proportional to the amount of light it receives. When considering light (natural or artificial) 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 the 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 1000 f.c. depending on the afore-mentioned factors and the location at which the reading is taken. Suffice to say, in most cases light is the limiting factor for the growth of house plants 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 artificial 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, artificial 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. Fluorescent lamps can be found that emit abundant amounts of blue and red (e.g. plant lights) 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. Inadequate light quantity can be compensated for to a certain extent by increasing the duration of the light. Since we cannot control sunrise and sunset the duration of light exposure can only be manipulated though the use of artificial lighting.
Water. More house plants are killed from over-watering than for any other reason. That said, a very common question asked by house plant owners is, "How often should I water my plants?" The answer to this question is very difficult and varies from situation to situation. Light, temperature and relative humidity all affect the rate of water use by house plants. 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/age of the plant and the size/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 two-thirds 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 house plant use unless it has been soften by a contact process water softener. The latter imparts a high degree of salinity to irrigation water which can damage roots and harm growing medium structure.
Air. Plants derive carbon dioxide used for photosynthesis from the air. However, it is the amount of water vapor in the air that has a greater affect on house plants than the amount of carbon dioxide. One of the biggest changes house plants must adapt to from the outside world is the extremely low relative humidity characteristic of most indoor settings. Many house plants are native to tropical rainforests where the relative humidity is quite 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 Desert. Relative humidity affects the rate at which plants loose water (transpire). The lower the relative humidity, the greater the rate of water loss by a plant will be. One way the humidity around house plants can be increased 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 humidity around house plants is to place them their pots on trays filled with pebbles and water.
Temperature. Since temperatures fluctuate in nature, most house plants 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 65-68o 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 (e.g. from 72 to 68) probably will result in healthier house plants as well. Plants growing in above-optimal temperatures (especially at night) tend to become spindly in appearance due to increased rates of respiration. Care must be taken to make certain plants are not located in front of heating (cooling) ducts for the air discharged from these ducts is well above (below) the optimal temperature and injury or death are quite possible. Additionally, although locating a house plant in a sunny, south-facing window might increase the amount of light it receives, such a location can get quite hot for the plant which has no means of cooling itself as due animals.
Fertilization. House plants need mineral fertilizers in order to grow just as outdoor plants do. A water-soluble fertilizer containing nitrogen, phosphorus and potassium is preferred over slow-release types since the latter tends can be erratic in the release of its nutrients depending on temperature and state of hydration. Most fertilizers formulated for house plants are water soluble and complete (meaning they contain nitrogen, phosphorus and potassium) . Since growing conditions in a home are much less ideal then out-of-doors, house plants need less fertilizer than do outdoor plants. As a general rule, fertilize house plants only during the months of spring, summer and fall, since the reduced light and temperatures of winter results 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 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, house plants will remain welcome additions to the home for many years.
REVISED: October 11, 2011