Healthy Houseplants

Published: October 2, 2015

As the outdoor gardening season draws to a close, avid gardeners often turn their attention to caring for those plants growing in their homes. Attractive and constantly changing, plants add a softness of line and provide a bit of nature indoors during the dormant months of winter.  Fall is an ideal time of the year to add to one’s collection of houseplants, since many retail outlets offer very attractive prices on them at this time of the year.

The fact is there are no houseplants in nature. All plants in nature grow out-of-doors and have basic needs for light, water, carbon dioxide, temperature and plant nutrients. Correct amounts of each are needed for maximum growth and varies according to species. Horticulturalists have selected certain species of plants that are able to withstand the austere, low-light and arid conditions characteristic of indoor settings and named them “houseplants.”

Let’s look more closely at those environmental parameters needed to keep the average houseplant healthy.

Water

More house plants are killed from over-watering than from 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 age/size of the plant and the size/type of container 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.

When 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 remaining overly moist by taking the water back up by capillary or “wick” action.

Water quality also must be considered when watering houseplants. Any potable water is considered safe and satisfactory to 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.

Light

Since “light is life” to green plants, improper light intensity ranks close to improper watering as a frequent cause for failure with houseplants.  A plant in proper light is better able to withstand the high temperature and low humidity commonly found in homes.  The amount of light necessary for good growth varies with different species of plants. 

Houseplants grown for their foliage plants are generally divided into those suitable for low-light areas, moderate-light areas and bright-light areas.  Only a few plants can tolerate dimly lit rooms.  Most foliage plants do well with light at a north window, daylight with no direct sun, or sunlight diffused through a lightweight curtain.  Plants that require full sunlight should be put in a south window.

Plants need to become acclimated to a new location.  An abrupt move from a low-light to a bright-light location may result in bleaching or “burning” of foliage. Therefore, any lighting changes should be made gradually.

The leaves of houseplants gradually orient themselves toward light for maximum light absorption, especially in low-light areas. Most plants can be kept from getting one-sided by turning them once a week.

Temperature

Proper temperatures for plants are often hard to find in the average home, since thermostats are set for “people comfort” instead of plant well-being.  Since temperatures fluctuate in nature, most houseplants are able to tolerate modest fluctuations in temperature. Providing indoor temperatures between 65 and 70 degrees F is considered ideal for most species. In winter, the temperature near windows may be cooler than elsewhere in the house. If drapes are drawn around a plant placed near a window, the temperature may be too cool. On cold nights, check temperatures close to windows. Some tropical houseplants can suffer “chill injury” if the temperature falls below 40 degrees F.

In contrast, avoid placing houseplant near windows that have hot air registers or radiators directly below them.  Hot air blowing on a plant often causes leaves to brown on the edges and, occasionally, to drop or die.

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 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.

A relative humidity between 40 and 60 percent is best for most plants but is difficult to attain indoors. Furnace or room humidifiers can help achieve that goal. If a humidifier cannot be used, watertight trays placed beneath the plants and filled with sand or gravel kept constantly moist can help increase humidity around the plants. Pots must be placed on, not in, the wet sand or gravel. Although many references recommend hand misting of houseplants, the practice is of dubious value. Misting plants with a spray bottle has a positive effect on increase the relative humidity only for about 30 minutes. After that, the plant is exposed to the ambient relative humidity in the room in which it resides. I question whether or not even the most avid plant lover would have the dedication needed to mist their plants by hand every half-hour.

Fertilization

Newly purchased plants have been well-fertilized in the production greenhouse or outdoor nursery.  They seldom need additional fertilizer for a several weeks. However, when required, fertilizing once a month is adequate for most houseplants that are located in ideal conditions and producing new growth.  Most often, this is during the spring, summer and fall months.  Houseplants do not need fertilizer in winter when no new growth is apparent. Do not use fertilizer in an attempt to stimulate new growth on a plant located in poor growing conditions. Lack of growth is more often due to improper light or watering than to nutritional deficiencies. In such cases, adding fertilizer may actually cause additional injury by increasing the salt content of the growing medium.

When fertilizer is needed, a water-soluble fertilizer containing nitrogen, phosphorus and potassium is preferred over slow-release types. The latter can be erratic in the release of their nutrients, depending on temperature and state of hydration.  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. Always read and follow label directions when applying fertilizer.

Repotting

Plants just brought home from the greenhouse seldom need immediate repotting. Many will not require potting for quite some time. A newly acquired plant must make adjustments to its new environment, and repotting immediately puts added strain on the plant. The time to repot is when the plant becomes pot-bound, this is, when the plant’s roots are too extensive for its container. Pot-bound plants may need to be watered more frequently and may grow poorly. When repotting is called for, a soilless growing medium should be used.  The latter often consist of blends of sphagnum peat, vermiculite and perlite. There are many commercially available peat-lite mixes that are ideal for houseplants.

In closing, houseplants are popular indoor decorations. 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.