Reviewed August 2000

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Contents

• Watering
• Lighting
• Temperature
• Humidity
• Fertilizing
• Repotting
• Insects and diseases
• Summer care
• Durable houseplants
• Diagnosing cultural problems
• Cultural preferences of plants often grown in the home

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Figure 1
The Aglaonema, or Chinese evergreen, can survive in fairly low light.

Caring for Houseplants

David Trinklein and Ray R. Rothenberger
Department of Horticulture

To many people, a home is not complete without attractive potted plants. Proper care of houseplants helps increase satisfaction and enjoyment from them and extends the blooming period of many flowering plants.

Most potted plants purchased from the florist have been grown in greenhouses under ideal conditions. When they are placed in home environments designed for people, not plants, they need good care to adjust to the new environment.

Watering

Houseplants are probably killed or injured more often by improper watering than by any other single factor. No general schedule can be used for watering all houseplants. Size of plant, pot, light, temperature, humidity and other conditions influence the speed with which the soil mass dries out.

When to water
In general, flowering plants need more water than foliage plants of the same size. Never water any plant unless it needs it. Soil kept either too wet or too dry causes plant roots to die, which leads to poor growth or death of the plant. Never allow plants to wilt, and never allow them to stand in water for long periods of time.

Learn to gauge the moisture content of the soil by its color and feel. As the soil surface dries it becomes lighter. Under continued drying, the soil begins to crack and pull away from the sides of the pot. When severe drying occurs, some damage already will have been done to the roots. Soil kept too moist becomes sticky and slimy, thus inviting root rots and other disease problems.

Kinds of water
Ordinary tap or well water is usually satisfactory for plants. Chlorine and fluorine often added to city water do not harm plants. Rain water or melted snow are excellent. Water run through most water softeners, however, should not be used continuously for watering potted plants.

How to water
Plants may be watered from either the top or the bottom of the pot. If you prefer watering from the top, use a watering can with a small spout and keep as much water off the foliage as possible. Each time, wet the entire soil mass, not just the top inch. Add water until it comes through the drainage hole in the bottom of the pot. Discard water that remains beneath the pot one hour after watering.

Watering from the bottom ensures thorough wetting of the soil mass. Place the pot in a pan or saucer filled with water, or dunk the pot in a bucket of deep water (just below the rim of the pot). When the top of the soil becomes moist, the entire soil ball should be wet. Remove the pot, allow it to drain and return it to the saucer.

Salts may form a white accumulation on the soil surface if plants are watered regularly from the bottom. Occasional watering from the top helps wash out the salts. Don't allow the soil to reabsorb any water that has been run through the soil to leach out salts. Surface salt accumulation may become too heavy to remove in this way. When this happens, scrape off the surface soil and replace it with fresh soil. Try not to injure plant roots.

Drainage
Potted plants should always have good drainage. Occasionally the drainage hole may become clogged by roots. Check it by pushing a finger, stick or pencil into it. Even though drainage from the pot may be good, pot coverings may hold water. Pots wrapped in waterproof foil or placed in deep planters should be checked occasionally for standing water.

Plants with "wet feet" soon look sick -- leaves yellow or drop, flowers collapse and normally healthy white roots turn brown. Any or all of these can result from stagnation of the water, too little soil oxygen and development of diseases which rot the roots.

Lighting

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 of many homes. The amount of light necessary for good growth varies with different types of plants. See Figure 1.

Flowering plants
All flowering plants need moderately bright light. Plants kept continuously in poor light will have spindly shoots, few flowers, yellow foliage, poor flower color and often little or no growth.

South, east or west windows are excellent for most flowering potted plants, with the possible exception of African Violets and related plants, which prefer a north window. Plants in bloom should be kept out of direct sunlight since the flowers will heat excessively and collapse more quickly.

Light in the average room, away from windows, is not bright enough for most flowering plants, even when ceiling fixtures are kept on.

Fluorescent lights located fairly close to houseplants will improve growth when plants cannot be placed close to windows. When artificial lights are used, place them about one foot above the top of the plant, and keep them on for about 16 hours each day. Extra fertilizer, water or repotting are not cures for insufficient light.

Foliage plants
Foliage plants are generally divided into those suitable for low light areas, moderate light areas and high light areas. Only a few plants can tolerate dimly lit room interiors. Light at a north window, daylight with no direct sun or sunlight diffused through a lightweight curtain are suitable for most foliage plants. Plants that require full sunlight should be put in a south window.

Abrupt change from a location in low light to one in bright light may be damaging. Plants can become acclimated to one location. Leaves gradually face toward light for maximum light absorption, especially in low light areas. Moving the plant disrupts this orientation, and light is not used as efficiently for a period of time. This is especially true of large plants.

Moving abruptly to more intense light also results in bleaching or burning of foliage, especially in direct sun. Any changes should be made gradually. Many 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 house. A hot, dry atmosphere shortens the life of flowers. Flowering potted plants should receive temperatures from 65 to 75 degrees Fahrenheit in the day and 55 to 60 degrees Fahrenheit at night. To get the most out of flowering potted plants in the home, move them to a cool spot at night.

Foliage plants are more tolerant of high temperatures, but they thrive at temperatures between 65 and 70 degrees.

In winter, plants placed close to a window may have cooler temperatures than those elsewhere in the house. If the drapes are drawn behind these plants at night, the window temperature may be too cool. On cold nights, check temperatures close to windows. Some tropical foliage plants can be injured at temperatures below 40 degrees Fahrenheit.

Do not put plants at windows that have hot air registers or radiators directly below them. Hot air blowing on the plants often causes leaves to brown on the edges and occasionally to drop or die.

Humidity

Air in most modern homes is extremely dry during the winter. A furnace or room humidifier can help plant growth. If one cannot be used, watertight trays placed beneath the plants and filled with constantly moist sand or gravel help increase humidity around the plants. Pots must be placed on, not in, the wet sand or gravel.

Misting over the leaves daily can help a plant overcome the stress of low humidity. Plants needing constant high humidity such as orchids or gardenias are best kept in kitchens or bathrooms where humidity often runs higher. A relative humidity between 40 and 60 percent is best for most plants but is difficult to attain in the house.

Fertilizing

Newly purchased plants have been well fertilized in the greenhouse. They seldom need additional fertilizer for a few weeks. If plants are to be discarded after flowering, there will be no benefit from fertilizing. Plants to be kept on in the home should be put on a regular fertilization program.

When to fertilize
Fertilizing once a month is usually adequate for most houseplants that are producing new growth or flowers. During midwinter (December and January) when no new growth is apparent, fertilizer should be withheld.

Do not use fertilizer 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.

Drop of lower leaves, overall yellow-green color or weak growth may indicate a need for fertilization. Since these same symptoms may result from poor light or overwatering, evaluate all conditions before fertilizing more than normal.

Kind of fertilizers
Water soluble, complete fertilizers have been formulated for houseplants and are available from many garden shops, florists and nurseries. They are easy to use. Since formulations vary, be sure to follow directions carefully. Do not apply more than directed. The roots of potted plants are quite restricted and easily burned by the application of too much fertilizer at one time.

Never apply liquid fertilizers to wilted plants. Water the plants first and apply fertilizer after the plants have recovered and the soil has dried slightly.

If soluble fertilizers such as 20-20-20 are available, these may also be used for fertilizing houseplants. Make a solution by mixing 1-1/2 teaspoons of this material in one gallon of water.

Some people prefer to use organic fertilizers for houseplants, but either organic or inorganic fertilizers or a combination of both will be satisfactory sources of nutrients.

Fertilizers that release nutrients slowly or over a long time period require less frequent application than liquid forms. They are available in beads, pills, spikes and other forms. Never exceed amounts suggested by the manufacturer's directions.

Repotting

Plants just brought home from the greenhouse seldom need immediate repotting. Many will not require potting for some time. A newly acquired plant must make adjustments to its new environment, and repotting immediately puts added strain on the plant.

When a plant is potbound (roots are too extensive for the pot) it may require too frequent watering and makes poor growth. This is a time for repotting.

A good potting mixture for most houseplants consists of a blend of three parts sphagnum peat, one part vermiculite and one part perlite. Many commercially available "peat-lite" mixes are ideal for houseplants. Exceptions are the epiphytic orchids, which should be potted in a very porous medium such as orchid bark, and cacti, which should be potted in equal parts of peat-lite mix and sterile sand. In most cases it is wise to avoid the addition of soil to a potting medium, as this often leads to poor drainage, overwatering and root diseases.

Acid-loving plants such as azaleas and gardenias should have at least 50 percent peat moss or other organic material in the soil mixture. With good care, these plants can be grown successfully in peat moss with no soil added.

Sterilize soil mixes before using them to avoid contamination by insects, diseases and weed seeds. In one method, moist soil is heated in a 200-degree oven for about 30 minutes or until it is heated through. Cover the container or wrap soil in aluminum foil to keep it from drying out while being heated. Chemicals can also be purchased for soil sterilization.

When repotting, avoid excessive damage to the root system. Firm the soil gently around the root ball, but do not press so hard that the soil becomes compacted.

Allow enough space at the top of the pot so that water can be added easily. Water newly potted plants thoroughly, drain and do not water again until necessary.

Figure 2
Serious spider mite damage on Brassia, or Schefflera. Wash leaves with soapy water to begin control.

Figure 3
Scales are shown on the runner of this spider, or airplane, plant. When possible, cut off badly infected areas.

Figure 4
The mealy bug, shown here on Fatsia, covers itself with a white, cottony appearing wax that protects it from predators and pesticides. Rubbing alcohol will dissolve the wax and kill the pests.

Insects and diseases

Watch new plants carefully for development of insect or disease problems. If detected early, these problems often can be corrected easily before serious damage is done. If ignored or unseen, they may become difficult to control. The three most common and difficult houseplant pests are spider mites, scales and mealy bugs. See Figures 2, 3 and 4.

Summer care

During the summer, many houseplants can be revitalized if placed outdoors. Do not rush the plants outside too early in the spring. Late May is usually soon enough. Cool nights may injure some of them.

Move the plants to a sheltered spot on a porch, beneath a tree or behind shrubs close to the house on a mild day, preferably when the weather is cloudy. After about one week of this adjustment, they may be moved to a more exposed but sheltered spot for the rest of the summer.

Plants with large leaves should be placed where they get good wind protection, since their leaves are easily torn.

Potted plants dry rapidly outdoors. Frequency of watering can be reduced by submerging the pots in soil. This also keeps pots from falling over. Lift the pots occasionally to keep roots from growing out of the drainage hole in the pot and to prevent the plant from becoming established outdoors. Fertilize monthly, and check occasionally for insects or diseases that may attack them outdoors. Move them indoors by mid-September before cool weather returns.

Durable houseplants

Although all houseplants grow best with good care, there are a few that stand abuse more than others. Some of the most durable houseplants are snake plant (Sansevieria), heart-leaf philodendron (Philodendron cordatum), devil's ivy (Pothos), corn plant (Dracaena massangeana), Peperomia (Peperomia obtusifolia), cast iron plant (Aspidistra), dwarfpalm (Collinea), Chinese evergreen (Aglaonema) and spider plant (Chlorophytum).

Diagnosing cultural problems

Problems resulting from poor growing conditions in the home are difficult to diagnose. Often poor growth results from a combination of several unfavorable factors. See Table 1 for the most favorable cultural conditions for various common houseplants. The following list includes symptoms and causes of several cultural problems.

Lower leaves turn yellow and drop when touched

• Usually caused by overwatering
• May occur when a new plant is moved from greenhouse to a low-light, low-humidity environment

Yellowing and dropping of leaves at various levels on a plant

• Overwatering
• Poor drainage
• Tight soil
• Chilling
• Gas fumes

Tips or margins of leaves appear burned, brown or both

• Too much fertilizer
• Plant too dry for a short period of time
• Plant exposed to too low temperature for short period
• Use of softened water

New leaves of plant are small

• Soil too dry for long periods
• Poorly drained soil
• Tight soil mixture

New leaves with long internodes

• Not enough light
• Temperature too high

Leaves yellow or light green, weak growth

• Too much light
• Poor root system -- possibly from poor drainage, overwatering or tight soil

Explanations

Night temperature
May fluctuate several degrees above or below listing. Day temperature should be 10 to 15 degrees higher.

No direct sun
Low light intensity suitable. Direct sun may bleach or burn foliage.

Filtered light
Needs good light but protection from long periods of bright sunlight.

Bright light
Suitable for south window exposure close to or in direct sunlight.

Thoroughly wet
Daily watering generally required. May stand in water for brief periods.

Evenly moist
Frequent watering required, but must never stand in water. Soil surface should always feel moist.

Drench, then dry
Soak root ball thoroughly, then allow the soil to become fairly dry before watering again. Do not allow the plant to wilt, however.

Cultural preferences of plants often grown in the home

G6510, reviewed August 2000