New Mexico Master Gardeners
 

Caring for Plants in the Home

By Lynn Ellen Doxon

Extension Horticulture Specialist

Light needs Vary

Even among plants that naturally grow in the shade, light needs vary. There are four basic light categories for indoor plant grown in natural light.

1. Sunny areas get at least 5 hours of sunlight daily in the winter. A window facing south, southeast or southwest usually allows this much light to enter.

2. Semi sunny areas get 2 to 5 hours of direct sunlight a day. East and west windows will allow this much light to enter.

3. Semi shady places have bright, open light, but little or no direct sunlight.

4. Shady areas get no direct sunlight, but get enough light to cast a shadow. 

Trees or shrubs outside a window can significantly reduce the amount of light coming through a window. Smoky or hazy air, and dirty windows reduce the amount of light the plants receive. A window screen reduces light by as much as 30%. Solar screen film, tinted glass and reflective glass also reduce the light transmission. 

The Mini-Encyclopedia of  Houseplants at the end of this publication lists the light requirements for many common houseplants. 

If the space available for growing plants does not have enough light to cast a shadow, or a particular plant needs more light than is available, artificial light can supplement existing light. Bulbs of different light intensities can provide a plant’s need for light. If the plant needs 300 footcandle-hours, that can be provided by a lamp that furnishes 30 footcandles for 10 hours or a lamp that provides 60 footcandles for 5 hours. Most houseplants do well with 50 to 100 footcandles supplied for 16 hours a day. Shade-loving foliage plants can survive on 15 to 20 footcandles for 16 hours per day, although it is best to allow them a few weeks of somewhat brighter light every few months. 

To calculate the footcandles a plant receives, square the distance from the light source and divide the result into the candlepower rating of the bulb. A 500-watt bulb has a candlepower rating of 7800. A plant 10 feet from this source receives a candlepower rating of 78(7800/100=78). 

A plant that is too close to an incandescent lamp can be damaged by heat. Plants under artificial light perform best with a combination of fluorescent and incandescent light. The best ratio is 3 watts of fluorescent to 1 watt of incandescent. 

Temperature and Humidity

Most homes are kept at temperatures of  70 degrees or higher. This is too warm for many houseplants, especially flowering houseplants like hydrangea and cyclamen. Most tropical plants however, thrive at temperatures in the range of  65 to 70 degrees, and suffer if the temperature falls below 60 degrees. All plants do better with a 10 to 15 degree temperature drop at night. Plants produce surplus food and store it during the day, then consume the reserves during the night. If the night temperature is cooler, plants consume less of the stored food and have more reserves available for new growth. 

Outdoor relative humidity in New Mexico commonly ranges from 10 to 100% with the lowest relative humidity during the colder parts of the year. Heating this air further reduces relative humidity. Plants from tropical rainforests are most adapted to a relative humidity of 50% or more. 

A cool-vapor humidifier is a good way to increase humidity. If buying and operating a humidifier is too expensive, humidity around plants can be increased by grouping plants together, double potting or placing them on trays of gravel or pebbles and filling the trays to just below the bottom of the pots with water. Double potting involves placing the clay pot in which the plant is established inside a larger pot, ceramic pot or planter and filling the space between with moist peat moss. After double potting, water the soil in the pot one time just enough to over flow the inner pot slightly. Thereafter, water the peat moss around the inner pot. Be sure to use a porous pot for the inner pot. 

Watering

Never allow plant to wilt from lack of water. There is no definite interval for watering plants. These intervals depend on humidity and temperature of the room whether the pots are porous or non-porous, the moisture requirement of the plant and even the size of the container. Rapidly growing, large-leafed plants need large amounts of water. Slow-growing flowering plants and most foliage plants need less water, and are easily damaged if too much water is applied. When excess water cannot drain away the amount of air in the soil is reduced and the roots are injured. The plant’s growth is checked even if other conditions may be correct. The plant may drop some or all of its foliage. If all the roots are damaged, the plant will die. 

Few houseplants like to be constantly moist. Bamboo, calla lily and Chinese evergreen are among those that can withstand soggy soil. Others are listed in the Mini-Encyclopedia at the end of this publication. Most plants do best if the root zone is evenly moist. When the top inch or so of the soil is dry, it is safe to water these plants. Apply enough water so that some runs out of the drainage hole in the bottom of the container. This helps remove some of the soluble salts that may have built up in the soil, thoroughly moistens the soil ball and draws some fresh air into the soil. 

Do not use softened water to water plants. Softeners replace the calcium in alkaline water with sodium, which breaks down the soil particles, reducing the air and water space. When the soil dries, a hard, concrete-like crust usually forms. 

Many plants are sensitive to alkaline water. It is difficult to grow acid-loving plants in New Mexico because of the water’s alkalinity. If a white crust builds up on the soil surface in the pot, leach the soluble salts from the soil. Leaching is effective only in pots with holes in the bottom.

 The first step in leaching soluble salts is to remove the top inch or so of the soil. This top inch contains most of the salts because they are deposited there when the water evaporated. Next, apply distilled water until the pot is full. Allow it to drain completely. Immediately water with distilled water two or three more times, then replace the soil that was removed with new potting soil. 

Sensitive plants such as azaleas, camellias and gardenias should be watered only with distilled water, or with a weak solution of distilled water and an acid fertilizer in areas where water is alkaline. Even then, these plants do not do well in temperatures and humidity experienced in much of New Mexico and cannot be expected to be particularly long-lived. For those especially attracted to the acid-loving flowering plants, it may be best to buy them, enjoy them while their flowers are at their peak, then discard them. 

Soils and Fertilizers

Plants adapt readily to various soils if they have the basic requirement of a potting mix. These basic requirements are a structure that allows for adequate drainage and airspace, water holding ability and nutrients. A good soil mixture is composed of equal parts of good garden soil, coarse peat moss and perlite. If your garden soil is sharp sand, the resulting mixture may take on the consistency of concrete when watered. In this case, it is best to use a soilless mix. To make a soilless mixture equal parts of vermiculite and peat moss can be mixed with a small amount of complete fertilizer. In commercial potting soil a general-purpose mix can be used for all plants. It is not necessary to buy a special soil for each type of plant. Do not over fertilize houseplants. Frequent but dilute fertilization is best for houseplants that are actively growing. Any complete fertilizer formulated for houseplants will work. Follow the manufacturer’s directions. 

Houseplants do not grow constantly. They enter dormant periods, or periods of less active growth when grown in dim light, when days are short or when temperatures are not right. Most foliage houseplants stop growing actively in December and January. Other plants become dormant during summer. Do not fertilize plants during these slow growing periods. When new growth begins, resume the normal fertilization schedule. 

Potting

In repotting a plant, it is usually best to move the plant to the next larger size pot. A 3-to 4-inch pot is usually adequate for seedlings, cuttings or offshoots. 

Clean pots are important. Soak used pots in water and scrub them with a wire brush to get salt deposits off the surface. Soaking in a 5% chlorine bleach solution after scrubbing sterilizes pots for reuse. 

Clay pots are less likely to waterlog the plants than plastic pots. Standard flowerpots have holes in the bottom to allow excess water to escape. It is best to use this type, especially where the water has a high pH, so the excess water can flush away salt buildup as it drains from the hole. 

Watch houseplants carefully for signs that they need repotting, If growth is slow and spindly, the plant may need a larger pot. Repot any plant that needs to be watered daily. Either the soil mix is so porous that will not hold water, or the plant has filled the container with roots that quickly remove all the water the soil can hold. If roots are growing out of the bottom of the container, the plant should be repotted. Check plants that have been growing in the same pot for 2 years or more. Gently tap the plant out of the pot. If the soil ball is filled with roots, repot the plant.

To repot a plant, place a piece of broken clay pot, a bit of screening or a bottle cap over the hole in the bottom of the pot. Make sure the potting soil is slightly damp. Completely dry potting soil will not absorb water well, and soggy potting soil forms clumps and may be hard to work with. Gently knock the plant out of the container in which it is growing. Retain as much soil around the roots as possible. Place potting soil in the new pot to support the plant so the stem starts 1” to 1-1/2” below the top of the pot. Place the plant on top of this soil, holding the plant in the center of the pot. Fill in around the plant with potting soil, firming the soil gently. Water the newly potted plant thoroughly. It is usually best to water it once, let the excess water drain away, then water it again. The soil will settle and no large air spaces will be left around the plant’s roots. If necessary after watering, add more potting soil to bring the soil to the appropriate level.

Pest Control

Potting soil can be sterilized in the oven of a kitchen range. Moisten the potting soil, put it in a heatproof container and insert a meat thermometer. Heat the oven to 200 degrees. When the thermometer reads 180 degrees, most harmful organisms, weed seeds, insect larvae and nematodes will be killed. Allow the soil to cool naturally before handling it.

Insects often cause problems with houseplants. These insects can be classified by their method of feeding.

Chewing insects bite out part of the leaf, stem or flower. General-purpose houseplant dusts and sprays are available to control these insects. Use sprays according to manufacturer’s directions.

Aphids, red spidermites, white fly, mealy bug and scale represent sucking insects. Examine houseplants often for slower moving sucking insects such as aphids and scale. These can often be wiped off the plant or the plant can be washed under a cold stream of water in the shower if the insects are found soon enough. White flies are a particularly difficult pest to control and a shower may not be effective. A dust or spray should be used to control whiteflies, red spider mites and other sucking insects that are too numerous or persistent for the shower. Malathion controls many of these pests. Be sure to move the plants outside before dusting or spraying. Follow the manufacturer’s directions carefully.

Diseases other than root rot, which is caused by too frequent watering, are less common in houseplants. Problems that seem to be diseases are often caused by improper growing conditions, such as high temperatures, low humidity, drafts, too much or too little sun, or the leaves touching a cold window in the winter.

Many commonly available houseplants are listed in the following tables with their light, water, temperature and humidity requirements. The first part of the table lists those that do well under New Mexico conditions. The plants in the second part of the table may not do as well because they require acid soil or high humidity. 

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