Light, Temperature and Humidity

Light is an essential factor in maintaining plants. The rate of growth and length of time a plant remains active is dependent on the amount of light it receives. Light energy is used in photosynthesis, the plant’s most basic metabolic process. When determining the effect of light on plant growth there are three areas to consider: intensity, duration and quality.

Light Intensity

Light intensity influences the manufacture of plant food, stem length, leaf color and flowering. Generally speaking, plants grown in low light tend to be spindly with light green leaves. A similar plant grown in very bright light tends to be shorter, better branches, and have larger, dark green leaves.

Light exposure

Plants can be classified according to their light needs, such as high, medium and low light requirements. The light intensity received by an indoor plant depends upon the nearness of the light source to the plant. Light intensity rapidly decreases as the distance from the light source increases. Window direction in a home or office affects the intensity of natural sunlight that plants receive. Southern exposures have the most intense light. Eastern and western exposures receive about 60 percent of the intensity of southern exposures, while northern exposures receive 20 percent of the intensity of a southern exposure. A southern exposure is the warmest, eastern and western are less warm, and a northern exposure is the coolest. Other factors such as curtains, trees outside the window, weather, season of the year, shade from other buildings and window cleanliness also effect light intensity. Reflective, light-colored surfaces inside a home or office tend to increase light intensity , while dark surfaces decrease light intensity.

Directional Exposure:

Day and Night:

Day length or duration of light received by plants is also of some importance. Poinsettias, kalanchoes and Christmas cactus flower only when days are 11 hours or less (short-day plants). Some plants only flower when days are longer than 11 hours (long-day plants), while others are not sensitive to day length at all (day-neutral plants).

Day Length:

Increasing the time (duration) plants are exposed to light can be used to compensate for low light intensity, as long as the plant’s flowering cycle is not sensitive to day length. Increased light duration allows the plant to make sufficient food to survive and grow. However, plants require some period of darkness to properly develop and should be exposed to light for no more than 16 hours per day. Excessive light is as harmful as too little.. When a plant gets too much direct light, the leaves become pale, sometimes burn, turn brown and die. Therefore, protect plants from too much direct sunlight during summer months.

Supplemental Light:

Additional lighting can be supplied with either incandescent or fluorescent lights. Incandescent lights produce a great deal of heat and do not use electricity very efficiently. If artificial light is the only source of light for growing plants, the quality of light or wavelength, must be considered. Plants require mostly blue and red light for photosynthesis, but for flowering, infrared light is also needed. Incandescent lights produce mostly red and some infrared light, but very little blue light. Fluorescent lights vary according to the amount of phosphorus used by the manufacturer. Cool-white lights produce mostly blue light and are low in red light; they are cool enough to position quite close to plants. Foliage plants grow well under cool-white fluorescent lights, while blooming plants require extra infrared light. This can be supplied by incandescent lights or special horticultural fluorescent lights.


Most plants tolerate normal temperature fluctuations. In general, foliage plants grow best between 70 degrees and 80 degrees F. during the day and between 60 degrees to 68 degrees F. at night. Most flowering plants prefer the same daytime temperature range, but grow best when nighttime temperatures range from 55 degrees to 60 degrees F. Lower nighttime temperatures help the plant: recover from moisture loss, intensify flower color and prolong flower life. Excessively low or high temperatures may cause: plant stress, inhibit growth, or promote a spindly appearance and foliage damage or drop. Cool nighttime temperatures are actually more desirable for plant growth than high temperatures. A good rule of thumb is to keep nighttime temperatures 10 to 15 degrees lower than daytime temperatures.


Atmospheric humidity is expressed as the percentage of moisture to air.This is important to plants in modifying moisture loss and temperatures. There are several ways to increase relative humidity around plants. A humidifier can be attached to the heating or ventilating system in the home or office. Also, gravel trays with a constant moisture level can be placed under pots or containers. As the moisture around the pebbles evaporates, the relative humidity in the vicinity of the plants is increased.


Another means of raising humidity is to group plants close together. Misting the foliage of plants is not generally recommended because of the increased potential for spreading diseases. If a mist is used, it should be applied early in the day so that leaves will dry before the onset of cooler nighttime temperatures.

For details on specific light & temperature requirements see Selected Foliage and Flowering Plants

Comments are closed.