Interior Lighting of Plants

The environment in our houses dictates which plants will grow well and which ones will suffer. No other factor is more important in growing good plants than adequate light.

Plants differ greatly in their light requirements. Those with brightly colored leaves such as coleus and croton, flowering plants and succulents grow best when placed in areas where they receive direct sunlight. Others, like ferns, philodendrons and many other foliage plants, grow well under indirect light.

In a room poorly lighted by the sun, artificial lighting can be used to supplement or replace natural light. The incandescent bulb most commonly used in homes can be used to supplement natural light. Incandescent bulbs produce light primarily in the red and far-red regions of the visible light spectrum, but they are a poor source of blue light and therefore as a sole light source are not suitable for plant growth. Incandescent bulbs also produce a considerable amount of heat which could damage plants if the light source is placed too close. Fluorescent bulbs are several times more efficient than incandescent bulbs in converting electrical energy to visible light and will last longer than incandescent bulbs. Most plants grow well and are attractive in appearance when placed under a combination of cool white and warm white fluorescent bulbs. The cool white bulb produces high amounts of blue-light energy and the warm white bulb supplies the red light needed for good plant growth.

There are a variety of fluorescent bulbs available that were developed specifically for lighting plants. These "plant growth bulbs" are fluorescent tubes with a special phosphor-coating inside that transmit light energy primarily in the blue and red-light energy regions of the visible spectrum. Plant growth bulbs are ideal for plants. However, because they do not transmit green-yellow light, they jeopardize the appearance of home furnishings and human complexions. Cool white fluorescent bulbs are superior to plant growth bulbs in the amount of growth produced. This is attributed to the higher light intensity of cool white bulbs over plant growth bulbs.

Fluorescent bulbs are available in various wattages, sizes and shapes. In addition to the standard bulb wattages (15, 20, 30 and 40 watts), higher wattage bulbs may also be obtained in tube lengths from 48 to 96 inches. These higher wattage bulbs are called high output (HO), very high output (VHO), power groove, power twist, and super high output bulbs. Although the long tubular bulb is most popular with interior gardeners, circular, U-shaped and panel bulbs are available.

It is important to order fixtures for specific bulbs to be used in them. Standard industrial fixtures with white enamel or white porcelain reflectors are widely used by hobbyists. Fixtures that accommodate two 48 inch long fluorescent bulbs are the most commonly used and inexpensive to buy. Strip or channel fixtures may be used with reflectorized fluorescent bulbs.

Light reaching a plant will decrease with the square of the distance between the light source and the plant. That is, a plant two feet away from a light source will receive only one-fourth as much light as a plant one foot away. A fixture with two 40 watt bulbs should be placed 6 to 18 inches from the top of a plant. When high output bulbs are used, this distance can be increased.

For best results plants should be illuminated for 10 to 18 hours a day. Low cost timers available at most hardware stores are a great help in keeping a consistent light schedule.


Copyright by Dr. Bob's Gardening Tips