Understanding the Effects of Light on Plant Growth

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Summary: Plant light has 3 dimensions or qualities. Understanding these lighting dimensions, the type of plants grown and how all these factors work together can help you enjoy your plants much more.

As we begin looking at the effects of light on plant growth we need to “agree” on the four elements effecting plant growth:

  • Light
  • Heat
  • Water
  • Food

Although most home gardeners have some knowledge of the effects of heat, water and food on plants, they know even less about what light does to them.

To use an example, if a plant fails to flower because it receives too much or too little light, most people tend to blame the plant.

Light Dimensions or Qualities

Light has three “dimensions” or qualities. The first of these is what are called photoperiod – that is, the length of the day.

Few plants continue to grow when the day is shortened to less than eight hours, and anything under 11 hours is generally considered a short day. Thirteen hours or more is a long day.

Plant Light – 3 Lighting Classes

From the standpoint of light, we find three classes of plants.

First, there are some plants which make vegetative or green growth during short days and flower during long days. Typical examples are annual asters and scabiosa.

Next, there are others which are indifferent to day length, provided it is over eight hours. Included in this group are African violets and roses.

Finally, there are kinds which make vegetative growth during long days and flower during short light days. Poinsettias and chrysanthemums belong in this group.

These facts explain why some plants bloom only in the spring, why others flower regardless of the time of the year, once they start, and why others wait until fall to open their flowers.

Production and Natural Plant Growth Patterns

Unless we go to the expense of shading to exclude light, or to provide artificial lighting, whichever is needed, plants will not, as a rule, produce beyond their natural growth pattern.

A good example of this is the Chinese cabbage. Amateur gardeners often plant this vegetable in the spring, hoping to have edible heads by mid-summer.

Invariably they fail, and the reason is easy to understand, because Chinese cabbage is a long-day plant.

When planted in the spring, so that plants come close to maturity during the short nights of May and June, Chinese cabbage sends up a flower stalk without heading.

When sown after June 21, however, days begin to decrease in length, so plants reach maturity by early September. Since nights are then long enough to prevent flowering, plants form heads.

Exactly the opposite is true with chrysanthemum, a short-day plant.

Plants do not start to form buds until late summer, when the nights are lengthening. Actually, there are three different types of chrysanthemums, involving an intricate day-length and temperature relationship.

This explains why their flowering season varies so much.

Plants growing in an open field, where they receive light from all sides, flower later than those in gardens where they are shaded for part of the day.

The amount of shade is enough to cut off light either during the early morning or evening hours, thus shortening the day length.

Plants Light Intensity Requirements

Plants vary greatly in their requirements for light intensity.

Some tropical plants, including some houseplants (aspidistra), will survive under light as dim as 50 foot candles. The flowering reaction of other plants is even more sensitive.

As little as one foot candle can change a plant’s reaction and throw it into flowering, or prevent this response, depending on whether it is a long or short day plant. There are yet other plants that need 7,000 or more foot candles and refuse to flower unless exposed to full sun.

Variation in light intensity, as well as day length, accounts for the differences in the flowering of chrysanthemums.

In some autumns, when early morning mists will cut down both the duration and the intensity of light, these perennials will flower early.

In years of drought, with little moisture to produce morning mists, and with no clouds in the sky, both duration and intensity of light are enough to keep them from flowering satisfactorily.
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The sensitivity of some plants is amazing.

If certain annuals or perennials fail to bloom, check their surroundings. Often a grove of trees to the east or west, or tall buildings, will reduce morning or evening light intensities enough to throw them out of step.

The human eye is such a sensitive instrument for the detection of light and adapts itself so readily that it is a poor medium for judging whether light is adequate. We need a kind of light meter that will read the spectrum to which plants respond, so as to measure effective light.

Light Composition

This brings us to our third dimension of light, that of composition. Only the visible spectrum is useful to plants. Plants do not respond to invisible light rays in the ultra violet range, and invisible red is actually heat.

Ordinarily, composition outdoors is not a problem. When we grow plants indoors, however, composition immediately becomes important.

Artificial lights, either fluorescent or incandescent, a combination of the two is best.

Daylight white fluorescent tubes supply the blue end of the spectrum, while the incandescent supplies visible red that is lacking in the colder light.

Normal growth of foliage and flowers is favored by the blue end of the spectrum, while root formation is stimulated by the visible red end.

For rooting cuttings under artificial light, where the heat from incandescent bulbs is excessive, the warm white or soft white fluorescent tubes are used because they supply more of this red quality.

This is at best a sketchy explanation of the role of light in plant growth. It is presented with a hope that it will stimulate further study by interested gardeners.

Images: Forban300 | Sean McGrath

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What To Do This Month- August

August's lazy days are here, when even the most energetic gardener's enthusiasm is dampened by lethargy.
  • Water & Weed - Make it a point, at least, to water and weed in preparation for Autumn's cooler days and flowers.
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  • Pinch Petunias - Pinch back leggy growths on petunias. A boost with a liquid fertilizer will keep them flowering profusely until frost.
  • Transplant - Plants which have finished blooming may be transplanted or divided: Japanese and bearded iris, Madonna lilies, Oriental poppies, daylilies, Virginia bluebells, Trains and Spring-flowering bulbs whose clumps need separating.
  • Sow Seeds - Sow seeds of bush beans, endive, lettuce, spinach, dwarf peas, turnips and cress for late crops.
  • Red Spider - Watch evergreens for red spider infestations. Hot, dry weather promotes the mites.
  • Harvest Herbs - Herbs may be cut and cured in a dry, airy place, without exposing to the sun, before storing for the Winter. The best time to pick them is just before the plants begin to flower, any time during the day as long as the dew has disappeared. Learn how to preserve herbs from the garden to the freezer.
  • House Plants - Water house plants with liquid fertilizer and cut back straggly shoots to induce bushy growth. New plants may also be started from cuttings. Neem oil sprays will get rid of mealy bugs, scale and white flies.
  • Wildlife - If you want the birds to come to your garden, let sunflowers, marigolds, cosmos and other Compositor, especially in out-of-the-way places, go to seed. Goldfinches and other seed-eaters will find them unerringly.