How Light or Lack of It Effects Plants

When a spot of green grass is covered, it takes only a couple of days until all color is gone. When potato sprouts develop in a dark pantry or cellar they are white. Those growing in full light are green.


Light causes the difference and is of utmost importance to all green plants. They cannot live without it. The green coloring matter is called chlorophyll. It is produced only in plants or parts of plants exposed to light.

A normal plant growing in full light may develop a white stem with white leaves. It is not unusual for the green and white variegated sultana (impatiens) to have albino shoots.

This house plant roots easily from cuttings but I have never been able to get the white cuttings to make roots. Lily seeds occasionally send up white instead of green stems, but they wither and die without making bulblets.

Plants Feed Different Than Animals

There is much we do not understand about plants. We do know that plants require food just as animals do, but they have an entirely different eating system. They feed themselves by absorbing through their roots and leaves certain chemical elements found in water and in the air.

Light for plants is the fuel or source of energy that produces the chlorophyll which enables the plant to turn these chemicals into sugar and to store it for future use in the form of starch.

We might conclude that plants would make most of their growth during the daylight hours, but we are told that light retards growth that plants do most of their growing at night.

Lighting Effects On Plant Growth

We know that even though some plants require a great deal of light and others much less, that many, by changing their growth habits, are very adaptable to whatever we can provide them.

Take pansies as an example. When planted in a sunny location they are low growing and fairly compact. One fall the pansy plants were set much too close to a vigorous row of bearded irises.

The next spring as the irises increased in height, they shaded the pansies more and more. The pansies, reaching for the light, grew tall, leaning on the irises as they grew.

The distance between the leaves was much greater than on the pansies growing in stronger light and the stems lacked firmness. Had the irises been cut down, the pansies would have toppled over.

Leaf and Stems Adapting To Plant Lighting

Violas plant themselves all over the garden. Those that do not have to share light with other plants, grow compact and low. Those planted among taller neighbors grow higher and higher.

Thus we can see how plants can adapt themselves to unsatisfactory growing conditions by changing their leaf and stem structures. Those with soft thin leaves and delicate stems in the shade develop smaller and thicker leaves on shorter stems in the sun.

Some hardy fall asters growing in full sun have erect stems. The same asters shaded, were flat on the ground as they crept toward light.

It is interesting to observe shallow rooted plants like cosmos or marigolds that have fallen over during very wet spells. The stems that develop grow upward practically at right angles to the main stem.

This should not indicate that all plants require sunlight. There are plants that perform very unsatisfactorily and even die if planted in direct sunlight. We think of certain ferns as shade loving plants.

There are plants which prefer the shade of trees or northern exposures where they receive little if any sunshine. Many plants like a combination of sun and shade especially to be sheltered from the hot noonday sun, or filtered sunshine falling through the trees.

Hostas and lilies like Lilium Henryi bleach badly if exposed to full sun all day.

In African violets care, watch out for leaves burning if placed next to the glass in a sunny window. The new leaves in the center do not develop normally but remain small on very short stems. Burned spots appear. If blossoms are produced they are likewise small on short stems.

On the other hand, if the plants are in too dark a location, the leaf stems become long and straggly and light in color. Buds do not develop.

Place an African violet where it can get a bit of morning sunshine and strong direct light the rest of the day and leaves develop normally and you cannot keep the plant from blooming.

Artificial Plant Lights

Artificial plant lighting may be used to supplement daylight to promote growth of plants that require extra light. But we must not forget that some plants require extra hours of darkness.

Outside, we know that chrysanthemums wait until toward fall to make buds when the days are shorter and there are more of the darker hours.

Was your poinsettia slow in showing color before the holidays? It may have been because the plant was not in complete darkness from 14 to 16 hours out of every 24 commencing in early November.

If the plant was kept in a room where artificial lights were on, it should have been moved into a dark room, or covered with a dark cloth when the lights were on.

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