Most gardeners take note of shorter daylight hours in autumn and longer daylight hours in spring because they know that light-the intensity of sunlight, the length of daylight hours, and the type of artificial light you use to supplement sunlight-influences the growth and health of plants. Here are a few principles to help you understand how light affects your plants, both indoors and out.
Light spectrum
Light is made of energy waves that appear as different colors, depending on how much energy they emit. Red, orange, yellow, blue, green, violet, and indigo-the colors of the rainbow-are colors of light we can see with the unaided eye. But those aren't the only ones that exist. For example, even though the human eye can't see ultra-violet light, many plant-pollinating insects can-and some plants produce ultra-violet markings on their blooms to attract these insects.
Blue light
The blue side of the spectrum has more energy than the red side. Blue light is crucial to helping plants put out healthy leaf growth. While natural light and cool-white fluorescent lights tend to be high in the blue range, incandescent bulbs are low.
Red light
Red light affects flowering and plant reproduction, which is why it's thought that tomatoes grown over red plastic mulch have a higher yield and better-quality fruits. When plants don't get enough red light, they may not bloom well or at all. Both natural light and most incandescent light bulbs tend to be high in the red range. Cool-white fluorescent lights tend to have little red light.
Green light
Some gardeners think green light is the most important color of all, but plants use relatively little green light. Instead, they reflect it back at us, and that's why we see their leaves as green.
Brightness
It's not just the color, but also the amount of light that's important. For example, full-sun plants do best if they get 6 to 8 hours of full sun outdoors. If they don't get enough bright sun, they don't do as well. Likewise, some flowering houseplants may not bloom indoors because they don't get enough light. However, if shade plants get too much bright light, their leaves burn.
Short-day plants
The length of the day affects the way certain plants grow. Short-day plants need uninterrupted nighttime darkness for a specific period of time, in order for them to bloom, for example. The period of light differs from plant to plant. If the plants get long nights and short days, they often develop flowers (poinsettias and dandelions, for instance) or start to produce bulbs or tubers (in the case of some onions and potatoes). However, if short-day plants get short nights and long days, they generally just grow foliage.
Long-day plants
These plants are the opposite of short-day plants. Long-day plants need short nights and long days to bloom or produce bulbs, tubers, etc. Examples are lettuce and some onions.
Day-neutral plants
These plants don't care how long the day is-they bloom well regardless of day length. Examples are everbearing strawberries and African violets.
Phototropism
This term describes the way plants lean or bend toward light. For example, sunflower blooms follow the sun's course as it moves through the day. Indoors, movement is more apparent-potted plants often lean strongly toward a light source and, in severe cases, can even tip over. Turn indoor plants frequently to keep the growth habit attractive.