Greenhouses do a good job protecting plants from the cold. In fact, conditions inside an unheated greenhouse can mimic the climactic warmth of a higher U.S.Department of Agriculture Plant Hardiness Zone. But the warmth comes at a cost. The same membrane that keeps out the cold also creates a barrier to light.
How Much Light Is Lost?
The illumination that is lost in a greenhouse depends on several factors including the opacity of the roof and wall material, the orientation of the greenhouse relative to the sun, and humidity both inside and outside the structure. It's not uncommon to lose as much as 40 percent of the natural light inside a greenhouse. One way to measure relative loss is to use a foot-candle meter inside and outside the greenhouse and compare the two readings at different times of day.
Ways to Mitigate Light Loss
To retain as much natural light as possible, try to orient the whole greenhouse to expose the most surface area toward the south. Use the clearest glass or plastic possible. When the day is at its warmest, raise panels to give the plants more direct access to the sun's rays.
Plants and Light
Plants cannot live without visible light. The shorter, blue wavelengths of light are captured by chlorophyll molecules to drive the photosynthetic reaction that converts carbon dioxide and water into glucose and oxygen. The longest visible lightwaves in the red part of the spectrum are absorbed by other plant structures, called phytochromes, and are used to power hormonal changes that in turn trigger reproductive functions, such as flowers. There's a good reason why greenhouses aren't actually green. If only green light could filter through the membrane to the plant, the plant would die. Green light can't be absorbed and used by plants.
Artificial Light Sources
As long as the correct visible light wavelengths get to the plants, they will flourish, regardless of the source. "Grow lights," which appear somewhat purple when lit, provide simultaneous blue and red light, but they can be expensive. "Cool white" bulbs, ubiquitous in kitchens and bathrooms, appear bright white because they emit copious amounts of blue light. Incandescent bulbs take on a warmer appearance because of their redder light.
Optimal Artificial Light Conditions
Greenhouse lights make up for what's missing from the sun in cold weather and in the extreme north. For optimal exposure, lights should be suspended just a few inches above the plants. One red light source for every 10 or so blue light sources is usually sufficient for ensuring blooms.
Plants need light, but they also need darkness. During the night, they switch from food production to food consumption. If there is too much light, they can die. Plants vary in their day-length sensitivity, but generally 12 hours of light is sufficient.