Light in the greenhouse (Photo 1) provides the energy through photosynthesis that enables a plant to grow and develop. When sufficient and of the right kind, light makes producing quality plants relatively easy. When light levels are inappropriate, production becomes more difficult. However, simple techniques of light management can reduce production problems.
The important thing is: know your plant s light requirements. Plant requirements for light vary with species, acclimatization, and stage of development (Photo 2) , (Photo 3) , (Photo 4) . You must know what light levels serve your plants best under a variety of conditions. Is the plant one that requires high levels of light, low levels of light, can it acclimate to a different spectrum of levels? If it can acclimate, how long does it take for acclimatization to occur?
A grower can adjust light intensity to achieve desired growth. When light levels are too high, shading is possible. Greenhouse shading is generally done by one of two methods: an external application of shading compound or paint, or internally with a woven type of shading fabric. Each has advantages and disadvantages. External shading is most effective at decreasing heat buildup in the greenhouse. However, applying a uniform coating of shade is difficult, as is removal. Once the shade is applied, it is semipermanent. A series of dark, cloudy days even during the summer means that plants may be receiving insufficient light for a long period of time.
Internal shade helps to solve the problem of permanence. Shade material can be pulled and withdrawn as needed. The problem with internal shading is expense. However, most of the money is invested in initial installation with much lower long term costs (Photo 5) . Often, a grower can double- or even triple-dip into a mechanical system and use the same installation to apply blackout cloth for short-day plants and heat blankets during cold weather. Another drawback with internal shade is its inability to prevent light from entering the greenhouse and being converted to heat. Using reflective or white materials reduces the problem.
Compensating for light levels lower than necessary for good plant growth can also be done. One simple way is to give plants more space during low light periods. Avoid the temptation to crowd plants when light levels are low. Each grower must decide where the economic breaking point between quality and quantity occurs (Photo 6) .
Arranging plants in the greenhouse to match light availability to plant requirements often helps to improve production. Placing low-light plants in darker areas of the greenhouse, such as in shadows, may solve some of the problem, assuming the existing light levels are adequate for the plants.
Choice of plants and their arrangement in a hanging basket situation can make a big difference in quality. If possible, choose a low-light plant for the understory and a high light plant for the upper deck. If it necessary to use high-light plants in both locations, then arrange the upper plants to let sufficient light through and give adequate spacing to the lower plants (Photo 7) , (Photo 8) .
When arranging plants is not enough to compensate for low light levels, growers can use supplemental light. In the greenhouse, the most affordable lamps with adequate intensity and spectral qualities are high pressure sodium or metal halide lamps. Both are types of high intensity discharge (HID) lamps.
In most regions of the United States, sufficient supplemental light is provided if the light intensity from the lamps is around 150 umol (approx. 1140 fc. from HID) at plant height. Although this level is lower than most plants require for good growth it is enough to augment natural light. Using HID s to increase light intensity during the day or extend the day can encourage good growth. A word of caution, HID units can block natural light, be aware of the shadows they cast. The decision whether or not to use supplemental light depends on each grower SE situation (Photo 9) .
