Light Requirement Categories for All Plants
Milliwatts per square meter (mWm2) is the irradiance produced by one-thousandth of a watt of light energy beamed into a surface of one square meter.
3,000 mWm2: Lighting after flower induction (forcing) during the latter part of cultivation; growing long-day plants, giving a little growth stimulation as well.
4,500 mWm2: Cultivation of vegetables, flowers (for produce and flowers) through harvest; young spaced vegetative plants.
6,000 mWm2: For rooting and young plants while still packed under the HID; high light vegetables, flowers (for produce and flowers) plants.
7,500 mWm2: Rooting some nursery stock and some high-light plants.
9,000 mWm2: Expensive plants; fast and programmed crops; special cases. Producing this much supplemental light in a room is very expensive.
Growing tropical plants (orchids or bromeliads) indoors from equatorial regions, growers give their gardens 12 hours of light throughout the life cycle to replicate the less dynamic photoperiod of the tropics. On the equator, days and nights are almost exactly the same length the years round. When this method is used, plants tend to bloom when they are chronologically ready, after thoroughly completing the vegetative growth stage.
The average house plant is "night neutral" since it will grow and bloom well with six to eight hours of darkness. These plants require 14 to 16 hours of daylight. Examples of these plants include most gesneraids, oxalis, Exacum affine, many begonias, and most house plants.
Chrysanthemums, poinsettias, species columnea, orchids and some begonias, Christmas cactus and related plants will bloom only if given a long night of 14 to16 hours and less than 8 to10 hours of light per night.
Summer-blooming plants and vegetables need 18 hours of light per day and a short night of 6 hours. Geraniums, annuals, tuberous begonias and seedlings of almost all plants are in this category. In fact, African violets will double their blossom production if they are given 18 hours of light rather than 12 hours.
Foliage plants are raised for green foliage; flowers are not important. These plants can have varying amounts of light and dark and growth is not affected. Examples are rex begonias, aroids, palms, marantas and ferns.
From my book: Gardening Indoors by George F. Van Patten
Ron
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