INTENSITY
Here again we find the needs of plants vary and fluorescent-light setups vary accordingly. If possible, measure the light in your growing area. The readings of a photographic light meter—the same instrument you employ in your photography—can be translated into foot-candles. Or you can get a meter that registers foot-candles. For advice, consult your camera dealer, or check with your local power-and-light company. Here in Redding we find the Connecticut Power and Light Company vitally interested in artificial-light plant propagation.
Again “in general,” house plants that require “full sun” when grown in a window need 1200 to 1500 foot-candles of artificial light, and for fourteen hours a day. Foliage plants will get by with 500 to 600 foot-candles. At about 1000 to 1200 foot-candles many plants, and I’m thinking of begonias and gesneriads in particular, will be robust and floriferous.
Should you find it difficult to figure light intensity as suggested above, you might follow the formula worked out by an old friend on Long Island, Elaine Cherry (Mrs. Norman Cherry, the wife of one of our engineering friends). Her formula is easy to follow. “A single forty-watt tube will serve a space approximately four feet long by six inches wide.” Small plants that need intense light can be set up close to the tubes.
Here is a tip—ever notice how your television picture is dim but brightens appreciably when you take a dust rag to the surface of the glass? The same is true of your light fixtures. Wipe them off now and then. Clean tubes give more light than dusty ones, and new tubes give more light than old ones. When a tube darkens at the ends, that means it has seen better days and should be replaced. According to Mrs. Cherry, it is a good policy to replace tubes after five thousand hours of service and not wait for the dwindling light to curtail the rays your plants need. While you are at it, it’s smart to insert new starters.