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The Plant Doctor: Rotation of houseplants encourages even growth

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Unquestionably, the most selected site for maintaining houseplants during the fall and winter is the ever-present windowsill.

Those of us who have a choice will usually select a southern exposure in order to maximize the amount of sunlight and thus enhance both the beauty and growth of the plant.

In a relatively short period of time, however, the house gardener will notice that the plants will almost certainly begin to bend toward the window. The cause for this less-than-desirable growth pattern is something scientists call “phototropi­sm.”

The physiological process, phototropism (from the Greek “photo,” for “light,” and “tropos,” which means to “turn”) is controlled by the plant hormone auxin, which under certain conditions causes the cells to lengthen.

All plant cells have auxins. The auxins in those cells exposed to the sun are essentially disabled. The result is an accumulation of auxins in the cells on the shaded side. Auxins stimulate both the elongation and rate of growth of cells. Thus the cells on the shaded side will grow faster than those on the sunny side. The result is that the stem bends toward the window, or essentially in the direction of sunlight.

In nature, this adaptation is crucial. Plants growing in an underbrush may receive sunlight from only one direction, so the bending reflex allows the plant to move out of the shaded area and to receive sunlight vital for its survival.

At home, this behavior is not necessary and results in spindly, awkward-growing plants. The solution is quite simple. Rotate the plants a quarter turn every two weeks or so to minimize the effects of phototropism.

Ideally, if you can create a condition where the light source is directly above the plants, you will find that the plants grow upright, and the stems are thick and short. That is because the plant cells are growing equally on all sides of the plant.

If you are lacking a windowsill you can still provide, at minimum cost, what I call the “light garden.” You will need a fluorescent fixture that contains two fluorescent tubes (40 watts) and the reflector unit that holds them, as well as a simple frame to hold the unit over a planting box, a timer to provide 14 hours to 16 hours of light and placement no closer than 2 inches from the plants.

There is no need to purchase the special Gro-Lux bulb, which is good but expensive. The “cool light” or “daylight” fixtures will more than satisfy the lighting needs of the houseplants.

Questions or concerns regarding gardening or houseplants may be addressed by e-mail to Harvey.Goodman@att.net.

Updated 10:25 am, October 12, 2011
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