the urban gardener

As the days get longer, we know spring's a-coming. So do the plants, for light is one of the things that triggers growth in northern areas. For our outdoor plants, temperature is the other trigger that determines exactly which day they sprout.

On the equator, because the days are always 12 hours long, moisture is usually the trigger. As long as light, temperature and moisture remain constant, plants grow. When we grow plants indoors we can control water and temperature, but the intensity of light and the length of days change with the seasons, which is one reason why our plants often look poor during winter. By understanding light, one can use it to encourage plant growth, especially when starting seedlings. It's possible to grow perfectly good plants anywhere, even in a cellar or an interior office with no natural light; it's a matter of understanding how plants respond to light levels and lengths of days. It's possible to grow perfectly good plants anywhere, even in a cellar or an interior office with no natural light; it's just a matter of understanding how plants respond to light levels and lengths of the day. Many plants will do well under low light intensity.' Perhaps you've noticed that sometimes you buy a lovely big plant, take it home and all the leaves fall off. This is common, especially with plants from supermarkets or chain stores. Most of these plants have been shipped from some sunny southern clime, with thick, sturdy leaves that grew in bright sunlight. But in your house, even in a sunny window, light levels may be too low to keep those sun-grown leaves happy. New leaves will often appear, particularly if weak fertilizer is given regularly. The new leaves, because of the dimmer light, will be thinner, smaller and have longer internodes. Local nurseries usually acclimatize plants until they get used to our low winter light levels. Growth of indoor plants can be helped by supplemental light, especially leafy plants, herbs and for starting seeds. Flowering plants are much harder to keep blooming without bright outdoor light or a sunny windowsill. I have some basil plants under a 15-watt fluorescent light on my kitchen counter that have been growing all winter. I keep experimenting with other indoor plants, some in lighted windows, others in natural light with sun for a few hours. I wish I had some definitive results to report, but the only thing that seems true is that once a plant is acclimated to its spot, it grows. Fertilizer seems to make more of a difference in the rate of growth than light. Fertilizer also seems to help the flowering things, even with lower light. A lot of scientific experiments have been done to trick plants into making them think it was summer by supplying long hours of supplemental light. It worked best for herbaceous plants. However, one interesting experiment showed that azalea cuttings grew five times faster under a low light level in a windowless room (500 foot candles for 16 hours), than at twice the light (l,000 foot candles), and faster than in a regular greenhouse with the same supplemental lighting. This would seem to indicate that you can grow lots of things almost anywhere, using lights. But what kinds of lights? Some US Department of Agriculture experiments indicated that plants would grow best with a full light spectrum, containing enough blue light wave lengths (plants grow toward blue) and also enough red light (red makes them grow faster). A combination of a cool white fluorescent bulb (bluer) plus a warm yellowish fluorescent bulb (more red) gave the best growth and longest flowering. Halide lights were about the same. Plant lights (full spectrum) were no better. Also a cool white fluorescent light plus a regular incandescent yellowish light bulb can provide the red light, although incandescent bulbs get hot and can cook the leaves. I bought a two-bulb fluorescent fixture, and put in one cool and one warm bulb, but I can't see much difference from the one bigger cool bulb I had. Experiments seem to indicate that the electric lights kept on 16 hours a day make things grow better indoors because plants think its summer. But if the plants get bright winter sunlight, the supplemental light might not help as much, because, by sensing the short day length, plants may figure out it's really still winter.