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Beware of Streetlights' Effect On Fall Bloomers
By Norman Winter
Central Mississippi Research & Extension Center
Three-foot tall Country Girl chrysanthemums, six-foot tall Indigo Spire salvias and five-foot Mexican bush sages should have been pretty awesome for my garden last year. Unfortunately, there was not a single bloom despite all the lush green foliage. The blooms were practically nonexistent on my goldenrods and Joe Pye weeds, too!
With planting time at hand for chrysanthemums and fall blooming salvias, I don't want bloom failures to happen to you. Your bed preparation is critical, but perhaps just as important is a phenomenon called photoperiodism.
My bloom failure happened as a result of a requested security light. My security light was placed exactly where I wanted it. Unfortunately, my order was accidentally duplicated and the next day another crew showed up and unknowingly filled my order again. They put the light in the next obvious place, which happened to be over my perennial garden.
The light looked good in my large back yard, so I left it. I knew I might see a problem with the chrysanthemums, but the other bloom failures surprised me. I wasn't able to just look up a chapter on which plants don't bloom under streetlights.
Photoperiodism is the growth response of a plant to the length of day, or more precisely, the length of the light and dark periods. These responses may include flowering dates, vegetative growth, and formation of tubers, bulbs, rhizomes and buds. Most of the plants we grow fall under one of three categories: short-day, long-day or day-neutral plants.
Short-day plants are those that initiate flowers only when the day length is below about 12 hours. The most important consideration of short-day plants is the long period of uninterrupted darkness instead of the length of light. Short-day plants include chrysanthemums, some salvia and cosmos, angel trumpets (datura) and the poinsettia.
Long-day plants were believed to start flowering only in day lengths exceeding 12 hours, but recent thoughts have it tied to set amounts of darkness instead. Black-eyed Susans (rudbeckias), hibiscus and flowering tobacco (nicotiana) are popular long-day plants.
Day-neutral plants have some type of stimulus other than day-length that initiates the blooming process. One of the most common and somewhat dreaded day-neutral plants is the dandelion. Many of our favorite tropical plants are day-neutral. The color of light also has some effects.
Many gardeners never think about security lights or streetlights as having a detrimental effect on blooms, but lighting can be just as important as bed preparation. Look all around and see if your lights might have a photoperiodic effect on future perennials you want to establish.
If you buy chrysanthemums to show a little color and then discard after a season, the streetlight phenomenon will pose no problem. The problem will occur if you want to keep them for subsequent years as a perennial.