The Flowering Secrets of Plants
Did you know that in many plants, flowering — and bulbing — is based on the number of hours of exposure to light?
This fact of plant biology explains many mysteries. Understand it, and you’ll be a smarter and more successful gardener.
If you expose a chrysanthemum or poinsettia to more than 11 hours of light each day, it will never flower. The triggering mechanism that forces these species into flowering is exposure to no more than 10 hours of light. Those conditions of light are called short days and long nights. Interrupting their 14-hour night with even a flash of light can prevent flowering.
During long daylight hours, greenhouse growers cover these species with shade cloths to force them to flower out of season. That’s why chrysanthemums are available throughout the year.
Nature’s cycle of short days and long nights begins in late summer. This natural cycle enables us to enjoy fall mums and greenhouse growers to grow poinsettias without having to shade them.
In the fall, some chrysanthemums flower earlier than others. This range is possible because breeders have developed cultivars with different maturing periods. Chrysanthemums’ short-day classification further divides into six-week, eight-week, 10-week and 12-week cultivars. These numbers refer to the number of weeks from the time a plant is exposed to 10 or fewer hours of light until the flower buds show color. By selecting different varieties, you can have chrysanthemums flowering in your garden for many weeks.
Short-day woody plants include azaleas, rhododendrons, dogwoods, lilacs, spring-flowering roses and viburnums, to name a few. The flower buds on these plants are produced late summer and early fall for flowering in the spring.
Long-day plants flower all summer long. This includes bedding plants and woody ornamentals such as fuchsia, crape myrtle, hybrid-T and floribunda roses, some hydrangea and hibiscus, among others. When daylight hours fall under 10, the plants remain in a vegetative state of growth.
Many varieties of onions are also classified as long- or short-day varieties. For long-day onions to form bulbs, they must be planted in the spring and form bulbs when the days are long. Short-day onions — as well as garlic — are planted in the fall and form bulbs when daylight hours are short. Plant short-day onions in the spring, and you’ll only get green onions.
Ask Dr. Gouin your questions at [email protected]. Please include your name and address.