Photoperiodism & Flowering: Short Day Vs. Long Day Plant

Short Day

• When sunlight is available for fewer than 12 hours each day consistently, flower formation occurs in short-day plants. Using photoreceptors in the leaves, plants generate a flowering hormone called florigen in both short- and long-day plants that directs energy toward reproductive activity rather than foliage or root growth. Short-day plants, as a result, typically blossom in the early spring or late fall. For example, poinsettias (Euphorbia pulcherrima), native to U.S. Department of Agriculture plant hardiness zones 9 through 11, are known for their autumn and winter blossoms decorating patios and backyards; they are short-day plants.

Long Day

• In contrast, long-day plants need shorter nights to stimulate flowering. For example, California poppies (Eschscholzia californica), hardy in USDA zones 9 through 11, blossom profusely in the late spring and early summer, like most long-day species. Because of the variety of both short- and long-day plants, each species carves out a specific flowering time to increase pollination and reproductive success. If all plants flowered simultaneously, pollinators could not possibly fertilize every blossom -- many plant species could die out as a result.

Interruptions

• Because of their sensitivity to sunlight, neither short- nor long-day plants can tolerate any interruptions in their darkness cycle. For example, turning on an outdoor light at night for only a few minutes interrupts the cycle. As a result, your poinsettia will fail to flower. However, you can move the plant from light to darkness for several minutes and it does not affect the flowering potential; only darkness interruption causes flowering issues. Many other factors, from soil nutrients to local temperatures, affect flowering periods, but darkness interruptions cause the most significant failures.

Forcing

• To force your poinsettia to bloom earlier in the year, you can create a darkness pattern by covering the plant for more than 12 hours a day, tricking it to release its flowering hormone for early blossoms. Placing a long-day plant under fluorescent lights for more than 12 hours a day stimulates flowering as well. However, each plant species has different light needs; some need only a few days of darkness to start flowering, whereas others require weeks of alternating dark and light environments. For best results, check the specifics on the plant you're planning to force.