Morning glory, short-day plant
Plants are classified in many ways—annuals, biennials, perennials; spring, summer, and fall bloomers; sun-loving or shade-loving; native vs. invasive; kingdom, phylum, family, genus, species; deciduous vs. evergreen; edibles, ornamentals, groundcovers; herbaceous vs. woody; by USDA hardiness zones; and soil properties— acidic vs. alkaline or sandy, clay, or loamy.
Photoperiodism is another way to classify plants. Plants are classified as short-day, long-day, or day-neutral. Photoperiodism is a plant’s biological response to the changing lengths of day and night throughout the year, which allows them to synchronize developmental processes like flower formation and bud set with the changing seasons.
Short-day (long-night) plants bloom when days are shorter than about 12 hours. If exposed to longer days of light, these plants won’t set buds. Long-day (short-night) plants bloom only when they receive more than 12 hours of light per day. Asters, bachelor buttons, coneflowers, California poppies, strawflowers, and sweet peas are examples. Day-neutral plants, like amaranth, cleome, soapwort, and stock, flower regardless of day length. Some plants, like petunias, bloom regardless of day length but flower earlier and more profusely in long days. The key factor is not the length of the light period, but the length of the uninterrupted dark period.
Raspberry Sorbet Chrysanthemum, short-day plant
Agronomists work with agricultural crops to select varieties to ensure proper timing for flowering and seed set to maximize yield. Plant breeders work to develop new plant varieties that flower under specific day-length conditions making them adaptable to a wide range of global climates.
Horticulturists and commercial growers may need to manipulate day length in greenhouses to induce flowering at specific times for commercial purposes to ensure plants like chrysanthemums, Christmas cactuses, and poinsettias flower for holidays or specific market requests. Growers cover the poinsettia benches with black cloth or use light-proof curtains to block out all artificial light during the night. Consistency is crucial because even a short period of artificial light during the dark period can delay or prevent flowering.
I set out to find out which plants in my garden were short-day plants and discovered many of the fall flowering plants were short-day plants, including camellia sasanqua, camellia japonica, chia, four o’clock, garden chrysanthemums, morning glories, and pineapple sage.
Home gardeners can manipulate day and night length indoors with growlights or covers to encourage plants to bloom at times they normally wouldn’t. For short-day plants cover them for at least 12 hours daily for several weeks in late spring or early summer to simulate fall conditions and stimulate blooming. For long-day plants provide extra hours of light with light banks to bring them into bud earlier, before natural day length passes 12 hours.
Photoperiodism affects humans, too. Setting clocks back in fall and setting clocks ahead in spring influences our circadian rhythm.
Pineapple sage, short-day plant
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