Short day plants are species that flower when nights exceed a specific duration, typically blooming in late summer, fall, or early spring when darkness dominates the 24-hour cycle. Despite the name, what actually triggers flowering isn’t the short day itself but the long, uninterrupted night. Chrysanthemums, poinsettias, soybeans, and rice are among the most familiar examples.
Why “Long Night Plants” Is More Accurate
Every short day plant has a critical night length, a minimum number of dark hours it needs before it will flower. If the dark period falls short of that threshold, even by a small margin, the plant stays vegetative. This distinction matters because even a brief flash of light in the middle of a long night can reset the clock and prevent flowering entirely. A plant getting 10 hours of daylight and 14 hours of darkness will flower, but the same plant getting 14 hours of uninterrupted darkness split by a few seconds of light at midnight may not.
Researchers confirmed this by exposing rice and other short day species to short pulses of red light during an otherwise sufficient dark period. The plants responded as if they’d experienced a long day and delayed flowering. Interestingly, following that red light pulse with far-red light reversed the effect, pointing to a light-sensitive protein called phytochrome as the key player.
How the Flowering Signal Works
Phytochrome exists in two forms inside plant cells. One form is active after absorbing red light (the kind abundant in sunlight), and the other becomes active after absorbing far-red light or sitting in prolonged darkness. During a long night, the red-light-activated form gradually converts back to its inactive state. Once enough of it has converted, the plant interprets this as a signal that the night has been long enough.
That signal triggers production of a small protein in the leaves that acts as a flowering hormone. Scientists call it florigen. Once produced, florigen travels through the plant’s vascular system (the phloem, which normally carries sugars) from the leaves up to the growing tip of the stem. There, it switches on the genes responsible for building flower buds instead of more leaves. The entire process, from detecting night length in the leaves to physically initiating flowers at the stem tip, is a long-distance communication system built into the plant.
Obligate vs. Facultative Short Day Plants
Not all short day plants are equally strict about their light requirements. The two categories make a practical difference for gardeners and growers:
- Obligate short day plants absolutely require long nights to flower. Without them, they will grow leaves and stems indefinitely but never bloom. Examples include African marigolds, hyacinth bean, and mina vine.
- Facultative short day plants prefer long nights and flower faster under those conditions, but they’ll eventually bloom regardless. Zinnias, cosmos, celosia, morning glories, and globe amaranth all fall into this category.
This distinction explains why some plants bloom reliably in any garden while others frustrate growers at certain latitudes. A facultative species like zinnia will flower in a Minnesota summer despite the long days, just a bit later than it would in autumn conditions. An obligate species planted too early may produce nothing but foliage until the nights finally lengthen.
Common Short Day Species
Short day plants span both agriculture and ornamental gardening. Soybeans are one of the most economically important examples. Flowering time in soybeans varies dramatically with latitude because day length changes more at higher latitudes. Varieties bred for northern farms have mutations in their phytochrome genes that make them less sensitive to long summer days, allowing them to flower and set seed before the growing season ends. Rice, another major crop, follows the same pattern.
On the ornamental side, chrysanthemums and poinsettias are the classic commercial short day plants. Their precise photoperiod requirements are what allow growers to produce chrysanthemums for fall sales and poinsettias timed perfectly for December. Other ornamental short day species include moonflower, creeping zinnia, signet marigold, and hiemelis begonia.
How Growers Control Flowering Year-Round
Commercial greenhouses routinely trick short day plants into flowering at any time of year by manipulating their light exposure. During the naturally long days of late spring through early fall, growers pull opaque black cloth or plastic over their crops to artificially extend the night. The material has to block light completely, since plants can detect levels below one foot-candle (roughly the brightness of a candle a foot away). Even small gaps or pinholes in the covering can prevent flowering.
The timing takes some finesse. Covering plants too early in the evening traps heat underneath, which can damage the crop. Many growers wait until 8:30 p.m. or later to close the blackout material, then open it again around 8:30 a.m. Others take the opposite approach, leaving plants uncovered during the warm evening and pulling the cloth just before sunrise, around 5 a.m., so the dark period is long enough without the heat buildup.
To prevent flowering when it’s not wanted (for instance, to keep poinsettia plants growing larger before triggering blooms), growers do the reverse. They extend the “day” with artificial lights or interrupt the dark period with a burst of light in the middle of the night. Even a short interruption is enough to reset the plant’s internal clock and keep it in vegetative growth.
Temperature Can Override Day Length
Photoperiod is the primary trigger, but temperature can modify or even override the response. Research on the model plant Arabidopsis (a long day species, but the principle applies broadly) showed that raising ambient temperature from 23°C to 27°C (about 73°F to 81°F) was as effective at inducing flowering as switching from short days to long days. High but non-stressful temperatures can accelerate flowering under short day conditions just as effectively as providing the “correct” photoperiod.
For short day crop species like rice and soybeans, this means that unusually warm growing seasons can shift flowering timing in ways that photoperiod alone wouldn’t predict. It also helps explain why the same variety can behave differently in two locations with identical day lengths but different temperatures.