Photoperiod
The duration of light and dark in a 24-hour cycle. Many plants use photoperiod as a trigger for flowering, bulb formation, or dormancy, independently of temperature or season.
Photoperiod is the length of the light period in a 24-hour cycle. Plants use this as a reliable environmental signal to time flowering, bulb formation, and dormancy - because daylength changes predictably with season while temperature does not. A warm spell in March might fool a temperature-sensitive plant into breaking dormancy prematurely; daylength is a more reliable indicator that summer is actually arriving.
The physiological mechanism involves phytochrome - a pigment that exists in two interconvertible forms depending on light exposure. The critical variable is actually night length (hours of darkness) rather than day length, though the two are inversely related. A “short-day” plant is more precisely a “long-night” plant: it flowers when continuous darkness exceeds a threshold value.
The Three Categories
Short-day plants flower when daylength falls below a critical threshold - typically when nights are longer than 12-13 hours. In North American gardens, this means fall. Examples: chrysanthemum, poinsettia, strawberry (for runners and dormancy), cannabis.
Long-day plants flower when daylength exceeds a threshold - typically when days are longer than 14-16 hours. In North American gardens, this means summer. Examples: spinach, lettuce, radish (these bolt in summer). Onions form bulbs in response to long days.
Day-neutral plants flower based on age, temperature, or other factors rather than daylength. They flower regardless of photoperiod once they reach maturity. Examples: tomato, cucumber, squash, most beans, day-neutral strawberries.
Why This Matters Practically
Onion selection is the clearest example. Onion varieties are classified as short-day (bulb in 10-12 hour days), intermediate-day (12-14 hours), or long-day (14-16 hours). Grow a long-day variety like ‘Walla Walla’ in Georgia, and the bulbs will never size up - Georgia’s summer days don’t get long enough to trigger bulbing. Grow a short-day variety like ‘Texas 1015’ in Minnesota, and it bulbs up in June while still small, before the plant has grown enough to produce a large bulb. Selecting the wrong day-length type for your latitude is a common cause of small or failed onion crops.
Garlic uses a combination of vernalization (cold requirement) and photoperiod to time its growth stages. Fall-planted garlic receives cold vernalization through winter, then responds to lengthening spring days to begin scape and bulb development.
Strawberry flowering types are photoperiod-controlled. June-bearing varieties flower on short days (fall/winter), producing a single heavy spring crop. Everbearing and day-neutral varieties have reduced or absent photoperiod sensitivity and flower repeatedly through summer.
Bolting in cool-season crops is partly photoperiod-triggered. Spinach, lettuce, and cilantro are long-day plants: as summer daylength extends beyond their threshold, they shift from vegetative to reproductive growth (bolt). Variety selection matters - “slow-bolting” varieties have elevated thresholds.
Photoperiod and Indoor Growing
Grow lights for indoor cultivation are often timed to simulate specific photoperiods. Seedlings started indoors in late winter benefit from 16 hours of light daily (long-day simulation) to support vigorous vegetative growth. Tomatoes and other day-neutral crops are less sensitive but still produce better under extended light periods as young plants.
See also: Day-Neutral, Short-Day, Long-Day