Short-Day Plant

A short-day plant is one that requires a daily light period shorter than a critical threshold - typically 12-13 hours - before it will flower, form bulbs, or enter dormancy. The trigger is more precisely described as “long-night” plants, because the physiological mechanism responds to the length of uninterrupted darkness rather than day length directly. Interrupting a long night with even a brief flash of light can prevent flowering in short-day species.

In North American gardens, days fall below 12 hours in late September and remain short through winter. Short-day responses therefore happen in fall and early winter.

Common Short-Day Crops

Strawberries (June-bearing types): June-bearing strawberry varieties initiate flowers in response to short days in fall. These flower buds overwinter in the crown and open in spring, producing the concentrated early-summer harvest characteristic of June-bearers. Plantings that don’t receive adequate short-day exposure may produce reduced spring crops.

Short-day onions: Varieties like ‘Texas 1015’, ‘Granex’, ‘Red Creole’, and ‘Crystal White Wax’ bulb up in response to relatively short days - around 10-12 hours. In the southern US (roughly zone 7 and warmer), these varieties are planted in fall or winter and bulb in spring before days get too long. Growers north of zone 7 should not plant short-day onion varieties: the shorter days required for bulbing never occur during the warm growing season at northern latitudes, and the plants will fail to bulb or bulb with small, poor-quality results.

Poinsettia: Requires 14+ hours of uninterrupted darkness for at least 8 weeks to produce bracts (the red “petals” are actually modified leaves). Commercial growers use blackout cloth to control this precisely.

Chrysanthemum: Classic short-day flowering plant. Garden mums flower naturally as fall days shorten. Florists can manipulate bloom timing using light exclusion.

Cannabis

Cannabis sativa and Cannabis indica are short-day plants that flower when the light period drops to approximately 12 hours. This is why outdoor cannabis flowers in late summer and fall in temperate climates, and why indoor growers deliberately switch light schedules from 18/6 (vegetative) to 12/12 (flowering) to trigger bloom.

The Night Interruption Problem

Because the actual trigger is uninterrupted night length, light pollution can disrupt short-day responses. Strawberry beds near bright outdoor lighting may not receive adequate dark periods, potentially reducing June-bearing fruit set. This is rarely a practical problem in typical home garden settings but can affect plants very close to high-intensity security lights.

Latitude and Short-Day Crops

Short-day responses limit the useful latitude range of certain crops. Short-day onion varieties are adapted to southern latitudes where planting and growing occur during shorter days; they’re inappropriate for northern gardens. Long-day varieties are adapted to northern latitudes where summer growing coincides with long days. Intermediate-day varieties work across a broader central latitude range.

See also: Long-Day, Day-Neutral, Photoperiod