Long-Day Plant
A plant that flowers, bolts, or forms bulbs when the daily light period exceeds a critical threshold, typically 14-16 hours. In North American gardens, long-day responses occur during summer.
A long-day plant flowers, bolts, or undergoes another developmental shift when the daily light period exceeds a critical threshold, typically 14-16 hours. As with short-day plants, the more precise description is a “short-night” plant - the mechanism detects uninterrupted darkness, and flowering occurs when that dark period falls below the critical threshold. Summer in North America, with 14-16+ hours of daylight, provides the long-day conditions these plants respond to.
Common Long-Day Crops
Long-day onions: Varieties like ‘Walla Walla’, ‘Copra’, ‘Yellow Sweet Spanish’, ‘Red Wing’, and most storage onions form bulbs when days reach 14-16 hours. These are the appropriate choice for northern gardens (roughly zone 5 and colder) where summer days exceed 15 hours. Planted south of their intended latitude range, they may not bulb, or may bulb only when days are too short to produce large bulbs.
Spinach: A classic long-day bolting response. When days exceed roughly 14 hours, spinach rapidly shifts to reproductive mode, sending up a seed stalk (bolting). This is why summer spinach production fails in most of North America - by June, days are too long for spinach to stay in vegetative production. Cool-season scheduling (spring/fall) works around this.
Lettuce: Similar to spinach, most lettuce varieties bolt in response to both heat and long days. The photoperiod response compounds the temperature response in midsummer. “Slow-bolt” varieties have elevated thresholds but will still bolt eventually under long days.
Radish: Spring radishes bolt quickly in summer. Long-day bolting is one factor; heat is another. Daikon and winter radish varieties have different daylength sensitivities and are planted in fall (shortening days) for this reason.
Practical Implications
Onion selection: Matching onion variety to latitude is essential. A general guideline: short-day varieties for zones 7-10 (planted fall/winter), intermediate-day for zones 5-6, long-day for zones 3-5. Planting the wrong type is the most common cause of poor onion performance.
Bolting management in leafy greens: Extending spring production of spinach and lettuce means planting as early as possible in spring (before days get too long) and selecting the slowest-bolting varieties available. Row cover can moderate temperature but doesn’t affect day length.
Fall production advantage: Cool-season crops like spinach and lettuce planted in late summer grow into shortening fall days, which delays their long-day bolting response and extends the harvest window. A fall spinach planting typically lasts weeks longer than a spring planting before bolting.
Garlic Scaping
Garlic sends up a flower stalk (scape) in response to lengthening spring days after vernalization. This is a long-day response. Removing the scape redirects the plant’s energy to bulb growth rather than seed production. The timing of scape removal - when the scape makes 1-2 spirals - is a key management step for hardneck garlic varieties.
The Latitude-Day Length Connection
Day length at summer solstice increases with latitude:
- 30°N latitude: ~14 hours
- 40°N latitude: ~15 hours
- 50°N latitude: ~16.5 hours
Long-day onion varieties are calibrated to these gradients. A variety requiring 16-hour days will only bulb well at latitudes where that day length occurs (northern US, Canada). At 35°N latitude, summer days peak at around 14.5 hours, which may not satisfy a 16-hour threshold.
See also: Short-Day, Day-Neutral, Photoperiod