Photoperiod i.e. ‘day-length’ in relationship to plants refers to the length of time that a plant is exposed to light in a day. Initially scientists thought that it was the amount of light per day that prompted growth responses in plants but later came to understand that is it actually the amount of uninterrupted dark that triggers most growth responses in plants. Day length can initiate vegetative growth, flower bud initiation & flowering, bulb formation, leaf senescence, etc. Plant growth processes can also be determined by temperature or day length and temperature together.
Plants can be divided into 3 main groups based on photoperiod response: short-day (<12 hours daylight), long-day (>12 hours daylight) and day-neutral/ intermediate day (not dependent on day length). Most plants stop growing when day length is less than 10 hours, even if ideal temperatures are maintained. Day length has to do with latitude. In Colorado, our latitude ranges from 37° to 41° N of the equator and since we are near the center of the US, our time between sunrise and sunset is moderate to southern and northern states.
Day length is an important consideration for choosing which vegetable and small fruit crops and varieties will grow best in your personal garden and what time of year they should be planted for best production. Day length can also determine when certain herbaceous perennials and houseplants will flower and when to prune woody flowering shrubs and trees.
Long Day Plants
(Day Length > 12 hours)
Short Day Plants
(Day length < 12 hours)
Day Neutral Plants
(Day length has no effect)
Yard long bean
For one, it has taken me several years to learn to grow onions successfully. In fact, I’m still working on it! Different varieties of onions grow better bulbs before flowering at different latitudes. Short day varieties form bulbs at latitudes below 35° N with day length from 10-12 hours a day; long day varieties form bulbs at latitudes above 35° N and 14-16 hours a day; day-neutral varieties of onions start to bulb around 12-14 hours of sunlight a day, given good top growth before then. Gardeners in Colorado should choose long day and day neutral varieties.
Another thing I’ve learned about the effect of day length on plants is from when I’ve grown leafy greens (spinach for one) through the winter under simple season extension covers. Even though the plants usually survived our freezing winter temperatures, there was always a period of time after the holidays through February where the plants didn’t grow at all. When the days would start to lengthen again in late winter/early spring, the greens would resume growth. However, then they would usually go to seed earlier than spring planted greens.
See the table above for a list of plants and their day length requirements for flowering. We may not want a vegetable plant to flower before harvest (radish for one) so will grow it by adjusting the timing we plant and harvest.