If you reside somewhere north of the equator but south of the north pole, you may have noticed the shortening and darkening of our days lately. In fact, the days have been getting shorter ever since June. Shorter days with less sunlight coincide with cooling temperatures and increased precipitation, all of which hinder plant growth and maturity to a certain extent. Anyone who has tended plants for even a few days knows that temperature can be a fickle friend and sometimes foe; but plants also have a significant relationship with light and dark – one that is more complex, and often not as well-understood by growers. A Brief Terminology Lesson In order to better understand how day length affects plant growth and what you can do to maximize or minimize the effects of photoperiodism based on your desired plant performance, first familiarize yourself with these important terms: Photoperiodism: The physiological reaction and/or developmental responses of a plant to the relative lengths of daylight and darkness it experiences. Photoperiod: The recurring cycle of uninterrupted light and dark periods a plant is exposed to; usually 24 hours, with varying ratios of uninterrupted light and dark periods. Long-Day Plant (LD): A long-day plant requires >12 hours of sunlight, or <12 hours of uninterrupted darkness, to produce a bloom or flower. Short Day Plant (SD): A short-day plant requires <12 hours of sunlight, or >12 hours of uninterrupted darkness, to produce a bloom or flower. Day-Neutral Plant (DN): Day-neutral plants do not initiate flowering based on photoperiods. Many of these types of plants instead flower after reaching a certain developmental stage or age, or in response to other environmental factors such as vernalization. These classifications of photoperiodism have been acknowledged by botanists for many years, but it was not until the mid-twentieth century that crop researchers first came to the understanding that it is actually the length of uninterrupted darkness experienced by a plant - rather than the length of daylight - that is the most crucial to its development. For example, a short-day plant that requires >12 hours of uninterrupted darkness to initiate flower production will still produce a flower if it is shielded from light for some time during its 10-12 hours of daylight exposure; it will not, however, produce a flower if it is exposed to light for a period of time during the 12+ hours of darkness it requires. Here is a list of some common annual vegetables and herbs and their classifications (SD, LD, or DN):
Crop Classification (Short, Long, Neutral)
Beets Long
Brussels Sprouts Neutral
Cabbage Neutral
Carrots Long
Corn Neutral
Cucumber Neutral
Fennel Long
Kale Neutral
Lettuce Long
Pea Neutral
Potatoes Long
Radish Long
Soybean Short
Spinach Long
Sunflower Neutral
Swiss Chard Long
Tomato Neutral
Turnips Long
  How Does Photoperiodism Affect Your Farm or Garden? In order to maximize a plant’s productivity, it helps to understand the light conditions under which it will experience vegetative growth (i.e. leaves) versus reproductive growth (i.e. flowering, or bolting). Because photoperiodism is what determines reproductive growth in some plants, an understanding of how these effects translate to plant growth can be a useful tool for growers to have. Hinona Kabu turnips flowering in a 2015 seed crop at High Mowing's seed production greenhouse. Turnips, a long-day crop, require <12 hours of uninterrupted darkness to initiate flowering. Take turnips for example, a long-day plant. Remember the definition of a long day plant: this means that with >12 hours of daylight and <12 hours of uninterrupted darkness, a turnip plant will initiate reproductive growth and attempt to flower. How does this come into play in your farm fields and garden plots? Say a farmer here in northern Vermont plants a crop of Purple Top White Globe turnips in the spring, around May 10th. This particular variety takes about 55 days to reach harvest maturity. 55 days from May 10th is July 4th. Let’s take a look at the recorded day length on each of these days in Wolcott, Vermont:
Date Day Length
May 10th, 2017 14h 37m
July 4th, 2017 15h 26m
  What will happen to this turnip crop as it experiences continuous photoperiods with >12 hours of daylight? It will attempt to flower – an undesirable result for a turnip, which is harvested for its root. When flowering, or bolting, occurs, the root becomes woody and unpalatable. However, if the farmer instead plants these same turnips in August, for a planned harvest in October, what would that look like?
Date Day Length
August 10th, 2017 14h 15m
October 4th, 2017 11h 32m
  As the length of day diminishes and dips back under 12 hours, the turnips are no longer at risk for experiencing the developmental response of reproductive growth. Thus, this variety of turnip performs with much better results in the early fall than it does in the late spring here in northern Vermont. You can use this information to determine the best day length conditions for each of your crops based on your location. There are many factors to consider in crop production, of which day length is just one. However, if you have ever attempted to grow a certain crop with all other factors being adequate (temperature, water access, soil fertility) and have experienced less than ideal results, day length could be a contributing factor. Use these classifications as a guide to help you determine when the ideal time of year is for planting certain crops based on your growing location. Considering the 10-Hour Day No matter its sensitivity to day length and darkness (LD, SD, or DN), a plant will not experience growth with fewer than 10 hours of daylight. Even if day-neutral plants like cucumbers, tomatoes and lettuce are grown in climate controlled environments with enough heat to grow and produce, they will halt their growth and remain dormant when day length dips below 10 hours. When the 10-hour day returns, plant growth begins again. These lettuces were transplanted into our greenhouse in northern Vermont in early October when daylight was still between 10 and 11 hours per day. Their growth is now halted as our days contain <10 hours of daylight. The plants will hold dormant in this protected culture until days with 10+ hours of daylight return to our region around February 1st. This factor is particularly important for growers seeking to extend their season with greenhouse production. There are supplemental lighting techniques that can help day-neutral and long-day plants continue to grow and produce during the shortest days of the year, including lengthening the day length and night interruption lighting. However, the cost of these techniques should always be taken into consideration when planning for extended crop production. For growers wishing to explore this as a production option, it is important to know the annual stretch of time when your location is experiencing <10 hour days; here is a sample of some U.S. cities and the dates of their days with fewer than 10 hours of daylight:
U.S. City Range of Dates With <10 Hours of Daylight
Portland, Oregon November 3rd—February 6th
Burlington, Vermont November 5th—February 5th
Detroit, Michigan November 8th—January 31st
Washington, District of Columbia November 16th—January 24th
St. Louis, Missouri November 17th—January 23rd
San Francisco, California November 20th—January 20th
Dallas, Texas December 16th—December 26th
  Day length is determined by your location’s latitude. On the equator, day length and night length are always equal. In the Northern hemisphere’s winter, days are longer the closer you are to the equator; in the Northern hemisphere’s summer, days are longer the farther you are from the equator. Remember: day length is just one of many factors to consider when planning your crops. A Note About Onions If you are an experienced grower of onions, you may be wondering where they fit into this discussion of long versus short day plants. Because onions are dependent on day length for bulb formation, they have their own classifications of long-day, short-day and intermediate-day categories. By the definitions of LD, SD and DN in relation to photoperiodism, all onions are long-day because they all require >12 hours of daylight to form a bulb. The distinguishing factors are as follows: short-day onions start forming bulbs with 11-12 hours of daylight. Long-day onions need 14-16 hours of daylight to start forming bulbs. Intermediate-day onions need between 12-14 hours of daylight for bulb formation. Thus, if you live in a location where the longest day length hovers around 14 hours but doesn’t get much longer, long-day onions will not perform as well as short-day or intermediate-day onions.