top of page

#BotanistLens: Summer solstice and photoperiodism

Happy summer! Come June end, here in the Mid-Atlantic, most seasonal gardens will see sprouts of summer color with Gladioli, Rudbeckias, Cosmos and many more bright colors. Day-length and length of darkness within a 24-hr period and how it impacts flowering time, is intriguing to say the least.

"The fact that the length of light and darkness in a 24-hour period has an effect on plants was researched thoroughly back in the 1900s, and the term “photoperiodism” was created to describe the phenomenon. The discovery of photoperiodism happened in 1920 when two employees of the U.S. Department of Agriculture discovered a genetic mutation in tobacco. Plants possessing the mutant gene bloomed in December instead of flowering in summer as normal tobacco plants do. Experimenting with artificial lighting in winter and artificial darkening in summer, they found that the mutant plants would only flower when exposed to the long nights that naturally occur in winter. They called them “short-day” plants" (

Classic short-day plants examples are: Cosmos, Zinnias and Marigolds. Common long-day plants examples are: Rudbeckias, Petunia, Salvia, etc. Some common day neutral plants (wherein the flowering is not regulated by photoperiod) examples are: Roses, Strawberries, Cucumbers, etc. Even though the terms 'long-day' and 'short-day' plants is commonly used, it is the length of darkness within a 24 hour period that affects bloom time.

Another fun fact: "Plants can further be described as having a facultative or obligate photoperiod response. Plants with a facultative response flower faster under a particular photoperiod but will eventually flower under all photoperiods. For example, a facultative short day plant will flower faster under short days but will eventually flower under long days as well. Plants with an obligate response absolutely require a particular photoperiod for flowering" (

Photoperiodism is manipulated for greenhouse production of many annuals and perennials. Either the vegetative growth is bulked up or the flowering time is modified. Good understanding of the length of darkness required for a particular species for maximum production value is extremely crucial for growers.

Change in length of darkness is also key for gardeners as they are indicators of what annuals and perennials will be in bloom and for how long. Currently at London Town, the Gladiloi, Boltonias, Crocosmias, Irises, Rudbeckia maxima and Jewel weeds are in bloom. Summer solstice is a reminder of nights getting longer and all the bright summer blooms that will initiate with this change in photoperiod. Here at London Town, we await the bright summer blossoms of Lilies, Asters, Mallows and many other gorgeous pollinators.

49 views0 comments


bottom of page