With the days getting longer, many local gardeners are ordering seeds from their favorite catalogs or beginning their starts in trays indoors. While many think it’s the length of the day, and how much light a plant gets, that determines how well it blooms, experiments have proven its the length of nighttime, the length of uninterrupted darkness, that determines the formation of flowers in most plants.
Oregon State University Extension Service horticulture expert, Kim Pokorny, explains that to understand plant flowering, you need to get a handle on “photoperiodism” – a plant’s response to night’s length. The experiments showed that if the length of darkness is interrupted, this impacts flowering compared to if the length of daylight is interrupted, no differences were shown.
According to Nicole Sanchez, of the OSU Master Gardeners Program, some plants absolutely must have a specific night length to bloom, for others, a specific night length is more likely to lead to flowering . Some plants are day-neutral, their flowering unrelated to night length.
A plant that requires a long period of darkness is termed a “short-day” (long-night) plant. Short-day plants form flowers only when day length is less than about 12 hours. Many spring- and fall-flowering plants are short-day plants, including chrysanthemums, poinsettias and Christmas cactus. If these are exposed to more than 12 hours of light per day, bloom formation does not occur.
Other plants require only a short night to flower. These are termed “long-day” plants. These bloom only when they receive more than 12 hours of light. Currently, days are getting longer by several minutes per day, and will continue to do so for the next few months. This initiates flowering in long-day photoperiodic plants such as carnations, Sweet Williams, snapdragons, sunflowers, as well as dill and its relatives, parsley, fennel, and cilantro.
Pokorny goes on to explain that “some plants form flowers regardless of day length. Botanists call these ‘day-neutral’ plants. Tomatoes, corn, cucumbers and some strawberries are day neutral. Some plants, such as petunias defy categorization. They flower regardless of day length, but flower earlier and more profusely with long days.”
Photoperiodism is commonly used in floral businesses.
Sanchez says, “Commercial poinsettia growers draw black-plastic curtains around their plants at night to block out accidental light sources. Conversely, interrupting the night with a few minutes of light to break up the cycle is used by greenhouse ornamental growers to control bloom time and deliver plants in peak bloom when consumers demand them.”
To get a jump on blossoms while waiting for the “long day” period to start, local gardeners can bring a long-day plant into bud formation and eventually bloom early by “adding supplemental day length” with the use of indoor grow lights a few hours each day. Next Fall, starting 8-12 weeks prior to the winter holidays, take this year’s trimmed poinsettia and start the “short day” period early by placing it in a dark closet to add supplemental night length.
By Stacey Newman Weldon
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