Most plants grow buds, flower, or lie dormant at specific times of year.
What Triggers These Seasonal Changes?
Temperature and rainfall often play a role, but the most important factor to many plants is the length of the day. At least, that’s what biologists thought until the 1940s, when the whole concept of day length sensitivity in plants was turned upside down.
Experiments showed that plants that seem to flower in response to day length actually respond to night length. In other words, what prompts irises to bloom in summer in North America isn’t longer sunny days, but shorter nights.
Here’s how biologists figured out how important nightlife can be to plants. They started with the cockle burr plant, which flowers when days are short. Midway through the day, the biologists exposed the plants to a brief period of complete darkness. What effect did this have on flowering? Absolutely none.
Plants At Night
Next, for several nights, in the middle of the night, they exposed the plants to a brief period of light. To the biologists’ surprise, this brief interruption of the darkness inhibited flowering. Clearly it was the long period of darkness and not the short period of light that caused flowering.
Although plants are sensitive to the nighttime part of the light-and-dark cycle, biologists and gardeners often still refer to plants as day length sensitive.
For example, poinsettias are called “short-day” plants because they flower in winter. A stickler for accuracy might call poinsettias “long-night” plants, because what prompts their flowering isn’t the short days of winter, but the long nights.