At the vernal (spring) equinox, the sun shines directly at the equator, making it one of two times in the year when days and nights are of roughly equal length. It often awakens a sense of hope and expectation from us, as we look forward to longer, warmer days ahead. But what about our plants?
“Photoperiodism” refers to the amount of time in a twenty-four hour period that we are exposed to light, and changing photoperiods can trigger a lot of physiological changes–in plants as well as mammals. Botanists initially thought that the amount of daylight a plant was exposed to determined whether it would form flowers. But subsequent experiments proved otherwise: It’s the length of darkness experienced by the plant that plays the most crucial role. Nevertheless, many flowering plants continue to be identified as “long-day” or “short-day”.
“Long-day” plants respond to increasing daylight and correspondingly shorter nights. As the number of hours of sunlight heads toward its peak, these plants switch from vegetative growth to reproductive growth, and the onset of flower bud formation is triggered. Many of our favorite summer-blooming native perennials (such as Rudbeckia) fall into this category.
“Short-day” plants are keeping time, too, and initiate flowering when day length is less than about 12 hours. In our pollinator gardens, asters (Symphyotrichum) are an example of this.
Spring or fall, the best strategy is to include both long- and short-day plants in your pollinator garden, to ensure that plentiful supplies of pollen and nectar are available all season long.
