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What makes plants bloom at different times of the year?
How do plants know when to do, what it is that they do? It’s not like they have a smartphone with calendar notifications. But if you observe a landscape from spring to fall, some plants flower early on, or middle of the season, or even right until they are killed by a fall frost. It makes one ponder the seasonality of plants. One Good Growing reader had such a question and posed it to us, “How do plants know when to flower?” When I posed this question to my wife, she said it must be “singing and love.” I certainly don’t want to discourage singing to your plants, but that’s probably not the trick to getting them to flower.
One way to separate flower timing is to distinguish between plant life cycles. In the world of flora, we have summer annuals, winter annuals, biennials, and perennials. All growing and flowering at different times of the year.
Evolution plays a significant role in flowering. If you haven’t picked up on it yet, nature is focused on competition over resources. Consider pollination. A plant may have more access to pollinators if they evolved a flowering time frame outside of when everyone else is flowering.
Spring ephemerals are a great example of this evolution due to competition. Woodland spring ephemerals, such as Virginia bluebells, are perennials that pop up early in the spring, bloom, grow, and photosynthesis to gather energy for next year all before the trees leaf out that would otherwise shade them out.
Lifecycles and evolution aside, what happens inside a plant that triggers flowering? Well, there’s quite a bit happening inside of the plant, and much of that related to flowering is driven by environmental factors. Plants do not have calendars, but they can respond to environmental changes, some being temperature, day length, and light quality.
Temperature and sunlight drive the chemical production occurring inside the plant leaves. These different chemicals trigger different growth responses. Temperature certainly has a seat at the flower power table, but light is the key for plants whose flowering is influenced by the photoperiod (duration of light and dark).
Not all plants have a photoperiodic response to produce blooms, but a few plants known for being dependent on photoperiodism include the fall-flowering chrysanthemum and New England aster. Carnations utilize photoperiodism, allowing greenhouse growers to manipulate the lighting, and is why florists always have flowering carnations available.
For years scientists thought it was exposure to light that triggered flowering in photoperiodic plants, but it has been documented that exposure to darkness plays more of a role in flowering. Ever had a plant near a streetlight or landscape lighting that flowered or grew differently than other nearby plants? You might be dealing with a photoperiod-sensitive plant.
Whether it’s triggered by darkness, lifecycle, maturity, or so on, let’s be happy plants have evolved to spread their blooms out over time so we can have something colorful from spring thaw to fall frost. Now, I need to serenade my mums.
■ Good growing Tip of the Week: Poinsettia, a tropical, must receive 6-10 hours of light and 14 hours of darkness (absolutely no interruptions to the dark!) to bloom and show off its colorful bracts.