What makes plants bloom at different times of the year?

flowerhead in the sunset
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As the chill of fall finally settles in, many Illinoisans find themselves outside cleaning up leaves, the garden, and landscape beds. 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?”

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. So what is the flower trigger? 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.

First Stop: Plant Lifecycles

One way to separate flower timing is to distinguish between plant lifecycles. In the world of flora, we have summer annuals, winter annuals, biennials, and perennials. Yes, this does not necessarily explain what causes a plant to flower but can be a general starting point.

When it comes to annuals (summer or winter) they have one season to reproduce. These plants do not invest much to create an expansive root system. Because making roots and making flowers costs energy. Annuals typically expend their energy on flower and seed production. A primary distinction is that summer annuals germinate in the spring, flower in the summer, and typically die by the fall. Winter annuals are pop up as they germinate in the fall, hang out under snow cover all winter and then flower and produce seed in the early spring.

Biennials are alive for two years. The first year is spent growing foliage and building the necessary energy for flowering. Then the second year a flower stalk shoots in the air to create seeds for the next generation and then the plant dies.

Perennials survive year after year, so they have time to invest in their root resources before needing to flower. When grown from a seed a perennial may not flower for its first two to three years. That’s because there’s a lot of work happening below ground on building a root system. Perennials often (but not always) flower in the middle to later in the growing season.

Competition

Yet even with these lifecycle categories plants are flowering at different times of the year. If you haven’t picked up on it yet, nature is pretty focused on competition over resources. One resource would be space. If every plant grew and flowered at the same time that would be quite a demand on the physical area, like soil. The same goes for pollination. A plant may have more access to pollinators if they evolved a flowering timeframe 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 photosynthesize to gather energy for next year all before the trees leaf out that would otherwise shade them out.

Diving Deeper: Photoperiodism

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.

As the seasons shift, temperature and sunlight drive the chemical production 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). Temperatures vary with the weather, but the cycle of the Sun overhead has been going steady no matter what the weather holds. This makes for a reliable method to plan out your lifecycle. That is if you were a plant rooted in one spot your entire life.

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.

We can categorize plants based on their response to the photoperiod. There are short-day plants, long-day plants, and day-neutral plants. The short- or long-day categories do not have specific day lengths. The names are as they suggest. Short-day plants bloom as the length of day is less than the length of night. Long-day plants bloom when the day is longer than the night.

Day-neutral plants do not depend on a photoperiodic response to initial flowering. These plants simply flower after reaching a certain point in their vegetative growth stages like corn or tomatoes.

Regardless of day or night length, a plant must reach ripeness-to-flower, which is a minimum vegetative state where the structure of the plant can support flowers and fruit and the plant has the necessary energy supplies to create viable offspring.

For years scientists thought it was exposure to light that triggered flowering, 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 differently than other nearby plants? You might have a photoperiod-sensitive plant.

Diving Even Deeper: Phytochrome and florigen

If you’re still with me, thanks! At this point, I’ve pulled out the old plant physiology textbook and flashcards. Yes, I kept my flashcards. Doesn’t everybody? Sifting through my flashcards on flower initiation, there are quite a few on light quality and phytochrome.

Now, this is a blog post, not a semester-long crop physiology class, but I know someone will have read to this point and thought, “What about phytochrome?” Okay, I’ll be brief. If you’d like to learn more, there are some great books out there that cover this in detail. Check the reference section below.

Examining a rainbow, we can see the visible spectrum of light - red to violet. However, on the edges of the rainbow is radiation we cannot see with our eyes, but plants can sense using a pigment called phytochrome.

Light quality or levels change throughout the year. On June 21, the longest day of the year for us in the Northern hemisphere, there is a lot of direct sunlight hitting the planet. As the planet shifts on its axis to autumn, the quality of sunlight also changes as the light waves pass through more of the atmosphere. Therefore, not only does the duration of light change, so too does the quality of light change.

This shifting pattern of red and far-red light is picked up by the pigment phytochrome which releases the plant hormone florigen in the plant leaves. Florigen moves to the appropriate location within the plant (often the branch tips) and alters the vegetive cells into flowering cells thereby forming flower buds. These may open immediately or wait until next spring. This all goes back to those plant lifecycles mentioned way back at the beginning.

Whether it’s triggered by light, dark, life cycle, maturity, and so much more, 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.

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!) for six to eight weeks to bloom and show off its colorful bracts.

References

Botany for Gardeners by Brian Capon

How Plants Work: The science behind the amazing things plants do by Linda Chalker-Scott

Lecture notes from Crop Physiology, SIUC, instructor Nathan Johanning

Textbook Plant Physiology 3rd Edition, by Lincoln Taiz and Eduardo Zeiger

 

MEET THE AUTHOR
Chris Enroth is a horticulture educator with University of Illinois Extension, serving Henderson, McDonough, Knox, and Warren counties since 2012. Chris provides horticulture programming with an emphasis on the home gardener, landscape maintenance personnel, and commercial landscapers. Additional responsibilities include coordinating local county Master Gardener and Master Naturalist volunteers - providing their training, continuing education, advanced training, seasonal events, and organizing community outreach programs for horticulture and conservation assistance/education. In his spare time, Chris enjoys the outdoors, lounging in the garden among the flowers (weeds to most).