Triggering plants to bloom

Ever wonder what exactly triggers a plant to bloom? Most of us probably understand it to be some combination of environmental factors such as heat and sunlight, along with the maturity of the plant. In fact, that is essentially what makes most plants flower. While many plants respond to a combination of heat and light, some plants are controlled much more strongly by one factor or the other.
 For example, plants like poinsettia, chrysanthemum, and Easter lily are so beholden to day length that one can make those plants bloom almost any time of year in a controlled environment by manipulating how many hours of continual sunlight vs. continual darkness they are exposed to. In college, I worked for a professor who made me do just this to trick fall-blooming chrysanthemums into blooming in May.

Other plants are controlled more by temperature, including cherries (both ornamental and fruit producing types), whose bloom time in the spring you can predict fairly accurately by measuring what are known as ‘chilling hours.’ This term refers to the number of hours a plant is exposed to temperatures above 32 degrees but below 45 degrees Fahrenheit, i.e., cold but not freezing temperatures. Once a plant has accumulated its desired number of chill hours in winter, it begins to initiate flowering.

Above: Yoshino cherry trees are beautiful during the 10-14 days in spring that they are in bloom.
Photo: J. Robbins, CC BY-NC-ND 4.0
Right: The Okame cherry tree
after bloom, in early spring.
Photo:  C. Tierney, CC BY 4.0

The three most popular ornamental cherry varieties in our area have long been ‘Okame,’ ‘Yoshino,’ and ‘Kwanzan.’ ‘Okame’ is the first to bloom because it requires the fewest chilling hours, roughly 300-400 hours, and thus accumulates what it needs the fastest. ‘Yoshino’ needs about 500-600 hours to bloom, and Kwanzan needs between 700-1000 hours. Here at the Pitt County Arboretum, our ‘Okame’ cherries bloomed about 3 weeks ago, while our ‘Yoshino’ cherries have just started their bloom within the last few days. Our one ‘Kwanzan’ cherry is still a way away from blooming.

While this concept helps predict when a tree will bloom, it also helps decide what trees might be appropriate or not appropriate for a given climate. Most fruit-bearing cherries need more chilling hours than our climate reliably provides, and thus they don’t typically perform well for us. While the three ornamental varieties do have some insect and disease pests and can be short-lived, they are generally better suited to our climate.

Left: The Kwanzan cherry is one of the showiest cherry trees when it blooms  in spring. Photo: Myrabella, CC BY-SA 3.0  Right: Leaves and blooms on the Kwanzan cherry tree. Photo: D.J. Stang, CC BY-SA 4.0

If growing any of these varieties, plant them in full sun to part-shade locations with well-drained soil. ‘Okame’ and ‘Yoshino’ have white to very light pink single flowers, while ‘Kwanzan’ has double pink flowers. ‘Okame’ has slightly more of a spreading branch structure and will grow 15 to 30’ high and roughly equally wide, while both ‘Yoshino’ and ‘Kwanzan’ will grow 30-40’ tall with a spread of 20 to 30’. None of the three do much of note other than flower, but each is tremendously beautiful during the 10-14 days of its spring bloom each year.
 
Matthew Stevens is the County Extension Director and Horticulture Agent for North Carolina Cooperative Extension’s Pitt County Center. If you have questions about this article or gardening in general, please contact the Pitt County Extension Master Gardeners at [email protected] or 252-902-1705.