We have two rhododendrons that have partially bloomed the past three years in late
October/early November. The flowers do not fully display, but at least half the buds do start
to open. The other rhododendrons around our house wait until spring to show their colors.
Do you have any thoughts about what we are seeing? Greg, Old Lyme
There are two primary environmental factors that affect plant growth and flowering of plants, temperature and daylight/darkness length. Many plant species have day length 'switches' that determine when they grow leaves, stems, etc. and when they flower. Photoperiodism is the term used to describe the plants reaction and response to the relative lengths of daylight and darkness.
In the past, scientists believed that daylight length determined whether a plant would form flowers. Over time, that proved not to be true. It is the amount of uninterrupted darkness that determines the formation of flowers on most types of plants. A plant that requires a long period of darkness is termed a "short day" (or long night) plant. Short-day (long night) plants form flowers only when daylight is less than 12 hours and darkness is greater than 12 hours a day. Rhododendrons and many spring and fall flowering plants are short day (long night) plants. Your rhododendrons are blooming now in response to the increasing length of darkness at this time of year that is similar to the length of darkness in the spring when they normally bloom.
Many of our summer blooming flowers and garden vegetables are "long day" (short night) plants. These bloom only when they receive less than 12 hours of darkness each day.
The very cold air and heavy frosts we have experienced this fall are responsible for stopping the complete opening of the flowers. Your partially opened flowers will freeze this winter and fall off. They will not form new flower buds for next spring blooming. Thank you for your great observation and question.
View As PDF
To view a PDF of this article, please
About Linda Lillie
Linda K. Lillie is the President of Sprigs & Twigs, Inc, the premier
landscape design and maintenance, tree care, lawn care, stonework, and carpentry
service provider in southeastern Connecticut since 1997. She is a graduate of
Connecticut College in Botany, a Connecticut Master Gardener and a national
award winning landscape designer for her landscape design and landscape installation work.