A Linda K. Lillie has been President of Sprigs & Twigs, Inc. for the last 25 years. She is a graduate of Connecticut College in Botany, an accredited NOFA Organic Land Care Professional, a Connecticut Master Gardener and a national award winning landscape designer for her design and installation projects. CELEBRATING 26 YEARS! Welcome to My weekly series! ASK PROFESSIONAL Linda K. Lillie of Sprigs & Twigs Hi Linda, 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. Do you have any thoughts about why they have partially bloomed in Oct/early November?... Greg, Old Lyme Dear Greg. 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. Sprigs & Twigs Very cold air and heavy frosts in the fall are responsible for stopping the complete opening of the flowers. Your partially opened flowers will freeze in the winter and fall off. They will not form new flower buds for next spring blooming. Thank you for your great observation and question. best EMAIL OR MAIL YOUR QUESTIONS TO: info@sprigsandtwigs.net or Linda Lillie, Sprigs & Twigs Inc, PO Box 245, Gales Ferry, CT 06335 SPRICS & TWIGS VOTED THE BEST BY DAY READERS EVERY YEAR! A Linda K. Lillie has been President of Sprigs & Twigs , Inc. for the last 25 years . She is a graduate of Connecticut College in Botany , an accredited NOFA Organic Land Care Professional , a Connecticut Master Gardener and a national award winning landscape designer for her design and installation projects . CELEBRATING 26 YEARS ! Welcome to My weekly series ! ASK PROFESSIONAL Linda K. Lillie of Sprigs & Twigs Hi Linda , 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 . Do you have any thoughts about why they have partially bloomed in Oct / early November ? ... Greg , Old Lyme Dear Greg . 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 . Sprigs & Twigs Very cold air and heavy frosts in the fall are responsible for stopping the complete opening of the flowers . Your partially opened flowers will freeze in the winter and fall off . They will not form new flower buds for next spring blooming . Thank you for your great observation and question . best EMAIL OR MAIL YOUR QUESTIONS TO : info@sprigsandtwigs.net or Linda Lillie , Sprigs & Twigs Inc , PO Box 245 , Gales Ferry , CT 06335 SPRICS & TWIGS VOTED THE BEST BY DAY READERS EVERY YEAR !