This year, Extension had a fair number of calls and emails from homeowners asking why they have seen lilacs blooming in late summer and early fall. Recently, I was at my parent’s house and walking around the yard with my mom. “Why are my azaleas blooming?” she asked. Sure enough, a few blossoms on each of her four 'Lemon Lights' azaleas had opened up. Later that week, I was up near Lake Vermillion and a few bunchberries were blooming here and there.
These are all spring-blooming plants, so why were they blooming in September?
Like so much in Minnesota, the weather has a lot to do with this phenomenon.
Environmental stress such as heat and drought may cause plants to respond in a variety of ways. For example, plants may flower and produce a great deal of seed, called “masting, " due to stressful environmental conditions. U of M Extension horticulturist Mary Meyer shared that plants that rebloom are termed “remontant” when they flower a second time in one growing season.
Neil Anderson, in the UMN Department of Horticultural Science, filled me in on how plants become remontant. Spring blooming plants produce or “set” their buds soon after blooming. If bud set is followed by environmental stressors such as heat or drought and then a cold period simulating winter, flower buds may break dormancy and open.
This can occur with many different perennial plants in the landscape when a plant’s minimum number of cold (chilling hours) has been met. Plant hormones that promote dormancy and prevent the buds from opening are broken down during the cold period, releasing the flower buds to open. Voila! Lilacs in autumn.
Looking back over the past growing season:
- 2020 was in the top 4 of the warmest summers on record for Minnesota. Heat and humidity levels set records in June and July followed by a warm, wet August.
- These warm conditions were followed by record cold at the beginning of September. (Source: Minnesota WeatherTalk)
This chilling weather was followed by a warm-up where some areas of the state saw temperatures in the high 80s.
The good news is that late blooming won’t significantly affect next year’s spring bloom, since it usually only affects a few, but not all, of the flower buds developed for next year.
It’s a good idea to note these kinds of observations in your gardening journal and record how the plants perform next spring. In the meantime, enjoy lilacs in autumn!