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Reblooming plants reflect the growing season

Hedge of lilacs in fall with a few random purple flowers blooming.
Lilacs blooming out of season are referred to as being "remontant." Photo: Allan Paul Eilen

This year, as last 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. 

These are 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. 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.

According to Minnesota Weathertalk it was an extremely hard summer for plants of all kinds:

  • 2021 had the fifth warmest summer on record for Minnesota. 
  • We had the tenth driest year in Minnesota history dating back to 1895.

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!

Author: Julie Weisenhorn, Extension horticulture educator

This is a revision of an article that first appeared in October 2020.

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