Did you see plants blooming this fall when you didn’t expect it? “Some of them were bred to do that,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum. “And some of them were just confused by the warm weather.”
Plants rebloom for a variety of reasons, but weather is a big factor.
A cool spell in early autumn, followed by temperatures in the 60s and 70s that persisted deep into October, tricked some spring-blooming plants such as forsythia into setting a few blooms this fall, six months too soon.
“Forsythia sets its flower buds shortly after it blooms in early spring,” Yiesla said. Unlike some spring-blooming trees and shrubs, it doesn’t require a long chilling period in order to bloom. “So just a little cool weather followed by a stretch of warmth was enough to make a forsythia think it was spring.”
Such stray blooms are not an uncommon sight when autumn weather is warm, she said. “Because the climate has changed to make our fall weather much more variable, I expect we’re probably going to see that kind of thing more often.”
Confused shrubs or trees will usually open just a few flower buds before cooler weather puts a stop to it. “Even if a rhododendron or weigela opens a few of its buds in October, there are plenty more left,” Yiesla said. “Enough flower buds will still overwinter for a satisfying show in spring.”
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Other plants sold in garden centers may bloom twice in summer and then keep on going into fall because they have been bred and selected for that trait.
For example, older varieties of roses bloomed only for a few weeks starting in June. Now, many cultivated varieties have been developed that will bloom again and may continue off and on until frost.
The Bloomerang® series of lilacs was also developed to bloom first in late spring and again a few months later.
“The later bloom in reblooming cultivars is usually more sparse than that first big burst, but you’ll still have a sprinkle of blooms through the season,” Yiesla said.
Some cultivars of perennials are bred to rebloom, and among them some, such as the familiar orange Stella D’Oro dwarf daylilies, will keep coming back over a long period. Some perennial plants, such as coreopsis, hardy geranium and phlox, will rebloom if you cut them back after they first flower. Others, such as bleeding heart and black-eyed Susan, are simply long-blooming by nature.
Common witch-hazel, a small tree native to the Chicago area, is one of the few woody plants that naturally comes into bloom in autumn each year, with wispy yellow flowers.
If you’d like more plants with repeat bloom or flowers that last late in the season, shop for them carefully. “The traits of cultivated varieties vary quite a bit,” Yiesla said, so you can’t assume that what was bred into one cultivar applies to any other cultivar or species. Not all roses will rebloom the way Knock Out® roses do.
Read plant labels carefully and investigate claims before you buy. “And if you get a few rogue blooms in fall in a place where you weren’t expecting them,” she said, “just enjoy them.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.