Why lilacs are blooming out of season in Iowa

Typically a harbinger of spring, lilacs have made an off-season appearance in Iowa this August and September.

Lilac bushes that lost their foliage over the summer have blossomed in pale purples as the season ended.

Just as stress brought on by Iowa’s 2020 hurricane-strength derecho led spring-blooming trees to blossom in the fall, the fragrant favorite, which normally blossoms in May, has been “tricked” into blooming for a second time.

Related: Derecho damaged trees bloom out of season

Aaron Steil, specialist at Iowa State University’s Consumer Horticulture Extension, notes that stressful growing conditions typically are the reason for untimely bloom.

“Occasionally, spring-blooming shrubs and trees, like lilac, crabapple, magnolia, or forsythia, will bloom out of season during the fall months,” Steil wrote. “This out-of-season bloom is usually triggered by stressful environmental conditions during the summer growing season such as heat, drought, severe defoliation from disease or pest, and/or heavy pruning.”

He noted that these spring-blooming plants set their flower buds for the following year in early summer shortly after the previous spring blooms fade.

While Iowa avoided drought this summer, other conditions can cause lilac bushes to drop their foliage, such as inconsistent moisture or over-saturated soil in the spring, which may lead to foliar diseases.

ISU Extension notes that fungal leaf spots alone rarely become severe enough to cause the decline or death of the plant. Removing and destroying leaf debris and pruning out dead branches may be the best strategy in reducing diseases in general for shrubs and trees.

More: Lilacs turning a whiter shade of pale

A lilac blooms in rural Chelsea, Iowa, in September 2024. (photo/Cindy Hadish)

“Stressful conditions put the plant in a dormant-like state and when the cooler temperatures of fall arrive, some of the flower buds are triggered to bloom,” Steil wrote on the Extension’s website. “This untimely bloom will not harm the plant, although there will be fewer flowers on the plant the following spring. Once fall blooms are observed, there is nothing that can be done to fix the issue. In future years, good care when there are stressful growing conditions will help reduce the chance of out-of-season bloom. This could include irrigating during periods of drought, managing severe disease or insect pests, and avoiding drastic pruning.”

Steil cited some spring-blooming species that have varieties or cultivars selected because they will bloom in fall – although that late-season bloom is not typically as floriferous as the spring bloom. “Bloomerang lilac (Syringa ‘Penda’) is one example,” he wrote. “These plants are intended to bloom in the fall and are often marketed and sold as special cultivars that regularly have blooms in both spring and fall. Fall blooms on these varieties are normal.”

Michigan State University Extension educators who have seen the phenomenon with apple trees during past autumns in their state, note that stresses such as drought, which causes growth to stop and start, can confuse woody perennials and cause out-of-sync bloom.

A similar occurrence happened after Hurricane Irene blew through North Carolina, when ornamental pear trees rebloomed after losing their leaves.

If you are considering replanting a tree lost in the derecho, see this list of some of the top native trees to plant in the Midwest.