Lilacs bloom in October on mostly bare branches with some new leaf growth.

We are seeing an uncommon phenomenon this year with a heavy rebloom on some lilacs throughout southern Wisconsin. The rebloom is happening on common lilacs (Syringa vulgaris), one of two lilac species native to Europe and the most common and beloved of the twelve recognized species worldwide.

This fall’s reblooming is due to weather during the growing season, which led to an extreme outbreak this past spring of lilac bacterial blight (caused by the bacterium Pseudomonas syringae pv. syringae). The bacterium persists in the environment, surviving in cracks and crevasses of lilac stems, and only becomes a problem when a wet rainy spring is coupled with cool temperatures around the time of flower bud emergence – as happened this year.

These wet, cool conditions continued through spring and the blight grew unchecked, causing early defoliation on all but a few disease-resistant common lilacs. The early summer defoliation caused the plants to go into a stress-induced dormancy (a deep sleep), similar to how they prepare for winter in late fall.

In early August, the temperature dropped after weeks of high temperatures around 90 degrees to highs closer to 70 degrees, and more than an inch of rain fell on August 6 after a stretch of dry conditions. Two weeks of lower temperatures and the heavy rainfall caused many common lilacs to break dormancy as if it were spring.

Longenecker Horticultural Gardens holds the third-largest lilac collection in the country. Because of the wide genetic variation in the species, some shrubs look like they’re in full spring display and others have only a few open flowers, or none at all. Lilacs set buds for the following year in June, shortly after they have stopped flowering in spring. If they rebloom from stress in the fall, they will not set new buds again until the next spring. This may diminish some of next year’s bloom display, especially on shrubs that are reblooming heavily right now.

Along with flowering, we are also seeing new vegetative growth, including bacteria-free, bright green leaves. The tender new growth and leaves will likely not harden off before the first killing frost this fall. But if freezing temperatures hold off for a while, the energy the plants gain through photosynthesis in new leaves could outweigh the energy they expended to break dormancy and resume growth. Fortunately, it will take more than one season of defoliation and reblooming to cause these extremely hardy plants to perish.

—David Stevens, Longenecker Horticultural Garden curator