Is it a trick or a treat? Lilacs blooming in the fall

October 4, 2024

Is it a trick or a treat? lilacs in the fall title photo of lilac blooms among small ornamental pumpkins of orange and white

During this first week of October, many of the plants blooming in our landscape are mums, asters, goldenrods, sedums, and ornamental grasses. But what if your lilac is blooming? A lilac blooming in autumn is unusual unless you have planted one of the new varieties bred to bloom once in the spring and then again during late summer or early fall.

When plants behave in unexpected ways, it is often a symptom of unusual growing conditions. In the case of a spring-blooming species producing blossoms in late summer or early autumn, plant stress can be the culprit.

Stressed out plants

Plants experience stress from a variety of sources. Biotic stress is a result of living organisms such as pests or diseases. Abiotic stressors are from non-living sources such as precipitation patterns, temperature variations, heavy pruning, and more. No matter the cause, stressful conditions during the growing season can trigger a plant response that mimics dormancy. This is a plant’s way of minimizing short-term damage so it can persevere long-term when growing conditions become more ideal.

During the summer, spring-blooming species such as lilac, forsythia, ninebark, and rhododendron develop shoots that include the flower buds for the following season. Typically, these plants enter dormancy in the autumn and remain dormant throughout the winter until spring arrives and blooms are produced.

If these plants experience high levels of stress after shoots are produced but before the onset of autumn, there is an increased potential for untimely fall blooms. The stress-induced state of dormancy followed by cooler temperatures and shorter days of autumn may trigger some of the buds to develop into blooms.

This year, our abundant precipitation early in the season provided adequate moisture for landscape shrubs. In west-central Illinois, during August and September, rain became scarce, and temperatures remained high. If plants were not provided with supplemental water during this period, plants experienced drought and heat stress. Although recent weather conditions remain dry, temperatures have dropped causing some plants to react by producing spring blooms despite the autumnal season.

What to do now?

If your spring blooming shrubs are blooming this autumn, there is not much you can do to stop the blooms from developing and otherwise healthy, established plants will not suffer long-term health effects. In the short term, you may experience reduced blooms in the coming spring since the buds have already produced blossoms.

In many areas of the state, the forecast remains bleak in terms of rain. To help drought-stressed plants, providing supplemental water is recommended. This helps rehydrate plants and soils in advance of winter, helping to minimize winter injury. To avoid runoff, use a soaker hose or slow drip to give water time to infiltrate into the soil profile. Continue to monitor soil moisture levels, and provide supplemental water when needed, until the ground freezes.

In future years, when rainfall is limited and temperatures are high, providing supplemental water to landscape plants is recommended. In addition, minimize other stress-inducing conditions by monitoring for pests and disease issues, implementing recommended pruning practices, and mulching the soil with untreated, hardwood mulch.

Good Growing Fact of the Week: Lilacs have a rich history in America’s home landscape. According to the Arnold Arboretum, George Washington and Thomas Jefferson, each made notes documenting their planting of lilacs.