Late Bloomers

Lilacs blooming in September? Apple blossoms in October?

Perhaps you have seen spring blooming plants flower this fall in your garden. It has not been uncommon in the past few years to see spring flowering shrubs, such as forsythia, magnolia, apples, crabapples and lilac flowering in the fall. The reason is usually stressful weather conditions such as drought or high temperatures. These conditions induce dormancy or reduced growth. When the conditions improve, with cooler fall temperatures and rainfall, the plants are stimulated to grow. And what is on the plants to grow are the flower buds which, for these spring blooming shrubs, are formed in June and July.

Stressful weather conditions have been common recently in the upper Midwest. Drought conditions have been the “new normal” for the past 3 years in Minnesota. And how about the heat? A “normal” of 13, 90-degree days was far surpassed with 33 for Minneapolis in 2023. The hot and dry conditions slowed growth and induced a dormancy for many plants.

Another issue has been a leaf spot disease on lilacs. With warm, humid nights and high humidity many lilacs developed brown leaf spots (identified as Pseudocercospora—see U of M Extension Lilac Diseases). With fewer or no leaves, these plants also went into a dormant condition with slowed growth. Then with fall cool temperatures and wet conditions, the plants were stimulated to grow and the flower buds opened.

The fall blooming phenomena does not harm the plant. Usually only a small percentage of the flower buds develop in the fall, and the majority remain dormant and flower normally next spring. Buds that develop and flower in the fall reduce the number of flowers for the following spring. These plants will still develop leaf buds and grow normally next spring, and it will set new flower buds in the following summer. Going into a dormant state with stressful conditions is a coping mechanism for the plants to escape and avoid growth when resources such as water are reduced. Lilacs, apples, and crabapples are cold weather plants that do not like high temperatures, they prefer to grow in northern climates, and you do not find them in warm climates; heat and high temperatures are not good for these more temperate plants.

There is one lilac, ‘Bloomerang^® Purple’ that was bred and selected because it typically blooms again in the fall.  This hybrid was introduced in 2009, it blooms in spring, takes a short break, and then blooms again in summer into fall. It is also more compact growing only to about four or five feet tall and wide.

If your spring blooming shrubs do bloom in the fall, just enjoy the flowers and make a note that you might not see as many flowers the following spring. Supplying additional water during drought may reduce the chance of fall flowers, but most of these shrubs are well adapted and resilient once established. And if warmer summers and drought conditions happen more often, we might see more spring flowers showing off in the fall.

Dr. Mary Hockenberry Meyer is a University of Minnesota Horticultural Science professor emerita. She is the Grass Collection curator at the Minnesota Landscape Arboretum and loves writing about plants and horticulture.