Why is my lilac blooming in fall, instead of spring?

FAQ
Question: 

Why is my lilac blooming in fall, instead of spring?

Answer: 

Occasionally, spring blooming shrubs and trees, like lilac, crabapple, or forsythia, will bloom out of season during the fall months.  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.  

These spring blooming plants set their flower buds for the following year in early summer shortly after the previous spring blooms fade.  Stressful conditions put the plant in a dormant-like state and when the cooler temperatures of fall arrive, some of the those flower buds are triggered to bloom.

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 the out-of-season bloom.  This could include irrigating during periods of drought, managing severe disease or insects pests, and avoiding drastic pruning.

Some spring blooming species have varities or cultivars that have been selected because they will bloom in fall - although that late season bloom is not typically as floriforous as the spring bloom.  Bloomerang® lilac (Syringa 'Penda') is one example.  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.