It is not unusual for some plants to blossom out of season. Magnolia, crabapple, lilac, and forsythia are notably spring-blooming plants, but stressful growing conditions can instigate a type of dormancy that pushes flowering to later in the season. Lilacs are a great example this year.
To better understand why this happens, it is helpful to know when flower bud formation takes place. As soon as the current flowers fade, trees and shrubs begin the process of making flower buds for next year. These buds are stored within the wood, fully formed just beneath the bark, during the remainder of the summer and extending through the winter. In a “normal” growing season (whatever that is!) flower buds will break dormancy in the spring and emerging buds will open. Environmental conditions and ill-timed pruning will disrupt the timing of flower bud emergence.
It’s important to note that the late flowering of a lilac does not indicate a dying plant. It does, however, point to something being off, whether it is manmade or environmental. One factor, pruning late in any growing season, is going to disrupt flowering the following year because mid-summer to late fall pruning removes flower buds that have already formed. Drought, deep cold, late frosts, and extreme springs are environmental factors that throw off the plant’s internal flowering clock as well.
For this year, the negative double-digit freeze of February followed by the cold wet spring segueing into hot summer conditions were the instigation of late flowering in lilacs. This abrupt variation in weather is difficult for us to handle but imagine being a plant stuck outside and weathering whatever Mother Nature throws your way! Unlike ill-timed pruning, which removes already-formed flower buds, weather conditions not conducive for flowering will keep flower buds dormant, delaying emergence until conditions are favorable for flowering.
Since late flowering can be indicative of plant stressors beyond our control (like weather), best management practices now will help them in the months ahead. Deep, infrequent irrigation, mulching with 2-4 inches of wood chips or shredded bark, and refraining from fertilizing are good practices, providing the support plants need to go about their normal cycles of emerging, growing, and flowering next year.