We have been seeing some trees and shrubs that normally flower in spring starting to flower. This is due to the unusually warm—actually very spring like—temperatures we have been experiencing. It’s most often seen on the very earliest blooming plants like forsythia and some flowering cherry species.
This phenomenon is not uncommon when we’ve had a brief cold period followed by very warm temperatures. Our normal high in early-mid December is around 50°F, but we’ve seen highs in the mid to upper 60s for the past few weeks. This causes some early bloomers to act like spring has sprung and start to flower.
The flowering itself won’t hurt the plant, but it does indicate that the plant is not fully hardened off and could be damaged by a sudden severe cold snap, such as evening temperatures in the low 20s.
So why doesn’t every plant burst into bloom after a few warm winter days? The reason is something called vernalization, which is the requirement certain plants have for a specific number of cold days before they will emerge from dormancy. The early bloomers have low or no requirement for a cold winter snooze, while other plants haven’t yet accumulated enough days below 45°F to begin blooming, no matter how warm temperatures get.
Vernalization is why only certain types of lilac, for example, will bloom well in our area; they are selected for having lower chill requirements than other lilacs. The same is true of many fruits; without enough chilling the flowering and fruit set may be poor.
Of course there’s nothing to be done to change the weather. You might as well enjoy the flowers now, as buds that open in fall will be lost for spring. In all likelihood you’ll barely notice the missing flowers in a few months when spring truly arrives.
WUNC talked to New Garden Gazebo's Nursery manager Jeremy Warren about the warm weather in late December. Listen below:
For more information on chilling requirements, this is a good article that is written about fruits, but the concept is the same for all plants with chilling requirements.