What is that rhododendron doing re-blooming in the middle of October? Well, actually, it might be more correct to think of it as jumping the gun on next spring rather than re-blooming.
Although it happens to some extent many years, there seems to be more reports than usual of blooming out of sequence this fall. Rhododendrons, crab apples and saucer magnolias are the most commonly reported species blooming this fall.
Spring-blooming woody plants initiate flower buds on previous year’s wood and rely on chilling to stimulate maturation of the flower buds. In other words, the flower buds require a certain amount of chilling before they break out of dormancy to open their flowers . Cool nights, like we’ve enjoyed recently, can provide enough chilling to result in plants blooming out of their “normal” sequence.
Different species and even cultivars within a species vary in their requirement for chilling units. In general, plants that flower earlier in spring require fewer chilling units than plants that bloom later. Plants, such as magnolia and some rhododendron, commonly re-bloom sporadically in summer and into the fall. There are some cultivars of rhododendron that have been selected for ability to re-bloom throughout summer and into early fall.
Though the total amount of bloom for the following spring might be reduced, this out-of-sequence blooming is not harmful, and, usually, there are still plenty of buds left to provide a spring show.