Unseasonably Warm Weather
St. Louis and much of the Transition Zone has “benefited” from unseasonably warm weather in November, after some good, cold weather in October. This weather pattern has totally confused our landscape ornamentals, especially beautiful flowering shrubs that bloom on old wood–such as the Bigleaf Hydrangeas (Hydrangea macrophylla cultivars). Plants in our region are programmed to go dormant, based upon either short days and/or cold temperatures. Think about this. To survive our cold winters, plants need to hibernate. Each species has a different chilling requirement (hours of cool/cold weather at a certain base temperature) and those plants that are adapted over a wide geography (such as the showy bigleaf hydrangeas) tend to require too few hours of chilling. As such, they are prone to making mistakes. A good example of how this works is the apple tree. At the risk of oversimplification, an apple tree needs 600 hours below 45 F. This is why Georgia grows peaches and very few apples!
The featured image shows an actual flower bud on the popular ‘Nikko Blue’ selection, taken just a few days ago. The buds emerged in mid November, only to be killed by one of our frosts. These buds should have opened next May! Those of you with these plants know how disappointing they have been these past few years. They were so gorgeous at the nursery! I did some research on the various reasons for flowering failure in this species. Breeders of these cultivars are working on improving heat tolerance, because prolonged hot weather decreases flower bud set. Another obvious need is better cold tolerance. For maximum success here in St. Louis, try to locate your bigleaf hydrangeas on an east or southeast exposure with good protection from winter winds. Shade after 1:00 PM or 2:00 PM is perfect. If you’ve got a brick or stone house, that will enhance your flowering success, because the stone will radiate heat during winter nights, increasing flower bud survival.
A good friend of mine grows nursery stock and I queried him for advice, too. He recommended caging the plants and filling it with straw mulch (or any loose mulch…not leaves…not hardwood bark) by early December to protect the tender buds. That would not have helped this year, but I may try it in the future.