Warm December Weather May Spell Trouble for Plants

By Dawn Pettinelli, UConn Home & Garden Education Center


Photo: Dawn Pettinelli

Usually this time of year the UConn Home & Garden Education Center is flooded with calls about the care of holiday plants. Not so this year. Instead many of the calls have been about the unusual plant behavior Connecticut residents are noticing in their gardens and landscapes. Why are my PJM rhododendrons blooming? What will happen to my bulbs that I planted in September as the foliage is starting to emerge from the ground? Should I be worried that my Johnny Jump-ups as well as the dandelions in my lawn are blooming? How about those magnolia buds – they are getting pumper so will this warm spell affect their usual spring bloom? My garlic bulbs are beginning to grow so should I do something?

The bottom line is, of course, Mother Nature rules. There is not a whole lot we can do about the vagrancies of the weather. We had an unusually mild December with Christmas Eve feeling more like an early autumn day than a winter one with temperatures in the northeast somewhere between 15 to 30 degrees F above normal. In many places the December temperature records were broken. What does this mean for our plants?

Despite the fact that dandelions, Johnny-Jump-ups and sweet alyssum are still blooming in lawns and gardens, there is no need worry about them. Many residents would be pleased if dandelions were thwarted by winter cold but do consider the pollinators that depend on them and go on to fertilize our food plants. Fluctuating temperatures will not harm dandelions, chickweed or other perennial weeds. Unfortunately they will just use the mild days to continue to grow. Cold hardy annuals like sweet alyssum, calendula and snapdragons may get killed over the winter or sometimes hang on especially if a nice snow cover materializes.

The foliage of spring flowering bulbs, like daffodils, is beginning to emerge from the ground especially in more sheltered areas. Since the energy needed for flowering this spring is stored in the bulb rather than dependent on the foliage, any damage to the early emerging leaf tips will not affect this year’s blooms. Those with live Christmas trees may want to lay the branches over bulb or perennial beds to serve as a winter mulch.

On my way to Boston a couple of weeks ago, I noticed cherry trees blooming. On the UConn Storrs campus a few of the rhododendrons are sporadically blooming. What I haven’t noticed is our native trees and shrubs breaking dormancy. These have evolved over thousands of years with our regional climate variations according to Dr. Carol Auer, UConn plant physiologist. So likely they are less prone to responding to unseasonable warm weather as opposed to non-native ornamentals that have not had this adaptive opportunity.

UConn Associate Plant Science & LA professor Dr. Jessica Lubell reminds us that “Plants go into dormancy in the fall and break dormancy following exposure to temperatures of 32 to 45. Different species have different requirements for hours if chilling. Those requiring less chilling hours may begin partial bud break in warm periods like this. Generally plants such as these may receive slight damage to some shoots, but will likely be fine.”

Dormancy in woody plants is first triggered each fall by the declining hours of daylight. Deciduous plants will shed their leaves. Just as important are chemical changes in the cells in wood and bark. These occur in response to a decrease in temperatures. Cells lose some of their water to improve their freeze resistance. Then the fats and proteins in the plant change into forms that can survive the freezing winter temperatures. The plant in a sense makes its own internal antifreeze.

In order to achieve maximum cold hardiness the woody plants need to be exposed to a gradual consistent drop in temperatures ending with several days of cold, subfreezing temperatures and in most of the state, this has not yet happened.

On top of that, temperatures above 60 degrees F may have a negative effect on a plant’s chilling requirements. So the $64,000 question as far as the survival of our woody plants over the winter will be what does the forthcoming weather hold in store for them? A rapid, steep drop in temperatures may spell trouble for some. We’ll just have to wait until spring to find out.

If you have questions about the strange behavior of your plants this winter or queries on any other home and garden topic, contact the UConn Home & Garden Education at (877) 486-6271 or ladybug@uconn.edu, or your local UConn Extension Center.