As long as they remain mild, warmer fall and winter temperatures are actually rather beneficial to most plants. No plant really needs temperatures of -15F (-26C) in order to survive: at most, the very hardiest plants tolerate such temperatures, but would be just as happy under warmer conditions. And in most areas, temperatures this year will still be low enough to meet the minimum requirements for temperate plants to thrive (many require 4 to 9 weeks below 45F [7C] in order to bloom, for example).
The Positive Side
Also on the positive side, if you have not yet harvested your kales and leeks from your garden, they should still be in perfect shape in most regions and in fact, sweeter than ever. And you can still plant fall bulbs if you have not already done so. If mild weather continues into January, some plants may even start blooming, including such spring bulbs as snowdrops, crocus and daffodils, plus hellebores and even a few shrubs.
The problem is instead that, when temperatures do drop (and winter temperatures closer to normal are predicted for February and March), the change may come about too quickly, before the plants are properly hardened off. A mild December and January followed by a severe cold snap in February could be disastrous for many plants.
This is because, to reach their full hardiness potential, plants need to gradually acclimate to cold temperatures. The ideal situation would be a fall where temperatures drop gradually, followed by an early winter where temperatures continue to fall day after day. This allows plants to harden off completely. But if temperatures continue to remain 5F (10C) above normal well into January, then suddenly nosedive to -15F (-26C) overnight, the plant’s cells will not yet be acclimated and may be damaged or killed. Of course, maybe temperatures will remain mild right through the winter, but that’s not what weather forecasts are predicting.
Also, snow is well known to have a significant insulating effect on soil and therefore plants (it protects their roots and dormant buds from extreme cold). It’s been too warm for it to build up in many areas that usually expect abundant snow cover, even in the mountains. In areas where snow cover is crucial to plant survival, one can only hope that snow will begin to accumulate soon.
Plant Enemies Have It Easy
Cold winters are enormously detrimental to harmful insects and other pests, causing mortality in eggs, pupae and overwintering insects. A mild winter, however, is often followed by a summer when the enemies of our plants are extremely numerous and cause major damage. Even weeds are likely to be particularly numerous in summers that follow mild winters, as fewer seeds will have been killed by the cold.
Effects On Different Plant Categories
A mild autumn and early winter followed by a sudden cold spell, or by see-saw temperatures that repeatedly go from mild to well below freezing, affects each plant type differently. Here are some examples:
Plants Grown Beyond Their Zone
Plants grown beyond their normal hardiness zone, such as a zone 6 shrub planted in zone 5, will be the most damaged, regardless of their category (tree, shrub, fruit, perennial, etc.), because their capacity to acclimate to cold is naturally inferior to that of plants of the proper zone. However, it is not too late to cover their roots with mulch or fall leaves, both of which are excellent replacements for the snow that may not be very abundant this year and this can help protect them.
Flowering Shrubs and Trees
Spring-flowering shrubs and trees may see their show severely reduced or even eliminated, as they bloom from old wood. That is to say, their flower buds are already present on their branches and thus the exposed buds, improperly hardened off, can be readily harmed by cold snaps. Already the buds on some forsythias, rhododendrons, azaleas and magnolias are beginning to swell, something they shouldn’t be doing until spring: that’s a sign they are already preparing to bloom. Once they reach this stage, there is no going back: they will not be able to harden off and their buds will in all likelihood die over the winter.
Many gardeners wrap such plants in burlap or geotextile in the belief this will protect the buds from the cold, but in fact, winter protection is useless against the cold: it will be just as cold under the wrapping as outside. Instead, the role of winter protection is to protect the buds from the drying effects of winter winds and that is apparently not a to be a major problem..
Summer- or fall-blooming trees and shrubs will likely suffer less that spring-bloomers, as they wait until spring, well after the cold of winter has dissipated, before initiating their flower buds.
Grapes, cherries, plums, apricots, peaches, raspberries, blackberries, etc. may produce much less if they are unable to harden off properly, as their flower buds (the future fruits) are present all winter and tend to break dormancy if the season is mild or if there are winter thaws. Most apples, though, need a long period of winter chill before they will break dormancy and are so unlikely to be negatively affected by a delayed winter.
As mentioned, some may start to bloom in January if mild conditions continue, but this does no harm to the bulb in the long term. Likewise the foliage of some bulbs (especially grape hyacinths) may already be starting to sprout, but once again, this is nothing to be concerned about. All will be able to bloom again the following year. Most spring-flowering bulbs will simply bloom as usual, although a bit earlier, if the winter remains mild.
Hellebore or Christmas rose (Helleborus spp.) is a very early blooming perennial and probably the only one that might just bloom out of season if mild conditions continue. If so, any flowers will likely be killed by the February cold and thus the spring show will be reduced or absent in 2016, but otherwise, this won’t hurt the plant. All other perennials are already fully dormant and well protected by the soil surrounding them, so they are relatively well protected from sudden cold snaps. This is especially true if you didn’t cut their leaves back in the fall, because their own foliage, even when dead, helps protect them against the damaging effects of winter. If you did do a fall cleanup, it would be wise to cover your perennials now with leaves or mulch.
A long, mild fall is actually beneficial to most lawns, especially the lawns that were recently planted, as this allows their roots to keep growing late until the season and thus better face the rigors of winter. In fact, in many areas, lawns are still producing new roots in a mild December. Also most lawn grasses used in northern regions are quite resistant to cold snaps. However, an exceptionally severe cold wave when the plants have not yet hardened off can still cause damage… and there’s not much you can do but fix any damaged spots come spring.
The Waiting Game
There you go! Keep your fingers crossed… and hope that a thick blanket snow is coming soon!
This text was first published on this blog on December 12, 2015. It has been revised and the layout updated.