As a gardener, you may not exactly live for winter. Even so, you’ve likely come to appreciate that colder weather serves some important purposes. For instance, some seeds require prolonged exposure to freezing temperatures in order to sprout next season. Heavy frosts also help to regulate the numbers of certain pest insects on plants in the garden come spring.
But winters generally have become much less predictable. According to the 2020 paper “Changing Lengths of the Four Seasons by Global Warming” published in the journal Geophysical Research Letters, “The four seasons of a year no longer have equal months, and their onsets are irregular.”
In fact, from 1952 through 2011, the length of summer in the Northern Hemisphere jumped from 78 to 95 days. Meanwhile, the spring, fall and winter seasons each lost days during that same period.
“Longer and hotter summers, shorter and warmer winters, shorter spring and autumn seasons are the new normal, and this kind of trend may be unavoidably amplified in the future,” the authors note.
“With the average minimum temperature increasing, you’re going to just be less likely to have a crossover into freezing. And, so, you’re going to have fewer of those cold snap events,” says Toni Morelli, a U.S. Geological Survey research ecologist with the Department of the Interior Northeast Climate Adaptation Science Center. She’s also an adjunct assistant professor in the department of environmental conservation at the University of Massachusetts.
Still Cold—but Different
That’s not to say cold snaps are gone for good. Instead? Over the last few years, some areas of the United States experienced extreme cold snaps after early warming periods.
“This is a huge concern with the stone fruit tree growers but also with other gardeners in that you get your perennials coming out of dormancy and then they get hit with what feels like an unseasonably cold event,” Morelli says. “But, really, it was the warm-up that was unseasonable or atypical, and the cold event is more normal.
“It’s just that it happened after a warming event, which is not how it used to be.”
A loss of winter snow—along with its natural insulating properties—is another aspect of our weirder winter weather. “That can really be a problem for our trees, since their roots are exposed to the freezing temperatures in a way that they weren’t before,” Morelli says. “Because we’re getting either less snow or we just get these occasional melting events, so the snow pack is just not as consistent as it used to be.”
If you have potentially sensitive trees, you may need to add some extra root protection to help get them through. In some cases a temporary layer of mulch, burlap or straw may be able to do the work that a nice blanket of snow once did.
Fewer—or shorter—periods of freezing likely will cause some gardeners to lose the ability to grow certain plant varieties altogether.
“The types of species that we’re going to see declining are going to be those that have had distributions that have historically been restricted to a relatively narrow climatic branch—potentially cold-adapted species that are losing the cold that they need to survive,” says Caroline Williams, an associate professor at the University of California, Berkeley’s department of integrative biology.
“If they haven’t had the cold that they required in the winter, plants with a strong vernalization requirement are not necessarily going to be able to accelerate their spring emergence.”
Thanks to changing weather patterns, seeds requiring a period of two or more months of consistently cold temperatures might fail to sprout altogether. You can try to salvage the situation by cold-treating certain seeds yourself.
To manufacture winter, mix seeds to be cold-stratified with sterile, moist growing medium. Refrigerate this in an airtight container.
After the requisite time has passed, remove from the fridge and try sprouting the seeds. (Just remember that some varieties take to this treatment more readily than others. So germination rates will vary.)
Eventually, you might need to switch to a plant variety that’s tolerant of a wider range of temperatures—and temperature fluctuations—instead. “If you’re planting something at the very southern edge of its range, then that’s a good sign that it’s probably not going to do as well in future conditions, “ Williams says.
“Because, typically, organisms prefer conditions from near the center of the range.”
New Climate Opportunists
Some plants that don’t require a distinct winter period in order to break dormancy may thrive within the new climate paradigm. “Many plants are able to advance their phenology and come up earlier in the season,” Williams says.
Unfortunately, many of these happen to be invasive. “Invasive species, almost by definition, are more opportunistic than other species,” Morelli says.
Only a small percentage of non-native plants become invasive, the ones that do are more adaptable to changes in their environment. “They’re actually going to be even better at responding to earlier springs and milder winters and taking advantage of those changes than a lot of the non-invasive, non-natives or than the native species,” Morelli says.
Sometimes described as “thermal generalists,” plants and insects that are less choosy about their living conditions may be in a better position to thrive. “They have very broad distributions and do well in a wide variety of climates,” Williams says. “For that reason, those are often pest species.”
Keeping a step ahead of the invasive plants and pest insects in our gardens may require extra weeding or weed suppression techniques. Placing cardboard between plant rows and along garden paths can help. So can interplanting with beneficial insect-attracting marigolds or low-maintenance greens such as kale.
Getting to know the different insects in the garden—and particularly their egg-laying preferences and habits—will also be increasingly important. If you want to prevent exponential growth of certain insect pests, hand-picking and disposing of their eggs before they have the chance to hatch can make all the difference.
Pests & Pathogens
Just as cold-adaptive plants may lose out with more warm days and fewer cold snaps, so, too, might some beneficial insects.
“Cold-adaptive insects have a chilling requirement during their winter dormancy. And if that chilling requirement is not fulfilled, then they can experience delays or disruptions to their spring timing as well,” Williams says.
Winter frosts also help to knock back the numbers of insects problematic to plants. But with fewer—and shorter—cold spells on the horizon, some pests will gain a stronger foothold by hatching out earlier and in larger numbers than they once might have.
“Things that have short generation times, large population sizes and a lot of genetic diversity—they’re going to do better than things with long-generation times,” Williams says. “Insects that have multiple generations per year like, say, fruit flies or some kinds of mosquitoes are going to be able to adapt faster than ones that have one-year or multiyear generation times where you only see one emergence of adults every year.”
Many insect pests are themselves vectors for disease. So we may also have to keep a closer eye out for bacterial and fungal diseases.
Besides regularly monitoring the number and types of insects we see on our crops, we can mitigate the spread of plant pathogens by spacing plants farther apart and promoting better air circulation with judicious pruning.
For better or worse, many tropical plants and insects are on the move—along with our traditional climate zones.
“The presence or absence of freezing forms a hard limit on the distributions of lots of tropical-adapted organisms,” Williams says. “And, so, what we’re seeing is a sort of creeping ‘tropicalization’ of the temperate zones … where there’s a transition between freezing and not-freezing temperatures.”
From mosquitoes and fire ants to the African dung beetle, many organisms are quickly expanding their ranges. “It’s sort of a movement of tropical organisms poleward towards temperate zones that’s allowed by this lack of freezing,” she continues.
Even beneficial monarch butterflies have changed their habits.
“They’re actually overwintering in some temperate regions now like along the northern Gulf of Mexico,” Williams says. “There are lots of shifts both in detrimental and beneficial species. So you can expect to see things that you haven’t been familiar with seeing in your garden.”
Taking photos and jotting down notes in a garden journal can help you to get a better handle on these new arrivals. And once you determine friend from foe? You can actively recruit—or repel—them as needed.
“By attending to the sort of habitat and environment that you provide, you can encourage the presence of beneficial insects that you might want to encourage to actually shift to your garden,” Williams says.
“You can provide for the things they need for their life cycle, in terms of host plants or nectar sources. And, if you have some idea in your particular area of what the nasty invaders moving poleward might be and what sort of requirements they have for their life cycle, you might be able to find ways to discourage their establishment.”