The redbuds are blooming, daffodils are pushing up through the soil, and the past two weeks have seen a spate of 60-degree days in Philadelphia.
With three weeks remaining in a winter that’s ranged from mild to unseasonably warm, plants and trees are waking up early, giving the region a glimpse of spring awakening that seems incongruous with the calendar.
“It’s normal February activity — if you live in South Carolina,” said Vincent Marrocco, director of horticulture at Morris Arboretum in Chestnut Hill.
According to the arboretum’s bloom calendar, the region is three weeks ahead of schedule this year. The sap is already rising in plants that shouldn’t be awake yet. Soil temperatures are approaching late-March norms, Marrocco said, which means gardeners can expect weeds to begin popping up much earlier.
Poison ivy, which thrives in heat, has gone from being a minor nuisance to a full-blown pest over the 30 years he’s been at Morris.
Katie Jacoby, a horticulturist at Bartram’s Garden in Kingsessing, tracks the temperature by counting the number of “growing degree” days — when conditions are ripe for plant growth. Last year at this time, she had clocked 11 such days; this year, the total is already three times that.
Warmth isn’t the only trigger for a plant’s growth — rainfall and sunlight can also be factors — but as winters get warmer, there’s more potential for trees and their pollinators to fall out of sync.
A bloom-killing frost could rob migrating birds of the seeds they rely on later in the year, Jacoby said. And if the beetles that pollinate magnolia trees haven’t yet emerged from the ground to serve their role when the trees’ flowers fall to the ground, it could have a cascading effect in the long term.
“It’s concerning because of what could happen down the road for those species, but also for trees in their own quest for a genetically diverse future,” Jacoby said.
In the short term, native plants are well positioned to navigate even the most unusual swings in late-spring weather, per Marrocco, but plants have evolved over eons to prepare. Many have a natural ability to survive a frost by moving water from within their cell walls to areas between cells, where freezing and expansion can’t destroy cells, Marrocco said. Daffodils, for example, evolved from mountainous regions, preparing them to handle the whiplash of a 40-degree drop in temperature taking place this week.
Non-native plants might not be so lucky. The roses at Morris Arboretum are beginning to leaf out, which concerns Marrocco. They aren’t well suited to survive a wintry cold spell, and many will likely die, he said.
When the arboretum collects new plants to grow, the quest now reaches to the Carolinas and Georgia – even as far south as Texas.
“Those are areas you wouldn’t traditionally think are home to suitable plants for the Mid-Atlantic area, but temperatures are changing and the plants that want to grow here are changing,” Marrocco said.
In some cases, a cold snap could injure trees like the magnolias and cherries that have begun growing earlier than usual. Jacoby lamented the fate of a non-native hydrangea species at Bartram’s that succumbs to late-season frost. Still, she kept an optimistic perspective.
“There’s always another plant that’s about to bloom,” Jacoby said. “That’s just the nature of nature.”
