Spring arrives in Philly with a bloom-threatening freeze in the forecast

As spring arrived officially at 11:06 p.m. Tuesday in Philly, once again winter appeared to be having second thoughts about leaving.

For the third year in a row, after a balmy February and weeks of serious spring flirtations, forecasters say a hard freeze is due Thursday and Friday mornings, with low temperatures below freezing even at Philadelphia International Airport and deep into the 20s outside the city.

This particular freeze is “kind of a scary one,” said Bill Cullina, executive director of the Morris Arboretum and Gardens, in Chestnut Hill, given that the trees and plant life around here are 10 days to two weeks ahead of schedule.

» READ MORE: Spring came mightily early to the region last year. Then came a freeze

And freeze impacts would be as much about those recent 70-plus degree days as the chill to come, he added.

Trees in the warmer parts of the city should escape the worst of it, said Ray Martin, a lead forecaster at the National Weather Service Office in Mount Holly, “but definitely in the outlying areas, it doesn’t look too good.” The weather service forecast called for widespread lows in the mid-20s Friday, even in Philly.

The cold could be fatal for some early blooms, said Cullina, and potentially a setback for the leaf-out processes of some trees. Yet again it would be especially dangerous for those gorgeous magnolia blossoms that have had the audacity to unfold ahead of schedule.

Said Cullina, “There’s nothing sadder than a brown magnolia.”

The weather around here in recent years has been messing with the trees, said Cullina, particularly stressing non-native species.

Volatility is a defining characteristic of this time of year, as the surging warmth battles for supremacy over the retreating winter. The trees and other plant life get caught in the crossfire. But the balminess of February and the first two weeks of March have fooled some of the trees into premature behavior, said Cullina.

» READ MORE: Winter, we hardly knew ye, and it's been wanting for snow in recent years

“It’s not so much that it’s abnormally cold around here,” he said. “It’s just that we went through an abnormally warm period first.”

Forecast low temperatures Thursday and Friday in Philadelphia would 5 to 8 degrees below normal. But they follow a warm stretch last week, when temperatures shot up to 75 degrees twice, more than 20 degrees above normal.

With early warmth, trees “start to lose their cold-hardiness,” he added. With a freeze, incipient leaves can get damaged, and that can push back re-leafing by a couple of weeks.

This is not the first time he has seen this movie. Tracking with the warming of the planet, six of the 10 warmest winters in the period of record in Philadelphia have occurred in the 21st century.

» READ MORE: Last winter also was quite mild, followed by a freeze

Last year, after the temperature reached at least 60 degrees on seven days in February, it dropped to 28 at the airport on both March 19 and 20. The year before, it hit at least 60 on six days in February, and fell to 26 on both March 29 and 30.

“Same old, same old,” said the weather service’s Martin. “Another whack at the magnolias.”

Cullina said that the native trees that have experience with this meteorological yo-yo should be fine, but this likely will be a challenge to Asian and other non-native species not as accustomed to this level of atmospheric caprice.

He expects the cherry blossoms to hold up, and most of the daffodils.

“Some of the spring wildflowers take pretty amazing cold,” he said.

But as for the likes of Asian woodland plants, mayapples, and jack-in-the-pulpits, “If they get nipped, ... they may not come back this year.”

If you have blooms you want to protect, Cullina recommends covering them with a blanket or two. A covering, he said, could make a difference of 4 to 6 degrees Fahrenheit.

» READ MORE: These charts and maps show how springtime in Philadelphia is getting earlier and warmer

If they live to see Friday afternoon they should be fine. They’re due for a watering Saturday, as rain is likely. Then, the government’s Climate Prediction Center sees the odds favoring above-normal temperatures in the Northeast Monday through April 2.

If the trees show evidence of damage, it wouldn’t hurt to add a little fertilizer around the roots. Trees do have their limits, and the last few years have been stressful, with hot summers and post-warmth freezes.

A tree has to use up its stored energy “to re-vegetate itself,” Cullina said. He added that Morris has lost several magnolias in recent years.

“The trees do have reserves,” he said. “The emergency funds aren’t infinite, and if you have too many emergencies, the funds get used up.”