Warm weather means springtime in December for flowers and trees

It's great fun when we humans can wear our flip-flops and shorts in December. That thick jacket with the fur-trimmed hood is just a short trip to the closet away when winter inevitably wallops us.

But what of our befuddled plants? There are tulips, daffodils, and hyacinths poking out of the ground as if it's spring. Some cherry trees, roses, snowdrops (oh, the irony), forsythias, and rhododendrons are blooming. Tender green leaves are emerging from some mophead hydrangeas, a plant that really suffered in our recent cold winters and now seems confused by the warmth.

As January looms, can plant annihilation be far behind?

"Don't panic," advised Mindy Maslin, manager of the Pennsylvania Horticultural Society Tree Tenders project.

The good news, area horticulturists said, is that the plants will probably survive whatever nature throws at them, though those most native to our region will have a distinct advantage.

The bad news is that spring may not be quite as pretty as usual. The new leaves and flowers will die when winter literally nips them in the bud. Most of these early-blooming plants won't make new flowers because the buds are set in summer or fall. However, there will likely still be plenty of blooms at the proper time.

"The thing that concerns me is that it's been so warm for so long that we may be setting ourselves up for problems when a serious cold snap hits us," said Paul Meyer, director of Morris Arboretum. Mahonias and camellias are blooming near his office.

"All could be well if we continue to have a mild winter."

Experts said premature growth is not all that unusual in some species. There does seem to be more of it this year. The risk of damage rises as we get closer to January because the plants are best able to withstand frigid weather when they've gotten used to the cold gradually.

Longwood Gardens, which has about a quarter-million spring bulbs in the ground, is keeping a close eye on daffodils that are poking out. "We're not on pins and needles yet except for some of the bulbs," said Jim Harbage, floriculture leader.

The bulbs themselves will be fine, he said, but if the weather stays warm long enough for the buds to make it to the surface, they could get zapped. Longwood may add some mulch if there's much more growth.

Meyer is worried about southern magnolias, crape myrtles, and camellias if the warmth sticks around.

What is happening here is a reflection of the varying ways that plants acclimate, or become dormant, for winter. The process is triggered by a combination of changing fall light and lower temperatures. When everything works properly, warmer temperatures in spring trigger new growth.

This region had some cold weather earlier in the fall before the recent warm-up.

Plants require varying periods of cooler temperatures - below 50 degrees - before they can begin spring growth, said Rich Marini, a Penn State horticulturist who specializes in fruit trees and woody plants. Many fruit trees need to have experienced 800 to 1,000 hours of cold before they'll flower. Other plants, particularly those that evolved in warmer climes, need much less.

It's not unusual for some kinds of early-blooming cherry trees to have a few blossoms in the fall, experts said, adding that they've seen more flowers than usual this year. The more the trees bloom now, the less they'll bloom in the spring.

The weather has been great for grass, Harbage said. "The turf has been beautiful," he said. "We're still mowing here." While the tall fescue on most lawns is likely safe, he said zoysia and bermuda grasses can deacclimate quickly. A sudden deep freeze could be hard on them.

There's not a lot you can do for your trees, but Maslin is a big fan of mulch. She recommends the 3-3-3 approach. Use no more than three inches of mulch but stay three inches away from the trunk. Cover an area that radiates three feet from the trunk.

Lena Struwe, a plant biologist at Rutgers University, has been noticing the weeds, because that's what she studies. "In the last couple weeks, have you seen all these dandelions flowering?" she asked. She's never seen so many at this time of the year. There are thousands on the university lawn. "There are all these fluffy balls right now," she said. She's also seeing hairy cress, which usually comes out in April, and veronica. Because these plants are fast growers, they may be able get in an extra growing season, spreading extra seeds for the spring.

Struwe is worried about how plants that have adapted over thousands of years to our seasons will ultimately respond to our warming, more unsettled climate. Plants that like warmer weather are already migrating northward. Kudzu, an invasive vine, has made it to Delaware and southern Pennsylvania.

Like others, she thinks most will be fine. "Plants are pretty resilient, she said. "The thing that kills plants off is really deep frost."

Meyer, who lives on the grounds of the arboretum, also has noticed the weeds. He spent some time last weekend weeding his own garden. He also planted some bulbs he picked up at an end-of-the-year sale. Many years, the ground would have been too cold for planting now.

He's got good reason to like the weather. The arboretum gets more visitors when it's nice out.

"My advice is, don't worry too much about it," he said of all the strange plant behavior. "Just sort of enjoy seeing these early previews of springtime."

sburling@phillynews.com

215-854-4944

@StaceyABurling