When the hottest year on record - 2015 - was capped off with the area's warmest December on record, no one was feeling the heat more than Lloyd Traven.

Traven owns Peace Tree Farm, a Kintnersville, Bucks County, nursery that's one of a handful of local operations tasked with manipulating thousands of bulbs, perennials, and trees to burst into a synchronized show of blossoms and foliage out of season, but just in time for the March 5 opening of the PHS Philadelphia Flower Show.

That work, known as "forcing" plants, is accomplished with carefully administered doses of fertilizer, light, and heat. But, for many trees and perennials, a prolonged cold spell is critical to kicking off that cycle - and earlier this winter it just wouldn't seem to come.

"This was an extraordinary fall. It was so warm through to New Year's, and things like roses and clematis that would normally get enough cooling just simply were not. So we're scrambling," Traven said. "There are crops we have right now that we're unsure whether they're going to bloom in time for the Flower Show. It's definitely been a dance this year."

The above-average temperatures this winter - forecast by the National Oceanic and Atmospheric Administration's Climate Prediction Center to continue through the spring - have already provoked frustration on ski slopes, angst among retailers of winter apparel, and confusion in gardeners unaccustomed to spotting daffodils in December.

How they will affect this year's Flower Show is less clear.

The show is themed "Explore America," in honor of the centennial of the National Park Service. Guests will enter through the Big Timber Lodge, a wood-and-stone structure surrounded with floral takes on totem poles, a 12-foot waterfall, and a landscape meant to evoke the forest's edge in springtime: a blanket of crocus, grape hyacinth, and echinacea below evergreens, birch trees, ferns, and shrubs.

The early-spring bulbs are doing fine, but the perennials and ferns are more problematic, said Sam Lemheney, chief of events for the Pennsylvania Horticultural Society, which runs the show as a fund-raiser.

"I actually just had a conversation with our grower. We had one set of ferns that haven't even started growing yet," Lemheney said recently. "Some of these plants require a cold hardening off point . . . and at Christmastime it was 70 degrees."

They may have to find a replacement for those cinnamon ferns, as well as some late-season rhododendrons and about 30 viburnum shrubs that, as of Thursday, were bare of leaves, let alone flowers.

"They might not make it," said Nate Roehrich, who oversees the forcing operations at PHS's Meadowbrook Farms, surveying the bare viburnum branches in one of his greenhouses. "I'm still rooting for them though. We've been misting them two or three times a day to try to get the buds open."

But getting replacement plants is tricky.

"With this being a woodland scene, we have less options," Lemheney said. "You can't go tropical. You can get things that are tropical that are look-alike plants, but it's sort of a last resort."

Still, Roehrich said many other plants are doing well. For the first time ever, he's forcing scillas, ostrich ferns, echinacea, and large numbers of cosmos. He has azaleas, witch hazel, and magnolias all poised for the big show.

Traven, who's working with a number of garden clubs and other exhibitors, said many of his plants are thriving. But his trumpet creepers and delphiniums didn't get enough cooling, and some grasses that required natural cooling are looking sparse. He's compensating by adding more plants and substituting other varieties.

"If the spreadsheet says natural cooling, you leave it outdoors. If it doesn't get cool enough, you keep saying, 'It will get cool next week,'" he said.

That doesn't mean the Flower Show in less than a month won't be resplendent - just that it will take extra work to get there. PHS alone spends $60,000 to $100,000 on forcing and plant materials each year, including the cost of backup plants. Exhibitors spend anywhere from $5,000 to $30,000 apiece on plant materials, according to PHS spokesman Alan Jaffe.

Glen Mills' Stoney Bank Nurseries, a longtime presence at the Flower Show, is one of the only exhibitors that still forces its own flowers. Joe Blandy, president of the company, said he was worried in December and early January. Now, though, he thinks the company's on track to produce a stunning rendition of post-wildfire Yellowstone, showing burned trees and new growth in the form of evergreen and deciduous ferns, shrubs and wildflowers like cosmos, poppies, black-eyed susans, and ammi, a Queen Anne's lace look-alike.

The nursery is also on track to produce plants for a number of other exhibitors, including large volumes of lupins, a notoriously difficult-to-force flower, for J. Downend Landscaping's rendition of Acadia National Park in Maine.

After 37 years of Flower Shows, Stoney Bank has a well-tested "forcing bible" with data on how to treat each plant. Of Stoney Bank's eight greenhouses, six are used for forcing for the show: Three are kept very hot, two warm, and one cold. Plants are moved between them to speed or slow buds.

Still, there are variables. "The cool greenhouse is at the mercy of mother nature. If it's warmer than normal outside, then we run into problems because then we can't cool azaleas that are coming on too quickly," said Hannah Deputy, a landscape architect at Stoney Bank.

The nursery also has a cooler box - a refrigerated trailer - for chilling shrubs and perennials. Trees won't fit, though. At one point, Deputy was worried enough about the trees that she counted the nights this winter when the temperature dipped below 32 degrees. There were about 40 of them.

"There were a couple things that needed that dormancy requirement and we weren't sure they had it," she said. "Thankfully, we had enough."