Minnetrista apple grower Lowell Schaper spent a cold, sleepless Monday night driving a tractor through his Minnetonka Orchards, trying to circulate warmer air and protect his budding trees from the chill.

Bill Jacobson hired a helicopter to churn the night air over his family's Pine Tree Apple Orchard in White Bear Lake, an experimental effort his father tried back in the 1960s -- "the last time we needed it," Jacobson said.

The chopper cost more than $600 per hour, "but it doesn't take too many bins of Honeycrisp [apples] to pay for that," Jacobson said. "It's scary, but what are you going to do? The helicopter makes you think you're doing something."

Minnesota's apple growers are on edge after two nights of below-freezing temperatures, threatening the delicate buds that formed weeks ahead of schedule, thanks to this year's balmy March. No one knows what, if any, damage has been done or how it will affect this year's apple crop, but many growers are going to extremes to protect their crops. "This could be devastating," Schaper said.

Spring bloom is a sensitive time for apple trees, according to David Bedford, research scientist and apple breeder at the University of Minnesota. "Well-bred apples from the University of Minnesota can withstand 30 below [in winter] when they're dormant," he said. "We've got that part figured out. But in spring, right before they bloom, is the beginning of their most vulnerable period of cold tolerance."

For a handful of days, from buds opening to full flowering, a few degrees can make an enormous difference, Bedford said. At 28 degrees, growers can expect about 10 percent of flowers to be damaged. At 25 degrees, the percentage can increase to 90 percent. "There's not a big span between minor and major damage."

A high mortality rate for flowers doesn't necessarily translate into an apple disaster, because the trees produce an over-abundance of blooms. Still, growers are concerned because it's impossible to see the damage right away.

This year's conditions are unprecedented, growers say. "We rarely have spring freezes during apple bloom in Minnesota," Bedford said. "Our normal spring is late and fast, and then we're on to summer."

'It's new ground for all of us'

April frosts are common in Minnesota, but they present a problem only in years like this, when the buds form so early.

"This is the earliest I've seen, and the coldest I can recall at this bud stage," said Schaper, who tried to prepare his budding trees for plunging temperatures by spraying them with micro-nutrients, in hopes of making them more cold-tolerant.

"No one's experienced weather like this -- ever," said Mike Dekarski, owner of Apple Jack Orchards, Delano, and president of the Minnesota Apple Growers Association. "It's new ground for all of us."

Although other fruits also are vulnerable -- including apricots, plums and grapes -- they're minor crops in Minnesota compared to apples, a $12.6 million crop in 2010 that produced nearly 16 million pounds of fruit, according to Minnesota Agricultural Statistics.

"It could be sort of a fruitless year," said Schaper. "Apples are the core of our business. If we don't have apples for U-pick, we don't have a business."

No apple shortage expected

Earlier blooming apple species such as Zestar! and the new SweeTango are especially vulnerable, Schaper said. Honeycrisp, Minnesota's most popular apple, is not yet in bloom, so it's at less risk for now, although a frosty night ahead could change that. "Any time temps get into the 20s, we're nervous," said Chet Miller, director of operations for Pepin Heights, a Lake City wholesale producer with 250 acres planted with 85,000 apple trees.

Even if Minnesota apple production is down this year, consumers aren't likely to face apple shortages or higher prices, Miller said. "We do have arrangements with other growers." Grocery-store chains would just buy their apples from other apple-producing regions, he said, although farmers' markets might be short on apples.

Still, many Minnesota apple species are ideally suited to local growing conditions and might not be as delicious when grown in other states, Bedford said. Take Honeycrisp. "It's a hometown kid," he said. "It just likes our climate."

Growers won't know the extent of this week's frost damage for at least a few days. The earliest clues of what apple season will bring will be revealed after growers cut into buds; brown or black inside indicates damage, while a green interior indicates a healthy bud that's capable of flowering and producing fruit.

"I haven't cut any open yet," Schaper said. "My hands are too cold."

Kim Palmer • 612-673-4784