MISSISSIPPI STATE -- A milder-than-usual winter may be nice for people, but it can mean trouble for fruit trees by causing them to bloom out of season.

Recent cold weather has not hurt this year's fruit production, even though warm weather has prompted some plants to bud. Generally, buds are not damaged until temperatures dip below 28 degrees for four or more hours.

Dr. Freddie Rasberry, horticulturist with the Mississippi State University Extension Service, said plants blooming out of season can limit fruit production by spreading blooms over several months instead of one. A freeze after a warm spell can also kill new buds and blooms, eliminating most fruit production for that year.

"Most fruit trees and blueberries need a certain number of chill hours, or time at temperatures below 45 degrees, to break dormancy and produce leaves and flowers," Rasberrry said. "I think most of the fruit trees and blueberries in Mississippi have enough chill hours, but with the warmer temperatures we've had, it's marginal this year."

Much of the problem comes from planting species not native to the area. A plant from the Coast requires fewer chill hours than does one native to more northern areas. A southern plant growing away from its native area may accumulate enough chill hours to bloom in January or February rather than in March and April when it should. If the weather turns cold again, that plant will suffer.

"The cold weather changes the level of hormones in the plant," Rasberry said. "Hormones control when the plant goes dormant and when it grows."

The number of chill hours needed by native plants in the state ranges from 400 to 1,400, Rasberry said.

"Growers should always plant varieties recommended for their area," Rasberry said. "If you take something that belongs somewhere else, it may or may not be able to adjust to the climate."

While nothing can be done for those plants that have already bloomed early or are caught by a late freeze, commercial growers have access to Dormex, a plant growth regulator.

"Dormex can replace lack of winter chilling and induce uniform bud break of leaves and flowers," Rasberry said. "It tricks plants into blooming and putting on leaves by adding 100 to 200 chill hours."

When used correctly, Dormex is a very safe substance that leaves no residue on the plants and breaks down into fertilizer. If used at the wrong time or in the wrong concentration, it can reduce crops.

In Mississippi, the Environmental Protection Agency has approved it only for experimental use. Any commercial growers wishing to use the product should contact the Bureau of Plant Industries, which would apply for a permit for the state to use Dormex on an experimental basis.

No producers in Mississippi are using the product, but it is available if a commercial grower decides to turn to it.