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Where's the Redbud Bloom?

Over the past 30 years, Minnesotans have enjoyed many mild winters. But the winter of 2013-2014 was a return to the winters of yore. In many parts of the state, the past winter was the coldest in either 35 or 78 years and it is a winter that will be remembered for long persistent periods of very cold temperatures. The persistent cold allowed deeper than normal frost penetration in soils even though snowfall was heavy and just as persistent as the cold temperatures. No matter what statistics you look at - lowest temperatures recorded in the state, average monthly temperature, number of days Minnesota reported the coldest temperature in the nation, amount and persistence of snow cover, soil frost depth, windchill conditions, number of nights with 0 degrees or lower - they all add up to one long, cold, snowy, difficult-to-live-through winter.

The past winter also took its toll on trees and shrubs. Winter burn on evergreens and salt damage on roadside white pines were severe. With the arrival of spring, cold injury to plants that are marginally hardy in Minnesota showed up. Vegetative damage can be seen in most of the repeat-blooming shrub roses whose canes died back to the ground. These plants are now busy sending up new canes from their crowns. Because repeat-blooming roses produce flower buds on this year's canes, these plants will still be able to bloom throughout the summer.

K. Zuzek, UMN Extension

Photo 1: Eastern redbud flower

D. Hansen, MN Ag. Exp. Stn.

Photo 2: Eastern redbud without flower bud winter injury

Winter injury to flower buds occurred among other plant species. Among marginally hardy plants that have been introduced to Minnesota from warmer climates, flower buds are often less hardy than vegetative parts such as leaf buds and stems. An example of this type of injury from the past winter is being seen on eastern redbud (Cercis canadensis). Eastern redbuds are small trees with lavender pink pea-like flowers that open to cover tree canopies in May before heart-shaped leaves expand. Flower buds formed during the growing season of 2013 should have provided us with beautiful bloom this spring. Some redbuds in the heart of the Minneapolis/St. Paul did bloom. But thanks to our low winter temperatures the more common scenario, especially in suburbs surrounding the Twin Cities and in colder outstate locations, was flower bud mortality and lack of bloom.

K. Zuzek, UMN Extension

Photo 3: Eastern redbud with flower bud winter injury

Redbuds are not native to Minnesota. Their native range extends throughout much of the eastern half of North America. In the Midwest, the range extends only as far north as southern portions of Iowa, Wisconsin, and Nebraska. The species is considered hardy to Zone 5 where average minimum temperatures fall between -10 degrees F and -20 degrees F, meaning it lacks the cold hardiness needed to survive and perform well in almost all of Minnesota. But decades ago a large number of redbud seedlings were planted at the University of Minnesota Landscape Arboretum and Horticultural Research Center. Some of these proved hardy enough for use in Minnesota landscapes. Over the last 20 years, seed from these trees have been collected each year and plants grown from this seed are sold as the Minnesota strain of redbud. Even with the improved hardiness of the Minnesota strain, redbud bloom is not 100% reliable and in the most severe of winters (such as the winter of 2013-14) flower buds are killed by low winter temperatures. Cold injury to eastern redbud flower buds used to be more common so that lack of bloom occurred every 4 or 5 years. With winter temperatures trending warmer over the last several decades, redbud bloom has been so much more consistent that this year's lack of bloom may seem unusual to all but the oldest few generations of Minnesota gardeners.

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