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WICHITA, Kan. – Some of Wichita’s spring 2012 daffodils have already bloomed. Statewide, the buds on certain landscape plants have swelled, and a few have actually flowered.
“This year’s early spells of unusually warm weather fooled those plants. Unfortunately, bud swell is a sure sign that plants have lost virtually all of their winter hardiness. So, between now and spring, extreme cold snaps could be damaging,” said Jason Griffin, K-State Research and Extension’s horticulture program leader.
Ironically, winter’s earlier cold weather opened the door for this problem, he said.
Deciduous plants must go through a certain number of “chilling hours” before they can bud and grow again. When the balmy days of January arrived, the plants that recently broke dormancy had already been through enough cold to meet their annual chilling requirement.
“Chilling hours" are those with temperatures between 32 and 45 degrees, Griffin explained. The number of hours required to meet a plant’s needs can vary widely, even within species.
“Depending on the variety, for example, apple trees’ requirements range from less than 250 chilling hours to around 1,700,” he said. “In general, though, plants adapted to colder climates require a longer chilling period than those adapted to warmer zones do. That’s why your hardiest plants are the least likely to be caught by an early warm spell or late freeze.”
This year’s outcome will depend partly on whether too-early bloomers also produced leaf buds.
“Losing a year’s flowers shouldn’t affect plant health,” said Griffin, “although it can mean losing a year’s fruit production. Losing leaves isn’t as common, because leaf buds are hardier. But, if cold kills the little leaf buds you’ve see along stems, your healthy plants should survive. They’ll just take a while to recover.
“From roses to grape vines and trees, woody plants have secondary buds that remain dormant unless and until plants lose their primary leaf buds. Secondary bud growth can make your plants unsightly. But, at least they can continue making food.”
Plant owners can’t do much to help vulnerable plants, other than keep drought from threatening their health further.
“Readily available soil moisture is always important. Roots can suffer drought damage during winter, too, and much of Kansas has been dry for quite a while,” Griffin said. “Plus, a tree with damaged buds will be even slower to recover if it also has damaged roots.”
Wintertime irrigation can be challenging, he warned. Frozen soil can’t absorb water. Cold soil can.
Griffin recommends that plant owners test for thawing by trying to push a rod, piece of rebar, or long screwdriver at least a foot into the ground. Just as that kind of rod stops when it reaches dry soil in summer, it will stop when it reaches frozen earth now.
“For winter irrigation to do much good, the water needs to reach 8 to 12 inches deep, across the plant’s root zone – or, comparatively speaking, at least as far out as a tree canopy’s drip line,” he said. “To achieve that, you have to irrigate slowly, slowly, slowly, so the soil can absorb every drop. It’ll take a while. Again, though, a rod will let you know when moisture has percolated deeply enough.”
Griffin also is director of K-State’s John C. Pair Horticultural Center near Wichita, where he specializes in nursery crops.
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