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Sarah Browning: Spring frost an unwelcome visitor
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NEBRASKA HORTICULTURE

Sarah Browning: Spring frost an unwelcome visitor

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Unseasonably warm, mid-70 to nearly 90-degree temperatures in early April lured trees, shrubs and perennials out of dormancy. Many trees and shrubs have been blooming for the last two weeks, only to be threatened by normal late April frosts.

• April 14-15, Lincoln nighttime temperatures dipped to 27 and 26 degrees, respectively.

• April 19-22, Lincoln had nighttime temperatures of 31, 32, 29 and 23 degrees, respectively.

Gardeners always have concerns following a spring freeze. What should you look for to determine if plants were damaged?

Vegetables, perennials

One way to determine the likelihood of damage to early season perennials is to look at the cold tolerance of early spring vegetables, which are actively growing during the same time frame.

• Very hardy: Can withstand freezing temperatures and hard frost (less than 28° F) for short periods without injury. Asparagus, collards, endive, kale, kohlrabi, lettuce, mustard, pea, potato, rhubarb, rutabaga, salsify, spinach, turnip.

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• Frost tolerant: Can withstand light frosts (32-28 degrees) without injury. Beet, broccoli, Brussels sprout, cabbage, carrot, cauliflower, celeriac, celery, chard, Chinese cabbage, Jerusalem artichoke, onion, parsnip, radish.

Early season perennial plants like bleeding heart (Dicentra spectabilis) and peony had put on several inches of growth in response to the warm temperatures. But fortunately, in many locations freezing temperatures did not dip low enough to cause serious leaf or stem damage, and plants had not yet developed flower buds, which could have been killed.

Freeze damaged foliage first develops a water-soaked appearance, then wilts and collapses, finally turning brown or black. If perennials were damaged, remove the dead foliage. New growth shoots will appear in a few weeks.

Did you plant potatoes on Good Friday? Potato foliage exposed to freezing temperatures becomes cupped, tattered and distorted, appearing very similar to herbicide damage.

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Can rhubarb and asparagus exposed to cold temperatures still be eaten? If either rhubarb or asparagus stalks were damaged by cold, they will turn soft, mushy and eventually become brown; damaged stalks should be pulled out and discarded. New leaves and asparagus shoots will soon develop. If the stems are still well-colored and firm, they are safe to eat.

If you planted early, tender plants like tomatoes and greenhouse transplants likely did not survive if they weren’t protected and will need to be replaced. Remember not to get too excited by early spring warm days and be fooled into planting tender plants early; in eastern Nebraska there is only a 10% chance of frost after April 29- May 12.

Woody plants, fruit trees

The new growth on many trees and shrubs can tolerate temperatures in the low 30s and upper 20s. Damage is most likely to appear if temperatures drop into the middle 20s or below, and, fortunately, it did not get that cold in the Lincoln area.

Damaged leaves at first appear water-soaked and may be darker green in color. Within a few days the leaves will dry up and may become black or brown, although sometimes they retain their green color for several days. Later in spring, as partially damaged leaves open, browning or leaf tatter caused by freeze injury to the leaf tissues while still in the bud may be seen. In fact this condition has a name -- oak tatters. When leaves fully enlarge later in the season, this old freeze damage injury may appear to homeowners as insect or disease problems, however, pesticides should not be applied to "control" the injury.

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However, flower buds are less cold tolerant than leaves and many trees --  magnolia, forsythia, ornamental pear, wild plum and crabapple -- were actively blooming during April 19-21's freezing nights. The flowers of tender plants like magnolia are usually the first affected and may have been killed completely -- turning brown and wilted -- but this should not cause any serious or lingering effects on the tree’s overall health.

Potential damage to fruit tree flower buds is a serious concern, since a significant crop reduction will occur if many buds are killed. Bud development stage for each tree type determines the likelihood of crop damage. For example, apple trees in the full pink stage -- the point at which flower buds are fully swollen and showing color but not yet open -- will suffer 10% flower death at 28 degrees and 90% flower death at 25 degrees. For more details on cold temperature effects on fruit trees, review the publications below.

• Picture Table of Fruit Freeze Damage Thresholds, Michigan State University

• Freeze Damage Depends on Tree Fruit Stage of Development, Michigan State University

Dead flower buds will become dark brown, shriveled or dry. Freeze damaged flowers and shoots on fruit trees provides an entry point for serious diseases like fire blight.

As with perennial plants, otherwise healthy and hardy trees and shrubs will develop secondary buds and re-leaf later in spring. However, new flower buds will not develop. Avoid pruning out damaged new growth until late spring to minimize the need for plants to use energy reserves sealing pruning wounds as well as beginning secondary bud development.

Also, avoid fertilization of freeze-damaged trees and shrubs. Fertilizer is not “plant food” and can worsen the condition of plants already struggling with the freeze injury. If a home turf receives regular fertilization throughout the growing season, and trees or shrubs are near turfgrass areas then additional fertilization is not needed or recommended.

Finally, follow best management practices, including regular watering and mulching, and control pests as needed during the summer to avoid additional stresses.

Sarah Browning is an extension educator with Nebraska Extension. To ask a question or reach her, call 402-441-7180 or write to her at sarah.browning@unl.edu or 444 Cherrycreek Road, Lincoln, NE 68528.

 

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