Late spring frosts can damage tender growth of plants. As plants leaf out and bloom they lose dormancy. The amount of damage depends on the temperature reached and the length of time at the temperature. It can be complicated. Temperatures may be lower at ground level than the measurements taken at a higher elevation. Cold flows downhill so frost may be more severe in low-lying areas.
In the case of corn, seeds that are in the ground only 24-36 hours and still taking on water can lose their ability to expand. The cells of the seed may rupture as they swell. Damaged seeds may fail to germinate or have arrested growth. They may leaf out underground, develop corkscrew roots and other problems. Seeds that have already fully expanded are more resilient to cold.
Corn plants that have already emerged may suffer damage to the exposed leaves. However, the growing point is still in the ground and protected until the plant has six leaves above the ground. Leaves are counted when the base or the leaf, or collar, is visible. These plants will continue to grow and send up new leaves and the plant will survive the frost. If the ground at ¾ inches deep reaches 28 degrees then the entire plant may be lost.
Fruit tree flowers have varying susceptibility to cold depending on the stage of bloom. Sensitivity increases as flower buds develop and open. For apple and pear trees, from first bloom to post bloom a 10% loss is expected at 28 degrees. For apple fruit, a 90% loss is expected at 25 degrees. Pears have a 90% kill at 24 degrees from full bloom to post bloom.
With peaches, a 10% kill occurs at 27 degrees when in full bloom and at 28 degrees when at post bloom. A 90% kill occurs at full bloom when temperatures reach 24 degrees at full bloom and 25 degrees post bloom.
Tender plants can be covered with blankets and sheets which will allow moisture to escape. Under these blankets and sheets temperatures can be 4-8 degrees warmer than the outside temperature, depending on the thickness of the cover. If possible, it is best if coverings do not touch the plants. They can be held above the foliage with a teepee structure or by using ½ PVC pipe or 9-gauge wire to form a hoop over the plants.
If it is necessary to use tarps or plastic be sure they do not touch the plants, or they can cause greater damage than if no cover was used at all. Plastic or a tarp over a blanket can increase the warmth retention. Coverings need to touch the ground to be effective. They work by trapping warm air rising from the ground.
Asparagus, collards, kale, spinach, peas, potatoes, and rhubarb are some of the vegetables that are very hardy when temperatures dip, while beets, broccoli, cabbage, carrots, and radishes are frost tolerant. Cornflower, pansies, primrose, dianthus and violets are very hardy, and black-eyed Susan, coreopsis, snapdragons and sweet peas tolerate frost. These plants may look wilted after a hard frost or freeze but will usually recover with warming temperatures.
Another potential tool in the box is the use of fans at low speed to drive off moisture and prevent the formation of frost. Fans at high speed can cause more damage. In our area, Derby weekend is generally considered safe for planting tender crops with a less than 10% chance of temperatures reaching 36 degrees or lower. The latest dates for dipping down to 36 degrees is May 15 for both the Paducah and Princeton weather stations.
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