As we look toward frost events this fall, there are some things you can do for your fruit plants before and after a frost or freeze.
There are two different types of frosts: advection and radiation. Freezes are different from frosts. We have a "freeze" when temperatures dip below 23 degrees F. A "frost" happens when moisture in the air during low temperatures causes ice crystals to form on surfaces, like leaves.
Within a single garden, some areas may be affected by frost while others are not. For instance, the ground may have frost while the leaves on plants do not. Plants closer to a building may be safe while those a few feet away are affected.
Some of our Minnesota-grown fruit plants are fairly tolerant of soft freezes (28-32 degrees F), but the fruit itself is easily damaged by frost. Let’s look at a few of the most popular Minnesota fruits.
Apples
Frost crystals can degrade the skin of the fruit, even if the inside of the fruit is healthy. A general estimate suggests that the fruit of an apple tree will be damaged if temperatures reach below about 28 degrees F for at least 4 hours (a "hard freeze"). But frosts can occur at higher temperatures than that, so apples can be damaged at 32 degrees F if there is a frost.
If your apples are ripe, harvest them before a frost. If they are far from being ripe, then you can either harvest them to use for processing or leave them on the tree and see how they do during the frost. If apples are damaged by frost, harvest them gently and then use them quickly.
Grapes
Grape clusters can tolerate temperatures above 30 degrees F. But like apples, frost on the berry skin damages the grapes. After a frost, the berry skin begins to soften because the cells in the skin have ruptured during the frost.
Grapes are susceptible to rotting after a frost. But if they don’t rot, they become sweeter as the berries dry and the sugars become more concentrated.
Strawberries
The leaves can handle a frost, but the fruit cannot. If you have day-neutral strawberries, harvest all ripe berries before a forecasted frost. Then, cover the plants with a sheet to protect them. Remove the sheet in the morning once the frost is over.
If you have June-bearing strawberries, don't do anything! In fact, they have to be exposed to repeated cold temperatures in order to go dormant for winter. Wait to apply straw until the plants are totally dormant, which is usually in November. Covering them with straw now could be detrimental to the plants because they are still actively growing.
Raspberries
Similarly to strawberries, the leaves can tolerate some amount of frost (and even some snow) but the fruit cannot.
If you have fall-bearing raspberries, harvest the fruit before a frost. The plant may keep ripening fruit after a marginal frost, but a hard frost marks the end of the season. If you have summer-bearing raspberries, do nothing. Be aware that a hard freeze will kill the leaves.
Blueberries and other summer-harvested shrubs
These plants were done producing fruit for several weeks or months now. A light frost is normal for late September or early October, and the plants are able to withstand it.
We would be more concerned about a hard freeze this time of year, as the plants are still green and have not put their energy reserves below ground for winter yet. As the leaves wither and fall off, the plants go into dormancy. Cold temperatures in late September and October help the plants transition to dormancy.
Author: Annie Klodd, Extension fruit educator
Related topics: Yard and Garden News Featured news
Original source can be found here.
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