It's mid October and gardeners are covering plants and pots with sheets at night to protect them when frost is predicted. Plants can be “nipped” by frost when fat in cells on the outside edges of leaves solidify and rupture the cell walls. This kills the cells and creates black areas. Frost doesn’t necessarily kill our landscape plants, but it does cause leaf damage.
A "hard freeze" is defined as four straight hours of 28°F. This will kill foliage on trees and shrubs and the parts of herbaceous plants above the soil. But what about fruits like apples? What if they are still hanging on the tree when a hard freeze is predicted?
Apple fruit starts freezing at around 28-28.5°F, so what if you can't get out before a forecasted hard freeze to pick your apples? Will they be alright after a night of freezing? The good news according to U of M fruit researchers is your apples should be okay provided the temperature doesn't fall much below 28.
The longer apples are exposed to temperatures below 28 degrees, the higher the chance that they will get damaged. A brief dip below 28 degrees may just weaken the apples enough to decrease their shelf life. However, several nights below 28 degrees are more likely to soften the skin and flesh of the apple, making the fruit unusable.
When should you pick frozen apples?
Frozen apples should not be picked until the fruit thaws out, as the frozen fruit will bruise and be unusable. Late fruiting apples like U of M's SnowSweet® and Haralson are more at risk of freezing because they are more likely to still be on the trees when a freeze hits.
After a freeze, leave the apples on the tree and wait until midday when they have thawed out. At 22°F, the fruit will freeze hard and cells will break down, making the fruit soft. If only a brief freeze happens and the fruit is still firm, use the fruit soon, as it may not store well.
Some people say that some later apple varieties need a frost to sweeten them. Sweetness is one indicator of maturity and harvest-readiness along with fruit size and color. However, apples will ripen and sweeten up without a frost.
Why don’t apples freeze at 32 degrees?
While 32°F is the freezing temperature for water, it is not the freezing temperature for most fruits.
Fruits such as apples, grapes, and strawberries are high in sugar. The sugar in the water and the fruit’s skin reduces the temperature at which the fruit freezes.
So if a very cold night is ahead and your apples are still on the tree, it's best to change your plans and get picking!