Frost Cracks

Frost cracks may look fascinating on small twigs, but they can destroy tree trunks.

Twig with frost crack (Kate Russell)

The first step to preventing frost cracks is to know your frost dates.

Frost dates

Frost dates are averages of when the first and last frost will occur in your area. Local frost dates provide valuable information to gardeners. You also need to know that those dates are only statistical averages. Frost may still appear before or after those dates, so be prepared.

Plants in winter

In nature, plants protect themselves from frost cracks and winter sunscald by absorbing large amounts of water from the ground and radiant heat from the soil. Even a blanket of snow provides insulation that stabilizes soil temperatures for the roots. Once the ground freezes, the water they need is locked up. Plants also go through chemical changes that help prevent death by freezing. Sometimes, those adaptations are not enough, and frost cracks can occur.

How do frost cracks happen?

Frost cracks can occur in two ways: the moisture inside the plant freezes too quickly, or the outer bark cools faster than the living wood. In the first case, the water stored within the plant freezes and forces its way out of those cells. The loss of moisture causes these areas to shrink and crack away from the unfrozen plant tissue. As temperatures rise, the tissue may be able to absorb enough water to close the crack. It will always be a weak spot, prone to pests, diseases, and re-cracking. Repeated cracking can result in a condition known as frost ribs. Frost ribs are permanently damaged areas that remain susceptible to pests and diseases.

These frost cracks tend to be vertical, exposing the cambium layer. They can be several feet long, though you may not see them until spring when the tree resumes growing. Frost cracks are more likely to occur at previous injury sites, providing a good argument for protective tree supports in high-traffic areas.

Frost cracks can also occur when the bark and underlying wood are heated during the day, causing them to expand. As temperatures drop in the evening, the bark cools faster than the internal wood, causing the bark to crack.

Frost cracks commonly occur on the west and southwest sides of thin- or smooth-barked trees, such as apples and pears. Beech, crabapple, horse chestnut, maple, and willow may develop frost cracks, too. Other plants are susceptible to frost cracks and winter sunscald, as well.

Caring for frost cracks

Once a frost crack has occurred, it will likely happen again. Caring for these injuries goes a long way toward protecting your tree from invading pests and diseases. First, resist the urge to paint sealant or tar over the wound. Sealants can trap moisture against the injury and increase the chance of decay, especially around branch collars. If a frost crack has jagged edges, you can help your tree heal by cutting a narrow strip (< 1/2”) around the wound with a sharp, sterile knife, creating smooth edges. Doing so will speed the growth of the cambium, which will develop into a protective callus.

How to prevent frost cracks

First and foremost, keeping your plants healthy and properly irrigated will help them protect themselves. Since healthy tissue and defective tissue expand and contract at different rates, maintaining healthy trees in the first place is your best insurance against frost cracks and winter sunscald. These other tips can make a big difference in how your plants weather the winter:

• Select varieties best suited to your microclimate.
• Whitewash trunks and exposed branches.
• Avoid fertilizing late in the growing season; wait until after the leaf drop.
• Leave frost and ice-damaged leaves in place, as they provide insulating protection unless it is a soft-stemmed plant. In this case, remove any damaged areas.
• Protect temperature-sensitive plants with blankets or other coverings. But keep the blanket from touching leaves and stems, and allow the material to hang rather than bunching it around the trunk. Doing this helps collect radiant heat from the surrounding soil.​

Remember, it is far better for your plants to have frost and ice protections in place before they are needed.