Frost cracks look strange on small twigs, but they can be devastating to tree trunks.
The first step to preventing frost cracks is to know your frost dates.
Frost dates are estimates of when the first and last frost will occur in your area. It is important to know your local frost dates. It is also important to know that those dates are only statistical averages. Frost may occur 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, plants are unable to get the water they need. 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 two ways: the moisture held inside the plant freezes, 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 they crack away from the larger, unfrozen plant tissue. As temperatures rise, the tissue may be able to absorb enough moisture to close the crack, but it will always be a weak spot, prone to pests and disease, 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 disease.
These frost cracks tend to be vertical, exposing the cambium layer, and can be several feet long, though you may not actually see them until spring, when the tree resumes growing. Frost cracks are more likely to occur where injuries, such as collisions, have occurred in previous years, 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 are most common on the west and southwest sides of thin, or smooth barked trees, such as apple and pear, as well as beech, crabapple, horse chestnut, linden, maple, oak, sycamore, yellow poplar, and willow. Other plants are susceptible to frost cracks and winter sunscald, as well.
Caring for frost cracks
Once a frost crack has occurred, it is likely to occur again. Caring for these injuries properly will go 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 are not effective at treating these injuries. Quite the contrary, sealants tend to trap moisture against the wound, increasing the chance of decay, especially near 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. This will speed the growth of 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 is how your plants weather the winter:
Temperature-sensitive plants can be protected with blankets or other coverings, just be sure to keep the covering from touching leaves and tender stems, and allow the material to hang, rather than bunching it around the trunk. This helps collect radiant heat form the surrounding soil.
Remember, it is far better for your plants to have frost and ice protections in place before they are needed.
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