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| Welcome to our tree column, “Ask your local arborist.” Each month, arborists from the Davey Tree Expert Company in Menlo Park discuss a tree-related topic from general care and planting tips to how trees can impact your property. If you have a tree-related question or an idea for a topic to discuss, email lifestyle@embarcaderopublishing.com or post in the comments section at the end of this column. |
When people think about winter tree damage, they often picture heavy winds or rain. On the Peninsula, the wnter sun can also pose a threat — usually a more subtle one. Bright winter sunshine followed by cold nights can stress tree bark and lead to two common problems: sunscald and frost cracks.
These injuries are more than surface-level problems. Over time, they can weaken trunks, create entry points for insects and decay, and compromise a tree’s long-term health.
What is sunscald?
Sunscald occurs when winter sun warms a tree’s bark enough to activate living tissues just beneath the surface. This most often happens on the south or southwest side of the trunk, where sunlight is strongest. When temperatures drop quickly after sunset, those newly activated cells can be damaged or killed.
Sunscald can appear as a discolored or sunken area of bark. In more severe cases, sections of bark can crack or peel away, exposing inner wood.
How frost cracks form
Frost cracks develop differently. As temperatures fluctuate, the outer bark and inner wood expand and contract at different rates. Repeated freeze and thaw cycles can build tension inside the trunk until it splits, often forming a long, vertical crack. These cracks may reopen year after year once they form.
Young trees and species with thin bark are especially vulnerable. This includes many ornamental and fruit trees commonly planted throughout Palo Alto landscapes. I’ve recently encountered a juvenile Oak tree that suffered sun scalding on the trunk due to too many lower branches being removed. Being left vulnerable to other pests, it then got attacked by borers and had to be removed completely.
Why Peninsula trees are at risk
Local conditions make these problems more likely. Winters on the Peninsula are mild enough for daytime sun to warm bark, yet cold nights still occur, especially during clear weather. That temperature swing is a classic trigger for both sunscald and frost cracks. Thin-barked trees planted in open, sunny areas are particularly susceptible.
Signs to watch for
Late winter and early spring are good times to inspect trunks and lower limbs. Warning signs include:
- Long vertical cracks in the bark
- Sunken or darkened areas, often on the south or southwest side
- Peeling or missing bark
- Exposed wood, sap seepage, or soft tissue beneath the bark
These symptoms should not be ignored. Open wounds can allow pests, fungi, and decay to move into the tree, leading to structural problems over time.
How to reduce risk next winter
- Use trunk wraps or guards
Light-colored wraps reflect sunlight and help prevent rapid temperature changes. Apply before cold weather and remove in early spring. - Mulch properly
A 2- to 4-inch layer of mulch helps insulate roots and stabilize soil temperatures. - Avoid late-season stimulation
Late pruning or fertilizing can encourage tender new growth that is more vulnerable to cold injury. - Maintain soil moisture
Watering deeply before a hard freeze can reduce drought stress and improve cold tolerance, even during dormancy. - Seek professional help
If you notice large cracks, extensive bark loss, exposed wood or signs that a tree’s structure may be compromised, it’s time to consult a certified arborist. A professional assessment can determine whether the tree needs pruning, monitoring, or additional support to remain safe and healthy.
George Reno is a local arborist at The Davey Tree Expert Company in Menlo Park, which serves communities on the Peninsula, including Menlo Park, Atherton, Palo Alto and Redwood City. Reno can be reached at 866-923-5658.
