Wednesday, February 15, 2012

Snap! Cold Damage to Landscape Plants

Warm Winter Weather
Landscape plants appropriate for the metro-Atlanta climate go through a cold acclimation process each fall to prepare for winter dormancy, but this year’s exceptionally warm winter has been counter-productive for in many plants. Until last weekend, warmer temperatures were enticing plant buds to break a month ahead of schedule. The 2012 Camellia blooms were spectacular, plus the response to the weather pushed many Saucer Magnolias into full bloom across the city.

Then the thermometer fell into the teens this past weekend. Everyone has frozen a head of lettuce by accident, but what causes the change is ice formation within the plant cells. Water expands as it turns to ice and bursts the individual cells, causing plant tissues to become brown and mushy.

This cold snap wasn’t very severe and produced only limited cold injury to some plants. The hardest hit were the flowers, the most fragile all plant parts. Camellia petals and saucer magnolia flowers turned to mush, although daffodils, witchhazel and hellebore flowers held up. Leaves, stems, fruits, trunks, (but hopefully not roots) of borderline hardy plants experienced some problems, along with new transplants and container-grown plantings.   

We also had plenty of wind last weekend. Windy conditions that accompany cold weather can cause plant damage through desiccation (evaporative water loss exceeds water absorption) causing plants to dry out. Marginal leaf scorch on broadleaf evergreens or loss of buds are telltale symptoms. When possible, Crabapple LandscapExperts will provide a deep watering before a cold snap to help landscape plants weather the weather. The rains of the week before helped greatly with this.

Bark splitting can be a problem when tender new bark is exposed to the first rays of sunlight in early morning. The LandscapExperts maximize the chances that landscape plants will survive the winter by carefully selecting our planting sites. For example, locating Camellias with a little early morning and afternoon shade helps with bark splitting.

Although not common in Atlanta, vertical frost cracks on the trunks of woody plants can occur in sections of the state when plants are exposed to extremely cold temperatures. A frost crack is a long, deep, narrow crevice running up and down the trunk of a tree. As temperatures cool down, the temperature of the trunk drops quickly and the trunk contracts and may split.

Care of Landscape Plants after a Freeze
Crabapple LandscapExperts will not rush to prune or remove your damaged plants, because while some plants will look bad, they are not really dead. We wait until the full extent of the damage can be determined to start our corrective pruning.  

Flower damage is immediately visible, but doesn’t threaten the plants. Foliage and shoot damage is noticeable within a few days, but it may be a month before all the damage can be ascertained. We scratch the brown outer bark to see if the cambium layer beneath (greenwood) is still green, toggle leaf buds, and watch for live green foliage to emerge. Once the damage has been determined, we will prune to eliminate any damaged parts.

Late-winter pruning is best. P. Allen Smith lists 3 benefits: late-winter pruning invigorates the plants for abundant growth when Spring does arrive; the wounds are exposed for only a limited time before new growth begins; and it is easiest to see what exactly needs to be pruned before there are leaves on the shrubs and trees.

Trees known as “Bleeding Trees” produce a large run of sap in late winter, and are best pruned later in the summer. Some of these include maples (well-known Sugar Maple), dogwoods, birches and elms.

And for a great specialty video on pruning tree peonies, watch Felix with Gurney’s. 

Camellia photos thanks to Geri Laufer (white = Shiro Chan C. japonica, red = Bob Hope C. japonica, Cold Damaged Nuccios Pearl C. japonica)
magnolia photo thanks to 

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