Overcoming Winter Scorch

Tim Johnson —  April 29, 2014 — 9 Comments

Gardeners are facing bigger challenges than usual this spring due to a “perfect storm” of weather conditions that scorched evergreens, protected plant predators, elicited heavy use of road salts, and encouraged snow molds. The scorch or burn that has left patches of brown on arborvitae (Thuja), yews (Taxus), boxwoods (Buxus), and other evergreens is the worst and most widespread I’ve seen in my 29 years at the Chicago Botanic Garden. Branch damage from voles and rabbits is also particularly bad this year, and heavy and prolonged snow cover also promoted snow molds, creating bleached-out patches of lawn. Road salts put additional environmental stress on our landscaping.

PHOTO: A boxwood hedge with the outer foliage killed by winter damage.

Although slow-growing, this boxwood (Buxus microphylla) should make a full recovery.

The bad news is that more plant damage is likely to appear once the weather is consistently warm, though many plants will recover from the long, hard winter. While plants may have to be severely pruned or removed altogether, the polar vortex has given us a few important reminders about growing in the Chicago area and could ultimately make us all better gardeners.

PHOTO: A completely brown, winter burnt white cedar.

Winter was particularly hard on this Heatherbun white cedar (Chamaecyparis thyoides ‘Heatherbun’).

During the cold winter months, evergreens continue to lose water vapor through their leaves or needles. The leaves must replace the water by pulling it up from the roots. But when the ground is frozen, the plants’ roots cannot absorb water to resupply the leaves.

If the weather turns warm and sunny while the ground is frozen, evaporation from the leaves increases and the water cannot be replaced. The resulting symptoms, discolored or “burned” foliage, tend to show up quickly in spring, when days are sunny and warm.

Bright winter sun and strong winds can accelerate evaporation, and it’s typical to see the worst burning on the west- and south-facing sides of a bush or tree. Signs of winter burn include needles or leaves that have turned golden or brown. Sometimes a plant has an overall yellowish or off-green color. Leaves may appear bleached. Salt sprayed up by passing traffic can exacerbate the problem and accentuate damage on the road-facing side of the plant.

Many evergreens—particularly fast-growing varieties such as yews—will be fine after a light pruning. Deeply scorched plants will require heavy pruning, leaving unsightly “holes.” Slower growing evergreens may take years to recover from a severe winter burn, and gardeners must decide on a case-by-case basis whether it’s best to remove the specimen.

Unfortunately, some evergreens will be a total loss. This is especially true for plants grown at the edge of the hardiness zone. To determine whether a bush is going to make it, look for new buds or lightly scratch a branch to look for signs of green wood. Patience is often a virtue in gardening, so if you have any doubt about a plant’s viability, give it some time. 

PHOTO: A browned Bosnian pine, with fresh green growth at the tips of its branches.

This Emerald Arrow Bosnian pine (Pinus leucodermis ‘Emerald Arrow’) also shows hard winter damage.

The deep drifts of snow and prolonged snow cover were a boon to such plant predators as voles and rabbits. The blanket of snow shielded voles, mouse-sized creatures that travel under the snow, from hawks and other predators, leaving the creatures free to gnaw on branches and trunks. The drifts also provided a stepladder for rabbits, which feed on top of the snow, allowing them to reach higher into bushes. Signs of rabbit damage include a 45-degree cut in branches. Severe rabbit damage often looks like a bad pruning job, but gardeners can improve the situation by evening out the bush. Branches that have been girdled—or chewed all the way around—are likely to die and should be pruned back.

The sparkling white drifts also promoted snow molds, which can leave large patches of dead-looking lawn. Typically, lawns will bounce back after raking and light fertilizer. Lawns damaged by salt spray might not recover as quickly, and strips growing along roads might need to be replaced altogether.

PHOTO: Grass showing winter damage.

Grass showing winter damage will recover fairly quickly with attentive watering and care.

It’s never good to lose a plant or shrub to the elements, but the polar vortex did provide the type of reality check that can lead to best gardening practices. In a relatively mild winter, Chicago-area gardeners may have success with plants growing at the edge of their hardiness, but these plants can be killed or severely damaged in typical USDA Zone 5 conditions. Perhaps you’ll think twice in the future before putting something less-than-hardy in the garden. Good mulching and watering habits, and planting in the spring to give plants an entire growing season to become established, will increase the vigor of your plants and may help minimize winter burn.

Another virtue of gardening is that it forces you to look forward. So keep last winter’s lessons in mind as you clean, prune, maintain, and perhaps replant this spring.

For more information about gardening post-polar vortex, go to our free public Plant Information Service: chicagobotanic.org/plantinfoservice

©2014 Chicago Botanic Garden and my.chicagobotanic.org

Tim Johnson

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Tim Johnson is director of horticulture at the Chicago Botanic Garden. Tim holds a B.A. in horticulture from the University of Nebraska.

9 responses to Overcoming Winter Scorch

  1. Hi Tim,

    Thank you for writing about the winter damage to evergreens! I own a garden design and maintenance business and we maintain many gardens in the suburbs of Detroit, (Zones 5-6).
    We are seeing more winter damage to our beloved boxwood and all other evergreens than ever before. Your advise to wait and see is reassuring, and I agree with you. We are pruning out the dead wood on the broadleaf evergreens, but often there is just too much winter burn damage on the boxwood to prune! We are applying HollyTone by Espoma, and hope for the best. What do you think?

    Love your Blogs! Thank you
    Alison

    • I have not personally used HollyTone by Espoma but think it may be of some benefit to the plants that you are caring for as it will provide a gentle boost of nitrogen. Take a close look at the remaining green foliage that is under the burned portions on the boxwood. We have discovered some boxwood that look okay from a distance after shearing off the damaged foliage. Upon looking more closely at the foliage, we have found it to be green but dried out and somewhat “crunchy” which indicates that the plant is dead and will need to be replaced.

      • Hi Tim, thank you very much for your response! Unfortunately I know exactly what you mean about the ‘dried crunchy’ stems and leaves on the box.
        Thank you! (Sorry we beat your team last night!)
        Yours,
        Alison

  2. Liberata DI CENSO April 30, 2014 at 3:17 am

    I have fig tee on my garden we cover on winter. Time, when I went to take off the cover the fig, the are all moldy. What can I do to take the mold off?

    • The mold should go away on its own now that there is good air circulation. Any foliage that is rotted would be of concern and should be removed. You could try washing off the mold using a garden hose too.

  3. What should I do to a scorched boxwood? Should I prune all the discolored leaves off?

    • It would be a good idea to prune off the dead portions though this can be time consuming for some plants. If there are sporadic dead leaves or small sections of dead leaves on live stems then less important to prune off the dead leaves unless the appearance really bothers you. I would focus on pruning out sections of the plant that have dead stems and leaves to increase the light getting to live foliage deeper in the plant.

  4. Is it possible that this past winter has killed a Wisteria vine (approx. 10 yrs old) that has otherwise been happily blooming for the last 4 springs? The vine shoots appear to have some “green” inside them, but bud formation on the vine overall is practically absent so far.

    • Yes, the winter could have killed your wisteria. It would be a good idea though to give it more time to see if some buds break. Also, watch the base of the plant to see if there is any new growth should the top of the vine die back.

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