During the gentle, warm, soft days of Autumn this gardener enjoys leisurely sauntering about the yard to say goodbye to the annuals and to wish the perennials a good winter’s slumber. “Goodbye lovely petunias. Thank you for bringing such beautiful color to my containers and for not getting those evil, decimating worms this year.” “Thank you weird jewel colored nasturtiums. I know you did your best, but unfortunately you didn’t bring much to the table…so this really is goodbye…” “Thank you little heucheras. You are my favorite little darlings and even though some of you return to the dark side – losing your brilliant leaf color – I will never desert you!” “Thank you wonderful… “ Ugh, then temperatures dropped like a stone, the snow arrived and there simply was no time for long, languid garden goodbyes. Sadly, many of the trees, shrubs and perennials were taken by surprise as well. Leaves remain on my sour cherry trees and several shrubs, but fortunately no branches have broken…yet… The extreme temperature swing may havetaken out both multi-stemmed redbuds and Gary the Ginko. When it warms up a bit…maybe next June?.. one can go out and check out next season’s leaf buds for damage and make an assessment as to whether survival is an option. So, to educate and share to knowledge… it’s off the internet, but I’m pretty sure it’s correct… here are some answers to both dilemmas -the quick freeze and the wild temperature fluctuations…but wait, there’s more! Also included is a bunch of horrible things that can happen to your outdoor sweeties during the winter – if you are new to gardening in Colorado, be afraid…be very afraid!
Why Didn’t My Tree Lose its Leaves? Deciduous trees lose their leaves every fall and grow new leaves each spring. Some usher out the summer with fiery fall displays as the leaves turn yellow, scarlet, orange and purple. Other leaves simply brown and fall to the ground. Particular types of trees sometimes lose their trees at the same time. For example, once a hard frost sweeps through New England, all the ginkgo trees in the region promptly drop their fan-shaped leaves. But what if one day you look out the window and realize that it’s mid-winter and your tree hasn’t lost its leaves. The tree leaves didn’t drop in winter. So why didn’t my tree lose its leaves, you ask. There are a few possible explanations for why a tree didn’t lose its leaves and both involve the weather. Some trees are more prone to leave their foliage attached than others, which is referred to as marcescence. These include trees like oak, beech, hornbeam, and witch hazel shrubs.
When a Tree Hasn’t Lost its Leaves? To understand why leaves did not fall off a tree, it helps to know why they usually fall in the first place. It’s a complex procedure that few people truly comprehend. As winter approaches, tree leaves stop producing chlorophyll. That exposes other colors of pigment, like reds and oranges. At that point, the branches also begin to develop their “abscission” cells. These are cells that scissor off the dying leaves and seal up the stem attachments.
But if the weather drops early in a sudden cold snap, it can kill the leaves immediately. This takes the leaf color directly from green to brown. It also prevents the development of the abscission tissue. This essentially means the leaves are not scissored off the branches but instead remain attached. Don’t worry, your tree will be fine. The leaves will fall at some point, and new leaves will grow in normally the following spring. A second possible reason that your tree didn’t lose its leaves in fall or winter is the warming global climate. It’s the dropping temperatures in autumn and early winter that cause the leaves to slow the manufacture of chlorophyll. If temperatures stay warm well into winter, the tree never starts making abscission cells. That means that the scissor mechanism isn’t developed in the leaves. Rather than dropping with a cold snap, they simply hang on the tree until they die. Excess nitrogen fertilizer can have the same result. The tree is so focused on growing that it fails to prepare for winter.
Winter injury to trees and shrubs. The frequency and severity of winter damage is determined by a number of factors, including the plant species or cultivar involved, the location and conditions under which the plant is grown, and the exact timing of weather extremes during the dormant period. Contrary to popular belief, plant damage is not generally caused by an unusually cold winter. Low temperature injury is more often associated with extreme temperature fluctuation than with prolonged cold weather.
TEMPERATURE FLUCTUATION: Acclimation to temperatures much below freezing results from exposure to slowly falling temperatures and other factors. Plants that are dormant but not fully acclimated can be stressed or injured by a sudden, hard freeze. Rapid or extensive drops in temperature following mild autumn weather cause injury to woody plants. Extended periods of mild winter weather can de-acclimate plants, again making them vulnerable to injury from rapid temperature drops.
LOW TEMPERATURES: Some species or cultivars of trees and shrubs are injured if temperatures fall below a minimum tolerance level. Plants most likely to suffer winter injury are those that are marginally hardy for the area or those already weakened by previous stress. Species such as rhododendron, holly, and some magnolias may survive several mild winters in the Chicago region before a more typical winter causes injury. Flower buds are often the most susceptible. If plants with marginal hardiness are used, they should be planted in protected sites, such as courtyards or sheltered areas. In general, low temperatures are much less damaging than rapid and extensive variations in temperature.
FROST CRACKS: Frost cracks, sometimes called radial shakes, appear as shallow to deep longitudinal cracks in the trunk of trees. They are most evident in winter at temperatures below 15°F. Frost cracks often, but not always, occur on the south or southwest sides of trees because this area experiences the greatest temperature fluctuations between day and night. A sudden drop in temperature causes the outer layer of wood to contract more rapidly than the inner layer, which results in a long vertical crack at weak points in the trunk. Once a frost crack occurs on a tree, it is likely to appear annually. Trees most susceptible to frost cracks include London plane, oak, Norway and red maple, horsechestnut, crabapple, walnut, linden, and willow.
SUNSCALD: An elongated canker found on the trunk of thin-barked trees, such as beech, maple, willow, white pine, and linden, is often referred to as “sunscald”. Sunscald often develops on the south or southwest side of trees following a sudden exposure to direct sun. In winter, the temperatures on the sun-side of the trunk may exceed air temperatures by as much as 20°F. This is thought to trigger de-acclimation of trunk tissue. The bark slowly darkens, turns reddish brown, and becomes rough. After a time, the callus tissue eventually cracks and falls away. Sometimes only the outermost cambium layer is damaged and a sunken area appears on the trunk. Affected trees often have sparse foliage, stem dieback, and stunted growth.
WINTERBURN ON EVERGREENS: A browning or scorched leaf tip on evergreen foliage in late winter and early spring is a form of winter injury. Browning usually occurs from the needle tips downward. Symptoms of winter burn are present on many narrow-leafed evergreens, such as hemlock, juniper, pine, and yew, and broad-leaved evergreens, such as boxwood and rhododendron. Winterburn is usually attributed to desiccation or loss of water through leaf transpiration. Winter sun and winds dry needles. Water in the stems and roots is frozen and unavailable to replenish the loss. A rapid drop in temperature after a warm sunny day can also cause further injury to the plant. Applying an anti-transpirant, also called antidesiccant, helps reduce transpiration and minimizes damage to the foliage. At least two applications per season, one in December and another in February are usually necessary to provide protection all winter.
SPRING FREEZES: Once spring growth has begun, a late spring frost can cause damage to de-acclimated woody stems, blossoms, and new shoots. Frozen, succulent, new tissue turns flaccid, appears watersoaked, and withers within a short time. Though symptoms resemble blight diseases, freeze injury appears suddenly after a hard frost, while diseases such as bacterial fire blight, juniper blight, and pine tip blight are progressive over time.
ROOT DAMAGE: Root tissues apparently do not acclimate to temperatures much below freezing and can be killed or severely injured by soil temperature below 15°F. This is especially true for shallow rooted plants. Fortunately, the presence of mulch, leaf litter, or snow cover insulates most soils sufficiently to prevent soil temperatures from falling much below freezing. Plants with frozen roots may wilt and decline after growth resumes in the spring.
SNOW AND ICE BREAKAGE: Heavy snow and ice storms cause damage by bending and breaking branches. Multi-stemmed evergreens, such as yews, arborvitae, and junipers, are often the most prone to damage. To protect these plants from limb breakage prior to winter, tie branches together loosely with strips of cloth or coated twine. Remove in early spring.
The branches of many hardwoods, such as Siberian elm, maples, and birch, may be seriously damaged in ice storms. Improper removal of ice or snow from the tree or shrub might increase damage. Heavy snow should be removed gently before it freezes to limbs and branches. Removing ice encased on branches can cause additional damage and should not be attempted. Instead, allow ice to melt off naturally.
SALT DAMAGE: Salts used for deicing pavements can cause damage to trees and shrubs. Symptoms of salt damage appear in spring and early summer and include browning of evergreens, leaf scorch, branch die back, and dead areas in turf. Branches and twigs can be killed from aerial deposits, and roots can be damaged from salt remaining in the soil. Salt will leach through well-drained soils, but damage can be extensive in poorly drained soils. Choose salt-tolerant species for sites where salt stress may be a problem.
GIRDLING BY ANIMALS: Mice and rabbits often damage young trees in the winter by feeding on the bark and girdling the trees. Damage occurs most commonly when there is prolonged, heavy snow cover, and food is scarce. Rabbits feed on the bark above the snow, while mice feed near the ground level. Mouse damage is usually more severe when the trees are surrounded by heavy grass, weed cover, or heavy mulch, so it is helpful to pull mulch away from trunks and branches. The most effective deterrent to girdling by mice or rabbits is to wrap the trunk and low branches of young trees with screen wire or hardware cloth from below the ground line to high enough above the possible snow line to prevent rabbits from reaching the trunk or branches. To help control mouse damage, maintain an area free of grass or weeds for a 1 to 2 foot radius around the base of the tree. Various chemicals are available to repel mice and rabbits, but are often not reliable in wet weather.
MINIMIZING WINTER INJURY:
- Select hardy species and cultivars.
- Avoid late-summer fertilization or pruning, which might stimulate new growth.
- Water trees and shrubs, especially evergreens, during dry periods until the ground freezes.
- Use mulch to conserve soil moisture and insulate the roots from cold temperatures.
- Protect evergreens from wind and salt spray with burlap screens.
- Apply anti-desiccant to evergreens starting in late fall, following label instructions.