If your evergreens suddenly have needle discoloration in early spring, they might have winter burn. This super common condition happens all over the U.S., but especially in areas with brutal winds and freeze-thaw cycles. It's frustrating, as the damage doesn't appear until weeks or months later, and once it happens, there isn't much you can do about it.
That's why winter burn is all about prevention, not treatment.
Above: Picea abies 'Susan' appearing healthy in the summer of 2021 and suffering from severe winter burn in the spring of 2022. I lost this conifer completely. I replaced the tree with a deciduous weeping beech.
How to identify winter burn
Winter burn is most easily identified by needle or leaf discoloration on the south-facing side of the plant or the wind-ward side of the plant. You could see browning or yellowing foliage, brittle or dry foliage, or needle or leaf drop.
The plant in question will likely look perfectly normal until about the time the daffodils and other spring-blooming bulbs emerge, when most plants are breaking winter dormancy.
(Source: Winter Burn – Wisconsin Horticulture)
Geographic areas most likely to experience winter burn
Winter burn is a specific type of winter damage that occurs when an evergreen plant loses too much moisture during fall, winter, and especially early spring.
Plants in northern areas with harsh winters, like in Canada, the northern United States, and parts of Europe and Asia, are more prone to winter burn. These regions often experience prolonged periods of a freeze-thaw cycle, sudden changes in temperature, and icy conditions.
You may also frequently encounter winter burn in high altitude locations and mountainous regions or prairie conditions with strong winter winds, especially if the area experienced an unseasonably dry fall.
Why winter burn occurs
Fascinatingly, winter burn isn't necessarily about how cold a tree gets. In fact, in laboratory settings, tissues of trees have been found to survive other-worldly cold temperatures.
Researchers studied dormant, one-year-old twigs of about 70 tree species across North America. The twigs were hardened off artificially for about one month through cold treatment, which triggered all the natural responses a tree undergoes in fall to go dormant for the winter. Then the twigs were artificially frozen to find their max freezing resistance. out of the conifers, those from the Northern U.S. and the Rockies survived between -60 and -80 degrees Celsius.
Another study found that white pine and American arborvitae could survive freezing below —120°C. The researchers hypothesized that it isn't the cold that kills these conifers, but instead dehydration from a combination of freezing soil and exposure to sunshine.
One complication of having evergreen leaves is that water can escape through them. This is a serious problem since liquid h20 isn't exactly easy to come by in the winter. That's part of the reason conifers have such interesting looking leaves: everything about them, from the size and shape of the needles to the waxy coating, are designed to reduce water loss. Even the stomata, or the tiny pore-like structures that aid in photosynthesis are recessed in chambers to protect them from the wind.
Despite the way conifers evolved to retain moisture, it isn't a perfect system. Water can still evaporate or transpire through the needles. Winter burn occurs when the loss of water in the needles surpasses the rate at which it can be replenished via the roots. Essentially, if your tree has winter burn, it went through a drought. In winter. While covered in snow.
Several factors contribute to this phenomenon. I garden on the great plains, and if you too garden in an extremely continental area, or far from the coasts or great lakes, then you know how variable yearly moisture levels can be. When we have an unseasonably dry fall, there is simply less water available to the plant going into winter. No water? The tree suffers from drought, just like in the summer.
Source: FS239E.pdf
For a similar reason, newly planted evergreens are especially susceptible to winter desiccation, as their root structures are small and relatively shallow, reducing the overall amount of water available. It also means they have less access to water below the frost line.
Source: FS239E.pdf
Additionally, winter burn may occur from cold, dry winds. Usually, winter burn happens from a combination of all these things.
You may have heard that conifers photosynthesize over the winter. From what I understand, this claim is somewhat true, but not to the extent that has been popularized. But it does seem like conifers can get a jump on photosynthesizing in the spring. While deciduous trees are working overtime to generate leaves, conifers are already photosynthesizing.
But for conifers that are actively photosynthesizing during this time, winter burn can be even worse. When a tree photosynthesizes, it opens its stomata, which let the tree transpire. This transpiration helps pull water into the roots via capillary action. But what happens when the stomates are open, but the ground is frozen solid? Rapid dehydration.
Just because conifers aren't photosynthesizing, doesn't mean that they have nothing to worry about in the dead of winter. In fact, they have to find a way to dissipate all that extra radiation energy from the sun that they aren't using to photosynthesize. If they don't, the sun will injure what's called their photosynthetic apparatus. This is especially true for conifers that stay green, rather than turning blue or silvery, during the winter. The green is caused by chlorophyll. Usually they deal with this excess radiation by dissipating it as heat and or by changing color from green to yellow or blue. When the tree can't keep up with the bright winter sun, the photosynthetic apparatus is damaged and winter browning occurs.
So if keeping their needles on during the winter is so damaging, why aren't all trees deciduous?
Even if there isn't much photosynthesis happening in the winter, evergreen needles are clearly an advantage in the spring and fall; by not having to form new leaves in the spring conifers get a quick jump on photosynthesis and can continue into the autumn long after the deciduous trees have shed their leaves. Perhaps more importantly, by not discarding their leaves every year conifers are able to conserve precious soil nutrients which are often limited in boreal forests because cold temperatures slow the rate of decomposition and nutrient cycling.
Conifers can open and close their stomata, or the pores on their needles. When they open their pores, they can both photosynthesize and pull water up from their roots via capillary action.
It's kind of their superpower--a spruce or pine tree can photosynthesize well before any deciduous tree by opening its stomata (like a pore of our skin), releasing water vapor through that pore, and in doing so, pull water through the roots via capillary action.
This incredible adaptation comes with a price: what if the stomata open while the ground is frozen? Without the ability to take up moisture from the roots, the plant loses too much water through transpiration and desiccates.
For this reason, it's easy to understand why plants growing in areas that often ice over are susceptible to winter burn: an area iced-over takes ages to thaw, meaning the plant is likely photosynthesizing before its roots have access to water. It also helps explain why it's so important to give trees, especially slow growing ones like conifers, plenty of time to establish themselves before fall: the deeper their roots, the greater the chance they can access moisture below the frost line.
Considering how deep the frost line can go in the coldest areas, it's no surprise that the toughest, most cold-hardy plants have incredibly deep roots.
Tree and plant root depths vary, depending on climate, soil and water conditions, including rainwater infiltration and ground water levels.
Ying Fan Reinfelder/Rutgers University-New Brunswick
It's also unsurprising that conifers growing in northern climates have to adapt to surviving in winters where their entire root system is completely frozen, especially as young trees. When this happens, despite a thick blanket of snow, the winter garden can act like a desert.
Loss of moisture via stomata is exacerbated by direct sunlight, indirect sunlight reflected from snow, and strong winter winds, making protection and siting critical for newly planted evergreens.
Site, treatment, and species selection makes a difference
Below are the factors that make a tree more likely to have winter desiccation.
An evergreen is at greater risk of winter burn if it is...
Planted in fall (rather than spring), meaning it lacks an established root system.
Exposed to south or southwest orientations.*
Excessively exposed to winter winds and/or on high hills.
Planted in dry, coarse, or poorly drained soils.
Stressed from drought, insect infestations, or disease problems.
Lacks protective mulch.
Ill-suited to the climate and conditions of the planting site.
Source: Irrigation in Forest-Tree Nurseries: Monitoring and Effects on Seedling Growth | SpringerLink
Conifers: Many coniferous evergreens, like pine (Pinus), spruce (Picea), fir (Abies), and arborvitae (Thuja), are susceptible to winter burn. Their needle-like leaves are more prone to moisture loss and damage from cold, drying winds.
Broadleaf Evergreens: Some broadleaf evergreens, including rhododendrons (Rhododendron), azaleas (Rhododendron spp.), and boxwoods (Buxus), can experience winter burn, especially if they are not well-established or if they are exposed to harsh winter conditions.
Newly Planted Trees and Shrubs: Newly planted evergreens, regardless of the species, are more vulnerable to winter burn because they often have less developed root systems that cannot efficiently take up water from frozen or dry soil.
Dry or Poorly Drained Soils: Evergreens planted in soil with insufficient moisture or poor drainage may suffer from moisture loss during winter, leading to winter burn.
Stressed Plants: Plants that have experienced previous stressors, such as drought, insect infestations, or disease problems, are more susceptible to winter burn because their overall health may already be compromised.
Lack of Mulch: Plants with roots that are not protected by a layer of mulch are more vulnerable to temperature fluctuations and moisture loss during winter.
Inappropriate Species Selection: Species that are not well-suited to the local climate and conditions may be more prone to winter burn.
How to protect plants from winter desiccation
As mentioned earlier, addressing winter burn is about prevention, not necessarily treatment. Here are a few things you can do to protect your plants.
Plant evergreens such as yew, hemlock, and arborvitae on north and northeast sides of buildings or in areas protected from wind and winter sun.
Construct a barrier of burlap or similar material on the south and windward sides of evergreens. Do not cover the plant completely or let the burlap touch the needles.
Water your trees.
Choose the right plants from the beginning.
Don't prune after August as pruning can induce the growth of new foliage.
Could size and drought tolerance be a predictor of winter burn resistance?
This is just a guess on my part, as I can't find a study to say whether this is true. But it does make sense from a few perspectives. For one, it makes sense that trees needing less moisture to thrive during a growing season would also be better prepared to handle winter drought from frozen roots.
In fact, in this totally fascinating article below in the Scientific American, the expert explains how you can predict whether a ponderosa pine will live a long life based on the aperture of its tracheids, or essentially the straw that facilitates water transfer. If any air enters the system, the aperture will close, sealing the tracheid. Researchers found that ponderosa pines with smaller apertures grew more slowly, and also lived longer lives than those without. Live fast, die young applies to trees, apparently.
According to the author, "Trees with smaller apertures resist drought better because when the torus is drawn up against them, they seal better. But smaller apertures also means water travels more slowly through the tree when the doors are open, slowing growth."
Based on this study, can we hypothesize that a plant's overall growth rate and drought-tolerance might be an indication of resistance to winter burn? I have no idea! From my own observations in my garden, whether the tree evolved in a climate with freeze-thaw cycles is a better indicator. However, i have always heard that dwarf trees are better suited for low-water and xeric applications, so perhaps this can help explain why.
Speaking of my garden, lets look at some successes and failures I can speak to personally.
Conifers and rate of winter burn in my personal garden
What grows well for me may or may not be suitable for your site conditions. I'll include some information about my garden and you can decide whether these trees will be a good fit in yours. You can compare your climate to mine in the following maps and draw some conclusions.
My garden:
Location: foothills of the Black Hills and Bearloge Mountains
Hardiness zone: 5a
Soil type: Silty loam
Rainfall: 22 inches per year, but varies from year-to-year dramatically
Summers: Around 85 degrees with a few weeks of 100+, low humidity, afternoon thunderstorms common. Severe weather like large hail and strong wind very common.
Spring/Fall: Extreme freeze-thaw conditions. For a long time, my town held the world record for biggest temperature change in one day. Our freeze-thaw cycle is lengthy, often starting at the end of September through December, and then March through May.
Winter: Our winters are unpredictable. Sometimes we experience 100 inches of snowfall, other years we're lucky to get half that amount. It isn't unusual to experience windchill of -40 degrees F, though our winter low is usually around -20 degrees F.
The Black Hills are a unique place, both geographically and climatically. Less than a mile from my garden is the arid steppe climate of the Great Plains, dominated by grassland and red clay. Only 5 miles from my garden are the mountainous conditions of the northern Black Hills, similar in many ways to the Rockies, albeit more mild. The Hills are smaller and tend to receive more water, although the overlap in plants that naturally grow in both spaces is incredible. Because of this overlap in native species and conditions, I would say that the metropolitan area most similar to my growing zone would be Denver, CO, with one major caveat: Dever gets about 11 inches of rain per year, whereas we get around 20 (although it's highly variable).
My garden is sort of annoyingly caught between extremes. It's far drier than what I consider the wet conditions east of the Mississippi, but nowhere near a desert climate. Meanwhile it's high above the coast at 3,500ft, but far from being considered a
high-elevation garden.
It's an island of small mountains in a sea of plains, where species of flora and fauna from east and west mix together to create astounding biodiversity, with plants flourishing here far outside of the rest of their native range. So while my growing area may very well be unlike anywhere else in the world, the fact that it has similarities to so many various types of climates (grasslands, forests, mountains, steppe, high desert) means that with a little understanding of the difference between our gardens, gardeners from varied climates may be able to extrapolate a lot of useful information from mine. At least, I hope so.
The next images show moisture levels and climate designations. Compare where your garden falls based on these parameters to help make educated guesses about which garden plants may be able to thrive in your own garden.
Exposed Site
(No windbreak from dry northern winds)
Conifers that have not gotten winter burn in fully exposed wind-prone site:
Picea glauca
mugo pine
rocky mountain junipers
ponderosa pine
Conifers on fully exposed site with negligible winter burn:
Picea pungens
Blue star juniper
Conifers I planted this year in fully exposed site *will update in spring:
fir
montana moss juniper
juniperous chinensis
Protected Site
(Protected from harsh wind by mature evergreens and privacy fence, protected from harshest sun from buildings)
Picea abeis
this species is perhaps the most disappointing--despite being in my most protected areas, p. abeis always gets winterburn. I've even lost a picea abeis 'susan' from winterburn.
Alberta spruce
eastern arborviate
Winter burn the first two years, now is negligible.
chameciparis pisifera
western arborviate*
yew*
boxwood*
weeping alaskan cedar*
pinus parviflora*
pinus densiflora*
cedrus libani*
pinus strobus*
*First year in ground, will update.
Further reading: mn_2000_ArbReview-00201.pdf (umn.edu)
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