From driving around the county, there is no other way to put it: some trees just look bad. In actuality, the number of damaged trees is much less than the healthy ones, but those are the ones that catch our eye. There are various reasons for dying trees. Currently, emerald ash borer is wreaking havoc to many ash trees. Beech blight disease is attacking beech trees. Many spruce, especially Colorado blue spruce, have rhizosphaera needle cast or another fungal disease often causing the tree to die from the bottom up.
But this year, one disorder very prevalent, especially on evergreen trees in landscaped areas, is winter burn.
As the name suggests, winter burn (winter injury) is not caused by any living organism, so it is considered an abiotic disorder. Each year, winter burn is common, but the combination of weather conditions last fall and throughout the winter seem to have accentuated the damage this year. Winter burn is often seen on plants like arborvitae, fir, hemlock, pine, spruce, false cypress, juniper, yew and some broad leaf plants like boxwood, holly and rhododendron. Symptoms of winter burn include sections of uniform browning on the plant canopy especially on foliage on the south, southwest or west of the plant. If the damage looks more like branch tip dieback that is scattered throughout the canopy, it may have a different cause.
Winter injury is often a result of evergreen plant leaves (needles) drying out during the winter. Because the ground is frozen, the water stored in the needles needed to have been taken up by the plant before it went dormant for the winter. Therefore, anything that limited fall water uptake can make a plant more prone to winter burn. Water loss can also be due to bright winter sunlight and winter or early spring freeze/thaw cycles. Additionally, if plants are not fully dormant when the weather gets cold, winter burn is more likely.
Recommendations for saving plants with winter burn varies by plant species. For arborvitaes, boxwoods, junipers and yews, the recommendation is to prune out dead branches all the way back to alive sections of the tree/branch. If you aren’t sure if a branch is fully dead, you can scrape back a bit of the bark to check for green tissue underneath, which indicates the branch is still alive. Obviously, with some of the damage we are seeing this year, pruning out dead branches may completely disfigure the tree or leave nothing behind.
Firs, pines and spruces can often grow out of the damage, so pruning may not be needed. However, if the whole plant is brown, it likely won’t recover and should be removed.
Uncontrollable winter conditions definitely contribute to winter burn, but there are many other factors that can make it worse; some of which we can control. To minimize the chance of winter burn in the coming years, use the following guidelines:
Make wise and informed planting decisions. Before planting, make sure the plant is cold-hardy enough for where you place it or that you place it in a protected location (e.g. east side of a building, in a courtyard etc.).
Do not plant evergreens after early October. It is generally best to plant evergreens in the early spring or late summer. If trees or shrubs are planted in the mid-summer, make sure they are watered frequently to prevent water-stress.
Do not prune or fertilize evergreen trees after late summer. All pruning cuts stimulate growth at that site. If a tree puts out new growth late in the year, it may not harden off before the winter and thus is more likely to be injured. Fertilizer applications have similar results. Soil tests can help dictate whether fertilizer is needed (or not) and when to apply it.
Properly mulch evergreens. A mulch base can help with water infiltration around the tree, protecting roots from injury, and insulating roots from soil temperature fluctuations in the winter. Recommended mulch thickness varies based on soil type: clay soils should have about two inches of mulch and sandy soils should have about four inches of mulch. Preferred mulches include loose shredded hardwood, pine or cedar bark, leaf compost or wood chips. Mulch should always be at least three inches away from the trunk of the tree to minimize potential disease and rodent damage.
Water plants well. As stated above, winter burn is often a result of the plant leaves/needles drying out. Appropriate watering during the growing season can help ensure the plant is well-hydrated going into the winter.
Protect plants during the winter. A barrier of burlap, canvas, snow fencing or other materials can help minimize the likelihood of winter burn. The material should be connected to stakes placed at least two feet from the plants to provide shade and a windbreak. Wrapping material directly around trees should be avoided because it can lead to plant diseases by holding in ice and moisture.
Despite some distinctive characteristics, tree diseases and disorders can be very difficult to identify by sight. The University of Wisconsin Plant Disease Diagnostic Clinic specializes in plant disease identification. Samples can be brought into the Door County Extension Office (421 Nebraska St., Sturgeon Bay) and they typically cost $20 plus shipping.
Questions regarding backyard tree health or other insect, weed or disease issues can be sent to the Master Gardener Volunteer Plant Health Advisers who volunteer in the local UW-Extension office to help answer questions. The Plant Health Advisers are available twice a week and can be reached at 920.746.5984 or [email protected] If possible, to aid us in a timely response, please submit pictures along with any question. More information about winter burn can be found at door.uwex.edu/horticulture.