Trees Not Overwintering - Solution Guide
The establishment of new trees can take up to 18 months, which will include their first winter. It is during this establishment period when trees are most vulnerable. The reality is that many factors can lead to trees not overwintering, making diagnosing the issue difficult. An important fact is that proper winterization of trees, like other woody deciduous plants, starts much earlier in the growing season than gardeners may realize. Below are some of the most common reasons trees can fail during winter and some valuable solutions for avoiding this disappointment.
Late Fertilization
Late feedings, especially with nitrogen, can interfere with the tree's ability to go properly dormant. Winter dormancy is not predetermined. It is an evolutionary process dictated by hormones in the tree, which are influenced by growth factors such as soil fertility and the availability of nutrients. If trees do not go completely dormant, or their dormancy is delayed or interrupted, they can suffer from dieback or winter kill.
- Solutions: Fertilize trees only in early spring when they are starting their second season in the ground. Newly planted trees should not require fertilizer if they are planted using adequate organic soil amendments. If a water-soluble fertilizer like Root & Grow® by Bonide® is used to help boost root growth, applications should stop by early July in colder zones. Mycorrhizal inoculants like MYKE® added at planting time increase the tree's capacity for establishment and natural strength.
Late-Season Pruning
Pruning stimulates growth. If woody, deciduous trees or shrubs are pruned too late in the season, that can initiate the production of growth hormones rather than dormancy-inducing hormones. This hormonal imbalance can leave trees vulnerable to permanent winter damage.
- Solution: Perform all necessary tip pruning of new bareroot trees immediately after planting, and all annual pruning of established trees should be done during late winter to early spring just prior to trees breaking dormancy. Root suckers or distinctive water shoots may be safely removed during the summer if needed, but this type of corrective pruning should be done as early in the summer as possible. If you want to fertilize your trees, you can do it right after they go dormant. That will ensure they have nutrients in spring when they start to grow.
Thawing & Re-freezing
Temperature fluctuations can lead to interruptions in dormancy. Warmer temperatures in late fall or even in the middle of winter can cause sap to flow, leading to damage or winter kill. Often, but not always, tree trunks can display vertical fissures or cracks or unnatural bulging when this happens.
- Solutions: Make sure any trees chosen are cold-hardy for your USDA zone. Weather patterns or extremes cannot be controlled, but mulching the root zone well into winter is an easy and beneficial step that helps stabilize soil temperatures. Wrapping trees with breathable fabric like burlap or a paper-type tree wrap can help protect outer bark layers from sun scald and wind burn and offer an extra bit of insulation during the winter.
Soil Kept Too Dry.
If the soil is too dry in winter, desiccation can lead to permanent winter damage. Water does not encourage growth, and proper soil moisture helps keep roots and limbs properly hydrated going into winter. Dry soils cause the roots to draw moisture down from the upper leader & limbs, leaving them susceptible to drying during periods of low natural humidity during the winter months, which can lead to physical damage or death.
- Solution: Maintain consistent soil moisture going into fall for deciduous & evergreen trees and shrubs. Apply ample mulch over the root zone to help maintain precious soil moisture.
Soil is Too Cold.
Too much cold during the winter can kill or damage tree roots. Evidence of this damage may not be known until the tree forms leaves in the spring when the new season's foliage quickly wilts and dies. This is often the cause, especially if the proper soil moisture is evident. Wilting and spring foliage dieback can also happen due to a simple lack of adequate soil moisture in springtime.
- Solution: Proper mulching around trees helps to insulate soil and roots, especially those of shallow-rooted species. Snow is a good insulator, but since snowfall is unpredictable, applying 3-4 inches of compost, shredded leaves, or bark mulch is the best preventative measure to avoid soil getting too cold.
Winterburn or Scorching.
This type of browning or winter scorching can be common on evergreens such as Arborvitae, Pines, and Juniper or broadleaf evergreens such as Boxwood or Rhododendron. The symptom of browning leaf tips migrating from the top of the plant working downward is a typical sign. This happens due to desiccation or drying of the tissues, which can be caused by a combination of intense winter sun and wind. During cold weather, even though these are evergreens, the plant's vascular system is dormant; therefore, any lost moisture cannot be replaced, and those leaves or needles die.
- Solutions: Maintain proper soil moisture going into winter. Mulch well over the root ball to help preserve precious soil moisture. Wrapping or protecting susceptible evergreens, especially from harsh winter winds, may be necessary and beneficial. Note that evergreen foliage can be susceptible to and become burnt due to deicing salts. Before planting evergreens near walkways or driveways, amend the soil appropriately to ensure proper drainage since excess salts can be leached more easily with applications of water if needed. Evergreens that do suffer from salt burn recover slowly and may become permanently damaged in severe situations.
Summary
Winter conditions can cause difficulties for plants just like for people, but newly planted or immature trees and woody shrubs are more vulnerable. Taking the proper cultural precautions, planting in the ideal location, and protecting trees as much as possible will usually get them through their first few winters without issues. Properly established and mature trees will become more capable of weathering whatever mother nature can throw at them as long as they are kept healthy and growing well. For young trees, use a tree wrap or tree spirals for the first couple of years until the mature bark starts to break through the juvenile bark. It also protects them from bark that can crack from cold winter temperatures.
