By Kathleen Cue, Nebraska Extension Horticulture Educator (Week of March 22, 2021)
Spring’s welcome temperatures give us a chance to walk the landscape, checking to see how our trees and shrubs weathered the winter. Rabbit feeding damage on burning bush, vole paths over the lawn, and browning of evergreen needles are some of the things you will notice.
Evergreens have the unique ability to photosynthesize when temperatures reach 45° F, which seems odd but can and does occur during our winters. Tree roots in frozen soil, as is often the case during our winters, means photosynthesis is using water that evergreens cannot replace. The result is brown needle tips that can encompass all the leaves on the south side of the tree. (The Northern Hemisphere’s tilt away from the sun during the winter months makes this possible.) The drying effects of bitterly cold winter winds also dries out foliage. Tree owners and tree services alike are reporting lots of tip browning of evergreen needles this spring, primarily of spruce trees.
The time to prevent needle browning is in the fall of the year. Tree owners should take note, making sure the soil is evenly moist prior to ground freeze. This ensures tree tissues are well-hydrated. The screwdriver test is the easiest way to check soil moisture levels when ground is unfrozen. A screwdriver blade difficult to push in is indicative of dry soil, while a blade that pushes in smoothly indicates good soil moisture.
Newly planted trees are the most severely affected by winter’s dryness, having less established roots to mine the water they need. Besides, watering, newly planted trees benefit from the application of an anti-desiccant in late fall. Anti-desiccants are spray-on products that contain wax or plastic to thinly coat needles and broadleaf evergreens, reducing the amount of winter water loss from drying winds and warm days. Anti-desiccants should be applied twice, once in late fall and a second application in mid-winter, and only on days in the 40-50° F range. More information about anti-desiccants is found here: https://ag.umass.edu/landscape/fact-sheets/protecting-evergreens-in-winter-qa .
There is nothing that can be done to force evergreen needles to re-gain their green color. Evergreens shed needles when the leaves are not photosynthesizing efficiently. The best thing to do now is to practice vigilance and water trees when there has been more than a month of no precipitation (or use the screwdriver test to check soil moisture levels). Refrain from fertilizing trees as this can deepen the effects of winter injury and summer drought. Snow melt can skew our perception of just how dry things are, indicating the importance of checking soil moisture. The U.S. Drought Monitor has large portions of Nebraska in the Abnormally Dry to Moderate Drought conditions: https://droughtmonitor.unl.edu/