It's not unusual for conifers to go into winter looking green and healthy, only to emerge looking brown, thanks to the drying effects of winter's sun and wind.
Here's how it happens: All plants lose moisture through their stomates -- the pore-like structures on the surface of their leaves or needles. But because deciduous trees and shrubs drop their leaves each fall, they don't lose much moisture in winter. Most conifers, on the other hand, hold onto their needles throughout winter, so they continue to lose moisture year-round. Unfortunately, their roots aren't able to pull in compensating moisture in winter because the ground is frozen. This results in brown needles, which may not be apparent until temperatures begin to warm in spring.
Some conifers are more prone to needle browning than others. Young or newly transplanted trees and shrubs seem most vulnerable. And weather conditions certainly play a role. But mulching, watering and creating physical barriers can help keep your conifers green.
Apply 3 to 4 inches of wood chip or shredded bark mulch over the root area of your conifers. Mulch keeps the soil from freezing as early in fall as it would otherwise and helps insulate roots from extreme cold and fluctuating soil temperatures. It also prevents the soil from drying as rapidly, which is important if we don't get plenty of snow cover. (The drier the soil, the more deeply frost can penetrate, damaging roots and compromising their ability to take in moisture come spring.)
You needn't remove the mulch. Instead, you can leave it in place year-round, replenishing it every year or two to maintain a sufficient depth.
Unless regular rainfall keeps the soil moist, you should continue to water your conifers every week to 10 days until the ground freezes. Fall watering alone isn't enough to prevent browning, but it is important, especially when rainfall has been scarce or spotty, as it has this year.
It's best to water conifers throughout the growing season, unless there's rainfall of an inch or more weekly. This allows plants to go into winter dormancy fully hydrated.
All conifers are more likely to develop brown needles when planted where they're exposed to lots of winter wind. Being exposed to bright winter sun exacerbates the problem.
Yews, hemlocks, and to some degree, arborvitae are particularly prone to damage from direct winter sunlight. Creating a physical barrier that will shield conifers from the sun can help. Rather than completely covering these plants, erect a barrier to the south/southwest side that will block the most intense sun. Lath snow fencing or burlap stretched between uprights should do the trick.
And if your conifer are growing old or scraggly from repeated winter die-back, consider replacing them with species that are known to grow better under sunny winter conditions.
You may have heard of people using anti-transpirants to protect conifers. Most anti-transpirants are sprays that consist of fine waxy liquid that plugs a plant's stomates, preventing moisture loss. The trouble is, they simply don't last long enough to be of much help here. In fact, some research suggests anti-transpirants remain effective no longer than a week or so and can only be reapplied when the temperatures reach 40 to 50 degrees -- which is not likely in most Minnesota winters.
Deb Brown is a horticulturist with the University of Minnesota Extension Service Yard and Garden Line. For help with garden, plant and insect questions, call the Yard and Garden Line at 612-624-4771.