Lynda Ellis, Extension Master Gardener in Anoka County, September 10, 2014
Do you remember the hard winter we had last year? After the long-awaited Spring 2014 finally arrived, we found some of our evergreens were looking worse for wear, with reddish-brown needles. Some plants were completely brown on top, though they might have been green at the bottom. They were victims of winter browning. Why did that happen? Can we prevent it happening again next spring?
Winter browning is usually due to freezing following a spell of warmer weather. The warmer weather causes needles to lose water (transpiration); the freeze prevents the water from being replenished from the roots. Often this injury occurs above the snow line. The part of the plant above the snow is warmed; the part under is protected.
The hard winter hurt many trees, but smaller evergreens may have benefited. A stand of low yew shrubs were badly browned in Spring 2012 and 2013; some died each year. They all were protected from winter browning by the heavy snow pack of this past winter and looked good in Spring 2014.
Winter browning does not necessarily kill the plant. The brown needles on a branch are dead and will fall off. But if the buds at its ends are alive, new needles will form. Wait to prune until after bud break, and only prune dead branches.
Occasional winter browning occurs in most evergreens, but repeated winter browning will weaken them and make them more susceptible to insect pests and disease. Don’t apply anti-transpiration sprays to reduce winter browning. Research has shown they are not effective for this purpose.
Instead, reduce winter browning through watering, mulch, and possibly planting different evergreens. Continue to water evergreens until the ground is frozen and you turn off the water to outside faucets. The average freeze date in the Metro area is December 6. Put several inches of mulch around evergreens, especially when young. Plant the more resistant black and white spruce or jack pines, rather than the more susceptible red and white pines, yews, arborvitae, hemlock, and Colorado blue spruce.
If evergreens are not badly damaged, they will again turn a healthy green after the last brown needles fall. For more information, see: http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/winter-injury-on-trees/ and http://blog.lib.umn.edu/efans/ygnews/2010/11/help-your-woody-ornamentals-su.html .