Garden & Landscape Tips

Diseased spruce trees
Like many backyard evergreens, these diseased spruce trees are dying from the bottom up.
George Weigel

You’re not alone if you’re seeing your needled evergreens go downhill. Gardeners throughout the Northeast and Midwest have been noticing branch diebacks and tree failures in their spruce, firs, pines, hemlocks, and other needled evergreens, especially in the past three years.

The Issues

Rather than a single plague killing everything in its path, such as the emerald ash borer that’s devastating ash trees or the new blight disease that’s deadly to boxwoods, the needled-evergreen’s troubles trace to a variety of issues.

Some of the issues are general, including hotter weather and soggy soil from excess rains. Others are species-specific, including needle cast diseases that attack Douglas firs and heat-loving spider mites which target the popular dwarf Alberta spruce.

One issue that has been grabbing attention recently is the demise of one of America’s favorite home-landscape trees, the Colorado blue spruce. This is the evergreen with the stiff, powdery blue needles and sleek, upright habit.

Several fungi have stepped up their infection of blue spruces the past two years, especially one called Rhizosphaera needle cast that causes browning of needles from the bottom of the tree upward. After several years of spreading infection, trees often die.

Colorado blue spruce
Here’s what a healthy Colorado blue spruce should look like.
George Weigel

Behind the Woes

  • In the case of Colorado blue spruce, the changing climate has been a key factor. Blue spruces are native to the western U.S. mountains, where it’s cool and dry.
  • Higher temperatures are increasingly stressing cooler-preferring species, which includes Douglas fir, Fraser fir, larch, and concolor fir, as well as the blue spruce.
  • The extreme rains many areas have had in the past two years have led to rotting roots, as most needled evergreens don’t tolerate wet soil for long.
  • Sometimes rotting takes months to become apparent, which explains why trees seem to mysteriously die the spring or summer after a rainy year.
  • Rain and humidity are also ideal conditions for many disease-causing fungi. Healthy trees may head off attacks, but ones planted in compacted soil or in poor sites struggle.
  • Extreme and erratic shifts in temperature have been another recent stressor, especially when temperatures nosedive in fall before trees have had a chance to slide into winter dormancy.
  • Bugs are another major threat to evergreen, leading to a host of issues. Bagworms, spider mites, bark beetles, aphids, scale, sawflies, borers, and adelgids are among the insects that commonly target different needled evergreens.
  • As with disease, bugs tend to gravitate toward plants that have been stressed or compromised by other issues.
Douglas fir with needle cast
Needle cast disease is causing the needles of this Douglas fir to brown and drop.
George Weigel

What to do?

Short of regular fungicide and/or insecticide spraying, which is both difficult and expensive on mature trees, there’s not a lot that can be done to save diseased evergreens. The best course of action is to keep your trees as healthy as possible with good cultural practices. Some of these practices include:

  • Watering deeply once a week if a drought strikes.
  • Not packing excess mulch on the roots or trunk.
  • Testing the soil to provide adequate fertilizer and pH conditions.
  •  Avoiding bark damage with weed-whackers and mowers.

Better yet is doing your homework at tree-buying time to pick the most trouble-free needled-evergreen choices and to get them in sites they prefer, which are generally full sun and loose, well-drained soil.

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