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What causes evergreens to brown?

There are several reasons for browning needles on evergreens, whether they turn completely brown or have needles with brown tips and green bases.

“There are several potential causes, but the correct cause may be difficult to determine,” says Joe Zeleznik, North Dakota State University Extension Service forester. “Insects, diseases, flooding or mild winter temperatures may all play a part. Applying treatments to fix the trees may not work, especially if the exact cause is not determined.”

Symptoms cover a wide range. In some cases, almost the whole tree is brown or it’s brown just on one side of the tree. On spruce trees, it might only be the needles that are underneath the branches that are affected, but not those on top of the branches. Sometimes just the needle tip is damaged but the rest of the needle is mostly green.

The reason that the symptoms are so varied is that the potential causes are so diverse:

  • A traditional example of winter injury is when needles above the snowpack are brown, while those below the snowpack are green.
  • Another cause is when sunny, warm days in late winter or early spring coincide with frozen soil. During this time, the trees lose water through their needles but can’t replace it because the ground is frozen. Tissues then dry and die.
  • In other instances, the warm weather may cause trees to deharden too early. Then, when cold weather returns, some trees can’t reharden quickly enough, so the needles die.
  • Tree health during the previous summer also affects a tree’s response to winter extremes. For example, aphid or mite damage might not be noticed during the summer but shows up the following spring.
  • Lack of insulating snow cover can cause damage to the root system of a tree.

“Nothing can be done directly to fix trees that are suffering from winter injury,” Zeleznik says. “The only thing to do is minimize the stresses that the trees face during the upcoming growing season. Also, just because needles were killed doesn’t necessarily mean that the tree is dead because the buds still may be alive.”

While environmental damage such as winter injury or flooding may be causes, insect and disease issues may also be playing a part.

“Several items are important to remember when treating trees,” Zeleznik says. “Make sure you know what insect or disease you are treating. If you don’t, the treatment is a waste of time and money. Timing is critical because applying a treatment at the wrong time will not work and is a waste. Always follow all label directions when applying pesticides.”

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