Oak decline management


Oak stands stressed by old age, overcrowding, severe drought and defoliation are vulnerable to attack by twolined chestnut borer and Armillaria root disease. Resulting decline is irreversible, but the following tips will help minimize excessive losses. (We recommend working with a professional forester, and remember not to harvest from April through mid-July if there is significant oak wilt risk in your area.):

  • Postpone harvesting or roadbuilding for at least three years after intense defoliation (over 50% of leaves eaten) and four years after drought.
  • Thin overly crowded oak forests when they are not stressed.
  • Diversify oak forests with other site-appropriate species, including other species of oaks if one type is dominant. Don’t be afraid to keep a lot of oaks -- they are predicted to do very well in our future climate.
  • Perform regenerative harvests in older oak stands, creating ample light conditions on the forest floor to promote young oak seedlings and saplings.
  • If a stand is declining:
    • Consider salvage-harvesting in declining sections of stands. If salvage is not a regeneration harvest (i.e. if only dead and dying trees are salvaged), harvest on frozen ground. Avoid operating in areas of the stand that are not in decline.
    • Wait to salvage firewood for at least one year after oaks have completely died. Waiting a year prevents you from spreading any potential oak wilt that you might not be aware of.
oak tree with orange leaves crown
Oaks in Morrison County growing in compacted soil show the symptoms of infestation from twolined chestnut borer. They could be infected by Armillaria as well.

Thinning during or shortly after a stress event can greatly increase mortality if a forest is susceptible to decline. It is important to stay out of stressed stands if you want to minimize decline. It isn’t possible to eradicate twolined chestnut borer or Armillaria from a stand by removing declining trees. Twolined chestnut borer outbreaks naturally decline on their own.

Yard trees

Prevent decline by keeping your oaks as free from stress as possible.


  • Mulch trees properly
  • Water when needed. If rainfall is inadequate, watering oaks weekly may be the best method to prevent decline. Water with a soaker hose under the drip-line for several hours.

Do not:

  • Add soil over roots
  • Fertilize when it isn't necessary
  • Drive heavy equipment under trees
  • Prune when not necessary
  • Allow herbicides to contact leaves
an ailing bur oak tree with almost no leaves
A declining bur oak in Wadena County.

If over 50 percent of the top half of a yard oak died in one growing season, it probably won’t recover. Cutting it down while it still has some life in it may be less costly than waiting until it is totally dead. Just remember, if cutting a living oak, avoid doing it April through July to prevent oak wilt.

For a high-value ornamental oak for homeowners with a healthy tree care budget, consider hiring a certified arborist to assess whether a slightly declining tree could benefit from trunk injections to prevent twolined chestnut borer, plant growth regulator treatments, or techniques to loosen compacted soil. Note that for every one of these treatments, we only recommend working with a professional arborist, and it is very important to still water during droughts to prevent decline.

Remember, decline is caused by multiple issues. Prevention is the best method to keep trees healthy.

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