Summer Drought Stress and the Damage You Won't See Until Next Year

What drought actually does

• Fine feeder roots die back first. They are the roots that actually absorb water, and they are concentrated in the top 12 inches of soil.
• The tree closes its stomata to conserve water, which also shuts down food production.
• Stored energy reserves get spent surviving instead of building defense compounds.
• The result is a tree that looks fine in September and starts declining two summers later.

The follow-on problems

• Hypoxylon canker. This fungus lives harmlessly in healthy oaks and turns lethal in stressed ones. The tell is a patch where bark sloughs off leaving a silvery-grey or tan dusty surface. By the time it is visible, the tree is usually not savable.
• Ambrosia beetles and borers. They target stressed trees specifically, detecting chemical distress signals.
• Sudden limb drop on large oaks during still, hot weather.

What actually helps

• Water deeply and infrequently, not daily. Soak the area under the canopy edge for several hours every week or two during a dry spell.
• Mulch the root zone two to four inches deep, kept well off the trunk. Never volcano-mulch.
• Do not fertilize a drought-stressed tree. Pushing growth on a tree that cannot support it makes things worse.
• Skip heavy pruning during drought. Leave the canopy it has.

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