Q: Why would my well established oakleaf hydrangea that has been in the same spot for eight years and has always bloomed for me suddenly be dead? Last year many of the stalks seemed to be dying. This spring it put up some new growth, but then suddenly those new stalks died also. What could have caused this?
A: Hydrangeas don’t normally die suddenly. They might slowly decline if planted in spots that are too dry or too wet. Leaves might turn yellow from lack of nitrogen in the soil. But sudden death of branches points to borer damage.
In early June I noticed that my neighbor’s hydrangea had two branches that suddenly wilted and turned brown. Closer inspection revealed two tiny “toothpicks“ emerging from the stem just below where the wilted leaves occurred.
I am familiar with the “toothpicks“ that show up on woody plants, like crapemyrtles, when they are attacked by Asian ambrosia beetles. They are particles of wood bound by minuscule webbing that are pushed out of the hole by the female beetles.
The “toothpicks“ I saw on the hydrangea were much smaller than those produced by the Asian beetles. They are smaller because they’re produced by smaller beetles: black twig borers.
The best treatment is to prune out dead branches a few inches below where you see dead leaves. This removes any insects present.
I am hopeful your hydrangea has enough life left to recover and make new branches.