Dark, round, 2–3" growths on branches = hickory gall wasp (Cynipidae) galls. Female lays egg on twig; secretes chemical causing plant to form protective chamber around egg. Larva develops inside; adult emerges, leaving woody gall.
No sprays, no pruning, no worry. The wasps are native, have natural enemies, and don't threaten the tree.
Enjoy the ecological interaction—your hickory hosts a tiny wasp nursery without consequence.
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