Plant Galls

A twig from a bur oak tree with several plant galls attached to it.

These weird growths on bur oak twigs are not acorns, but galls caused by a tiny wasp called a cynipid. Galls are common on plants, and form when insects, mites, or other organisms secrete chemicals that affect the plant's growth hormones and stimulate the plant cells to develop in unusual ways, creating a structure that provides a safe and nutritious haven for the insect or mite. Galls generally do not harm the plants they grow on.

The wasp lays eggs in buds of the oak, and the feeding activity of the larvae stimulates the tree to grow around the larvae, enclosing them. Small holes in the galls show where the wasps eventually chewed their way out after maturing.
Bur oak trees (Quercus macrocarpa) are not native to Colorado, but are commonly cultivated in gardens and along streets. You may find burr oak galls on trees in your neighborhood.