Acacia pycnantha generally grows as a small tree to between 3 and 8 m (10 and 30 ft) in height, though trees of up to 12 m high have been reported in Morocco. The bark is generally dark brown to grey—smooth in younger plants though it can be furrowed and rough in older plants. Branchlets may be bare and smooth or covered with a white bloom.
The flower heads in the summer, before hatching in May and June when the larvae induce the formation of the grape-like galls and prevent flower development. The galls can be so heavy that branches break under their weight.