No visible growth..what happens underground, we don't see.

Bamboo culms do not grow taller at any time of year after they initially come up and reach full, permanent height in the first several weeks of shooting. Rhizome growth is typically finished by fall.

^ I know there is no visible growth of what happens underground, and that the culms do not grow any taller after they come up. I'm referring to the roots, the rhizomes. Visible or not, is there any growth? Or expansion?

To repeat: “Rhizome growth is typically finished by fall.”

Yes, what I have seen myself indicates that like other hardy woody plants cold climate adapted bamboos have a rooting rush in fall that stops before winter.

Bamboo, like other plants, obtains its energy from photosynthesis via its leaves. When it gets cold, it goes dormant and stops growing. Some phyllostachys will survive in cold weather but will never reach their full height. That can only happen in a warmer climate.