Bamboo takes 5+ years to build a foundation, all underneath the surface and after this it grows fast above the surface.

“It takes bamboo about 3 years to develop its roots and establish itself in the ground. It takes 5 to 7 years for the bamboo to reach maturity and be ready for harvest.”

https://howchimp.com/how-long-does-it-take-for-bamboo-to-grow/