What kind of Bamboo is this? It's about 20+ feet tall, I live in AR. If I want to grow some of this, how can I start it so I can plant it in the Spring?
it's hard to tell from that picture.
Where is it growing? Is it close to where you live?
Dig up as large of a rootball as possible without disturbing the soil around the roots/rhizomes, and the rootball should have as many culms as possible, at least 2 or 3, and they can be topped just above the lower limbs if the culms are too tall/long.
If you dig up the rhizomes without the soil, do it when the plant is dormant such as now, or early next Spring but do not let the roots dry out from digging to transplanting, and do not let them freeze. Water well and you should be set for next Spring.
Probably not the case there but if you are below Zone 6 or 7 wait until spring to dig them up, if possible. And plant where the indefinite spread of the rootstock and seemingly random popping up of new tops will not be a problem.
Or surround the planting with an effective installation of correctly used bamboo root barrier.