I suppose you should be able to divide it up as a potato as you say--taking some eyes with each piece (though I've never done it this way--usually easily and cheaply available by me). No, it's not too late--they are FAST growers when happy. Just provide heat and LOTS and LOTS of water--they can even grow in water.
is Golden Bamboo (Phyllostachys aureus). Not sure any of the Phyllostachys will keep foliage really low down as the grove matures--if you've ever been in a mature grove--you'll understand what I mean. Black bamboo (P. nigra) will also grow well in the area, and although a runner, it is somewhat better behaved than aurea. My grove includes a mixture of P. nigra, aurea, viridis, nidularia and vivax). Of these, P. aurea most be the most aggressive/fastest. You might also consider Pseudosasa japonica--they will be much denser at ground level compared to Phyllostachys. their leaves are MUCH larger (so it's also a matter of taste--I tend to prefer the finer leaved varieties). Pseudosasa japonica is about as cold harder as the others I mentioned and also perform well in zone 7. You can try your local, better garden centers of HD's for the common bamboos (which will include clumpers--mostly Fargesia in our zone). But you can also try eBay. I've gotten many nice plants from there as well (most recently Giant Moso)--you'll just have to wait longer for that grove. If you can only get the runner Golden at your local nursery, it's not a bad species and will eventually form a large, attractve grove. --All the other species of runners I have, I have had to order on line and through the mail. Good luck!

Sorry about the early part of my posting--it looks like an earlier message I sent to another poster, somehow got combined with yours--weird! Anyhow, my posting should have started out saying that in zone 7, in the NYC/MetroNJ/LI area the most commonly available bamboo sold is Phyllostachys aurea (Golden Bamboo). The rest of the posting reads as I intended.