There are hundreds of different kinds of bamboo. It depends on what kind of bamboo you are using for your trellis. There are so many different sizes and wall thicknesses to choose from. It also depends on how it is cured after cutting. Most 'wild' bamboo does quite well when used as a trellis, finishing, depends on what you like, and yes, polyurethane can be used.

P. nigra(black bamboo) would do fine in your area but it is a runner. There are dozens of green clumpers to choose from that will do OK in your area. What size clump are you looking to grow, height and culm diameter?

Kt

Thank you, Kt. The vendor I emailed suggested Silver Stripe. It's listed as growing to 25 ft. As for diameter, 4 inches across would be fine. They recommended I allow 20 ft. I am planning to put it down at the bottom of my yard right at the edge of the woods. It is all dry sand, so I would need to amend the sand first. This particular vendor is in Va so is a near neighbor- anything hardy there would be hardy here. Their service is great- I emailed them 2x this wkend and they already answered both emails. Mid-atlantic bamboo.
I assume the bamboo needs to dry out before being finished. How long should I allow for it to dry out? And does it matter whether it is lying down or standing up? Thanks, Brandy

My Silver Stripe leans outward a lot and the culms aren't always very straight. It only gets to about an inch in diameter here.

Al Karr, or some of the other Bambusa multiplexes would be great.

The largest culm diameter clumping bamboo that grows here is B. Oldhamii with culms just over 4 inches thick.

B. Textilis is much more cold hardy, produces many more culms each season but the culms get to a diameter of about 2.5 inches.

Both would be great for thick-poled trellises. I would suggest the textilis since it is more cold hardy and needs little care.

Bambusa Lako, Dendrocalamus Atrovialacea and Asper Betung Hitam, as Socalboo suggested, are all great black bamboos, but they all freeze back here during the Winter in zone 8b, as they can't handle much below freezing, and never get any size to them, except in a large pot.

Mexican Weeping is a very nice bamboo, but mine doesn't get the brownish culms, they stay green. It also doesn't like too much water, but doesn't mind the high humidity.

I dry mine standing up in shade or under a ventilated carport or the like, just because it takes up less area. Under a tree is a good place where it is shadey. The slower they dry, the less likely there will be a large crack instead of a small one, since most will crack no matter what process you use to dry them.

Good Luck

Kt

Thank you,Kt, that is very helpful. Brandy