Plant it now; it should be well established by fall. Bamboo in the ground is hardier than bamboo in pots, and since they are 18 months old they should be plenty ready to plant. I'd plant them at least 4-5 feet apart. Moso is a timber bamboo and a runner, so don't plant them too close to one another.

Kudzu9, Thanks for the advice. I will get it out this week. My week off from work (spring break) will be spent in the garden!!!

elaine-
Have fun! Moso should grow well in your area.

I'm 2 zones colder than you and I've started a batch of over 50 moso seedlings about 13 months ago and I've planted a few a couple weeks ago. These seem to be doing better than the ones still in the pots, I guess due to better moisture retention, so go ahead and plant them.

Moso should get hardier each year as they become more and more mature and they are pretty easy to over-winter here simply with a tarp. They probably don't even need any protection here once established.