[Ans] The only working tea plantation in the us is located in which southern state?

"No..No..US not cultivating Tea", If it is your opinion then you should change your opinion now. 

Camellia Sinensis can be grown in warmer parts of the United States and currently the US mainland has a relatively large plantation with full mechanization in Charleston, South Carolina.

You may continue reading more with the below link:

https://en.wikipedia.org/wiki/Tea_production_in_the_United_States

Other Facts:

Commercial tea cultivation in the Americas was first attempted in 1744 in Colonial Georgia,

In 1880 the US Government hired John Jackson, an experienced tea planter in India, to cultivate tea plants planted 30 years earlier in Liberty County, Georgia

In 2003 Hawaii had an estimated 5 acres (20,000 m2) of land producing tea

East Texas Tea Company in Mount Vernon, Texas has started tea cultivation in 2009 and sell by private placement.