When I think about the longan varieties brought in here and tested like Biew Kiew, Chompoo, Dang, Daw, Fa Hok Chai, Fuhko No.2, Haew, Homestead No.1, Ilao, Kay Sweeney, Kohala, Ponyai, Saig Geeb, Skek Yip, Wai etc only 3 of them are grown in any numbers. Maybe to be sure about the most cold tolerant would be to look at the growing districts in China, pick the coldest area and drill down to what longan varieties they are growing.