My trees now are about 2 to 3 ft tall.
The best time to plant them would be when they are dormant, usually between December and March.
https://www.gardeningknowhow.com/ornamental/trees/tgen/how-and-when-to-plant-trees.htm
https://www.gardeningknowhow.com/ornamental/trees/chinese-parasol/chinese-parasol-tree-info.htm