Q.best time to plant Chaste Tree
Best time of year to plant these trees

Spring or fall are always good time for plantings but really anytime with proper care.
Well draining soil is very important for Chaste Tree, so make sure you have the proper planting site. Amending soil may be necessary if you soil does not drain well.
https://www.gardeningknowhow.com/ornamental/trees/chaste-tree/chaste-tree-cultivation.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/improving-garden-soil.htm