Good to hear...
For me... As long as I cut them back after fruiting they start fruiting all over again.
Nice thing too... the fruit is consistently large every time.
And the pruned branches root very easily as well.
I've been using Thai Dwarf Mulberries as rootstocks for grafting White Pakistani's and Shahtoots.
A very high percentage of takes.
And you were right... the Shahtoots are very vigorous compared to the Whites.
Thanks!
Kevin