Which mulberry variety is the most vigorous grower for you?
I have SH, IE, and Corral, all 4 year old grafts and growing healthy in Z6. SH is a monster, it suffers more from winter dieback, but always out grows the other 2. IE seems to grow “controllably”, good healthy growth, but not too much. Corral is growing healthy also, but considerably smaller, and more like a bush.
They all get “deer pruning”. SH responds by sending out a new shoot from the terminal bud that grows extremely fast. IE will do the same, but at a much slower growth rate. Corral will send out shoots from several of the last buds on the pruned branch, giving it a bushy form.