I don’t have had alot of success grafting Morus Nigra myself. But from my last attempts I start to believe an active growing rootstock is the key. In my climate Morus Nigra are aprox 2-3 weeks behind Morus Alba. So when Alba is growing actively here I believe I am good to go. That said it probably is the safer route to wait a little longer. I really did not have success grafting Morus Nigra early (February) or late (September). But you definately don’t need to wait until your other Nigra wake up. My earliest graft looking good was from dormant scion wood I cut out of the tree and immediately regrafted into the base of the same tree right after the rootstock showed some budbreak. That was in the first week of april. Budbreak of that Nigra (the top part) was aprox 2 weeks after that. The grafted scion stayed dormant a little longer and is starting to grow right now. I wraped the whole scion with buddy tape (sturdier than Parafilm M).
I did that graft cause the tree is grafted high onto alba and I don’t like that much.
I grafted M. nigra (Noir of Spain, Black Beauty, Kaester) on M. alba understocks when the latter started to push growth. Used bark grafts (two M. alba trees were planted one year ago but grew very fast and had thick trunks; I cut them low and top-worked with multiple bark grafts). One month later, all grafts are pushing good growth and seem to do well.
here’s a photo sequence of one of our nigra grafts on a male alba a few months ago. Will take another pic of overall growth tomorrow.
btw, second pic shows signs of drippy, sappy state of the alba rootstock stump which seems to be what nigra budwood prefers.