Mrs G. I used to have a Black Tartarian cherry at my old house. The tree was loaded every year because my Lapins cherry pollinated it. He needs to buy another sweet cherry variety to pollinate them. Tony

I do many multi-grafts and have not yet had virus problems. The only problem I have had with viruses is that G16 is very sensitive to viruses and I have lost many of those which I did multi-grafts to. Cherries can easily be multi-grafted. I have only done a couple myself but they are fine. The main trick to multi-grafts is to leave enough room so each variety has enough space. If not you will end up with a big mess: lots of disease, and lots of wood being produced with no buds on it. Today I was out pruning and of the 200 or so closely planted (and many multi-grafted) trees I pruned today nearly all were fine but a couple were too crammed and I needed to take major corrective action (bend over a tree to the side, remove a tree, etc). Scott

I am in Zone 5 and my nanking cherry blooms before the other sweet cherries and I see no overlape for pollination. Anyway you have a good combo on your cherry tree.

Yes, there are a few self-fertile sweet cherries ('Stella', 'Lapins', 'Sweetheart') but most others will need a second variety to afford cross pollination. And it doesn't have to be a second sweet cherry variety - pie or sour cherries are all self-fertile and will provide pollination services for other fruiting cherries.