Brooke M said
one of my cherry rootstocks has started leafing out... Can I still graft on it?

There has never been any rule against grafting on an active rootstock. In fact it is better to be active than dormant.

If it is for sweet cherry. The biggest challenge is going to be the condition of the scion where I think I can be a big help here thanks to older people that have helped me. Collect healthy scions around November to mid December and well before Christmas especially where it rains lots. Then store as cool as possible. For example if you can find a place to keep temperatures no more than 34F you can get results as high as apples and pears. Temperatures that stay below freezing can be stored lower, for example 20F.

I always kind of worry about sweet cherry scions handed to me that I have to graft at the HOS scion exchanges because of what I just said. This is why I always have my own sweetheart sweet cherry next to me to give out as part of the order.