Sorry to hear your Mel’s heritage is not growing as well as usual. Mel’s heritage here is a beautiful large beast. I have always been under the impression that Mel’s heritage is not rated to zones bellow 6. I would think the fact that it is growing at all is a good thing. I hope others in zone 5 have had better luck but I am not certain many are growing it in cold zones like that.

Here is one own root 3rd season, around 20 feet wide. So I think the issue is that it likes it warm. Maybe with some years it will be able to conquer your cooler zone.