I've been browsing online, looking at several nurseries that sell nut trees and seeds. One practice that seems to be quite common is to graft Walnut trees onto Black Walnut rootstock. This is done for both Butternut trees and Persian Walnut trees. It's made me curious. Why is this done? Black Walnut trees are pretty aggressive growers. Their roots go down real deep, real fast. Is this the reason? Does it make the tree more resistant to more infections? Also, does this mean the tree emits Juglone like a 100% Black Walnut tree? Or does it emit lesser amounts like, say, the Butternut that is grafted on to it?
It's done for the same reason other fruit trees are grafted to rootstock. It results in a low level of incompatibility. This prevents the tree from growing too tall, and through limiting the normal growth, diverts energy so the tree will begin producing earlier in its lifespan. But paradoxically, black walnut is also extremely vigorous, so will make the tree more vigorous in other ways, more walnut production. Black walnut also has more disease resistance. But this is not something that you will always want: black walnut also produces far more juglone, a substance that makes it difficult for other plants to grow in the soil near the proximity of a walnut tree. There is also a walnut hybrid that is used as rootstock.
