There is marked variation in the size, form, and quality of the nuts of different hickory trees, even when growing in the same neighborhood. Thus there is good opportunity for selective breeding, but unfortunately the hickory is of such slow growth that few experimenters have the courage to undertake its development. The hickory does not ordinarily bear nuts until it is ten or twelve years old.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 11 Chapter 5