You can fertilize in fall but because of Rubrum's vigor, I hold back to keep growth in check.
If you are growing for girth. You dont care about long internodes at the end of the branches, but you still want short internodes at parts of the plant youll be keeping becaue thats where the tree will back bud when you chop. So tight internodes at parts youre keeping.
I grow my trees slowly, and dont look for maximum growth all at once. Thats my decision and process. You can have yours.
rubrum add about 1/4" - 1/2" girth a year in a pot the year after a repot depending on pot size. They slow to about an 1/8" a year the year of a repot or once they get rootbound from my notes.