Does Speed-Gro fertilizer affect the growth of repeating crops?
Short answer: no according to the wiki
Long answer: Speed Gro only affects the rate at which the plant "matures" and not the rate at which it "produces crops". For most plants like pumpkins and melons, these are the same age so it doesn't matter, but since blueberries ,cranberries, etc all produce multiple times it is an important distinction.