Is it grammatically correct to say fall's planting time?

Yes. I can think of two (only one correct) possible meanings, though, depending on the usage: The complete sentence, "Fall's planting time" means, "The season of Fall is the time of year when we plant." This is not correct! Fall's shows possession , no conjunctions includ the Word "is". You could also say something like this: "Late September is Fall's planting time," meaning: "The time within the season of Fall when we plant is late September."