The best thing I can say is to garden as much as possible. Plant several things at a time and give them what they need often. Head to the Bazaar, and buy the highest seeds available for your current rank. The higher rank you get, the better the rewards, although it's gonna get harder and harder to rank up in gardening as you go. Hope this helps!
just plant plants from the crown shop or special seeds that are no auction.
just have to wait that lack of WEEKS though .... :( you'll probaly get there in about a month i got to rank 8 in 2 1/2 months. each harvest gives me 60-250 garden XP.