Very much so. At first, the rewards aren't too impressive, but there are a few plants out there that guarantee wonderful snacks (insert the 6,845,932,398th mention of Couch Potatoes and EMPs here), plants such as Sword Ferns and Silver Trumpet Vines for Treasure Cards, and so on.
Absolutely. Gardening is by far the best resource in the game. As mentioned above, you'll get loads of pet snacks, treasure cards, and gold. I rarely ever, if ever at all, have to farm on any of my characters with fully stacked gardens because they keep me at max gold. I also want to mention King Parsley for getting amber. I got over 100 amber from King Parsley mature and elder harvests in about 2 months.