i don't think so. i used to think that watering trees on the first day helped them grow, but when i posted that on here i got slapped down getting told that was the stupidest thing people had ever heard, lol. so if watering your baby trees doesn't help, that means that the only reasons we water anything in this game are 1. to grow red turnips, 2. with a gold watering can to get black flowers, and 3. when your flowers are dying to save them. i always water my stuff 2-3 times just in case i miss the first few... i doubt over-watering is possible.