I finally got the golden watering can!
When she says you need more green, start planting flowers everywhere you possibly can. You can never have too many flowers. Also, make sure you weed every day and there are no pieces of trash or random fruit laying on the ground. When you have a perfect town, a Jacob's ladder will grow. Keep at it until Pell tells you that your town is a great place to live. A couple of weeks later, when you go to ask her about your environment, she'll give you the golden watering can.