Reclaimed water is perfect for watering your flowers.  You don’t necessarily need a rain barrel, though they are great as well.  We have put a pitcher/basin in our shower for all of the overflow water that you do not use cleaning your body.  As well, during this humid summer time, any water collected in your dehumidifier is a great solution.  My mom even puts a watering can below the air condition (on the outside part), for the dripping water and her flowers are gorgeous… love it!