Mix 1 part washing soda to 1 part water directly into your bucket. You should be able to purchase washing soda at your local grocery or hardware store.
Dip your scrub brush into your bucket.
Scrub the flower stain with your scrub brush. Use firm pressure, and change the motion as you go. You may find that starting with a circular motion, then switching to a back-and-forth motion works best. If the stain is stubborn, apply the washing soda directly to the concrete and scrub.
Rinse the washing soda away with your garden hose. Allow the cement to air dry. If the stain is still noticeable, repeat.