Technically, the heart is located between our two lungs, not on either side. The flower would be feeling not the heartbeat, but the blood going through the lungs to get oxygenated.
Actually the heart is located farther left (thus comes the reasoning for the left lung being smaller than the right)so if the flower was in a pocket in the right spot that was to the leftish like the heart it could hear the heartbeat!
Actually, the heart is directly behind the sternum, which is the final bone directly in the middle of the ribcage. The ventricles, being larger, give it a sense that the heart is farther to the left. The heart is directly in the middle, but it curves to the left. If the heart were to be located farther to the left, then CPR would be applied to the left side, not directly above the sternum.
The flower would be feeling not the heartbeat, but the blood going through the lungs to get oxygenated.