The joke is a pun.  A 'petal' is a leaf like part of a flower that makes up the colorful bloom part of the plant.  'Petal' sounds like 'pedal' which refers to a gas pedal in a car in the phrase 'pedal to the metal' which means to go as fast as possible.
When the florist was running behind in delivering the roses , he had to put the petal to the metal .