Cup Plant Cup Plant flowers Cup Plant, Silphium perfoliatumCup Plant is quite tall, up to 8′ and easy to recognize by the way its large leaves form a cup around the stem. The flowers bloom in August.O’Neal’s study found a trace amount of Cup Plant pollen, which is included in the CP-42 seed mix. Cup Plant leaves surround the stem to form a cup that holds rainwater Cup Plants along a dusty gravel road, Dallas County