Thoroughly soak florist foam with water, and place it in the center of a pedestal bowl or cake plate. Cut a bucketful of hydrangea blooms. Begin pushing their stems into the foam. Place them around the bottom, and work your way up until it is completely covered. You can use a pencil to make holes in the foam if the hydrangeas' stems are too weak to penetrate it. Pack the blooms tight enough so you can't see the foam.