Sow buttercup squash seeds for big, thick walled winter squashes. Buttercup squashes have a turban-like shape. They are flat on top with a bulbous blossom end. The skin is usually dark green and quite hard. This squash belongs to the species Cucurbita maxima and will readily cross pollinate with others in its species, including the larger pumpkins, Hubbards, Turk’s Turban, banana squashes, and zapallitos.