robertianum: Petals (8.5-)10.5-12.5(-14.2) mm long, (2.1-)2.6-4.5(-6.1) mm wide; anthers purplish; middle leaf segment 16-37(-49)-lobed in distal half; mericarp with 1-2(-3) collar-like rings at tip

Other keys emphasize the hairs on the sepals as a distinguishing characteristic. All keys include anther color but since both species have yellow pollen, plants can be difficult to distinguish based on anther color alone. Geranium purpureum and Geranium robertianum have chromosome numbers