Little-Robin

Geranium purpureum is an annual (sometimes biennial), herbaceous, flowering plant. It has small flowers with 5 sepals and 5 petals, each petal being 6.3–8.4 mm long and 1.5–2.0 mm wide. The flower has 10 stamens, each with a yellow anther. In the center of the flower, a single fused style supports 5 stigmas. The fruit is a schizocarp, which splits into 5 one-seeded mericarps at maturity.

Identification. Geranium purpureum is often confused with Geranium robertianum, a close relative. The primary character that distinguishes the two species is flower size. The corolla of Geranium purpureum is approximately 7.5 mm in diameter while that of Geranium robertianum is at least twice that size (15–20 mm). (For comparison, a U.S. dime is approximately 18 mm in diameter.) An identification key for genus Geranium in California distinguishes the two species as follows:

Geranium purpureum: Petals (5.3–)6.3–8.4(–9.9) mm long, (0.8–)1.5–2.0(–2.6) mm wide; anthers yellow; middle leaf segment 5–20(–38)-lobed in distal half; mericarp with 1–4 collar-like rings at tip Geranium 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. …

Geranium purpureum ranges from Great Britain southeastward to the Caucasus, with native populations occurring as far south as central Africa. Non-native populations occur on all continents except Antarctica. Introduced populations are often found along railway corridors.

Geranium purpureum can not survive winters with an average January temperature of 4 °C (39 °F) or less.