Weed of the Month: Herb-Robert (Geranium robertianum)
Herb-Robert—also called Robert’s geranium, or, colloquially, “stinky Bob”—is both a winter and spring annual with light-green, deeply dissected leaves. In late fall the foliage turns red. The stems fork and are brittle at the joints. They are pubescent (hairy), reddish (especially under high light conditions) are can be up to 25 cm (10 in) long. The roots are shallow. The pink flowers have five petals that are 7-10 mm (1/4-1/2 in) long, often with a white stripe. Herb-Robert propagates by brown seeds about two mm long. A distinguishing characteristic of the species is the pungent odor of the crushed leaves, whence comes its “stinky” epithet.
Herb-Robert spreads primarily by seeds; a patch can produce as many as 3,100 seeds per square meter. They are ejected ballistically, in response to the drying of the capsule that surrounds them. Reports indicate that they may be ejected as far as 15 to 20 feet from the mother plant, which helps account for its very rapid encroachment. Connected to the seed is a sticky thread that attaches the seed to the underside of leaves or to passing animals or people. The seeds attached to leaves wash down after rainfall if they are not removed by someone brushing against them. Herb-Robert poses a threat to forest understories and plant biodiversity by displacing native species, especially herbaceous species; it was added to Oregon’s quarantine list in December 2009 as a Class B weed.
Herb-Robert has little root structure and pulls from the ground easily at all stages of growth. Because the brittle stems break easily, pull from the base of the plant to remove the whole plant and the root. It may also be controlled using a string trimmer in early to mid summer before fruiting.
Bleeding heart (Dicentra formosa) is a shade-loving perennial with dissected leaves similar to those of herb-robert, with distinctive, heart-shaped pink blossoms in spring.