WSU Extension

Hortsense

Weeds : Herb Robert (Robert geranium, stinky Bob) : Geranium robertianum
(revision date: 4/7/2021)

Family: Geraniaceae
Cycle: Winter or spring annual
Plant Type: Broadleaf
Use Integrated Pest Management (IPM) for successful weed management.

Biology
Herb Robert is a sprawling to upright plant reaching 2 to 12 inches tall, or taller if supported by other vegetation. It often grows under or through desirable plants. The leaves are deeply dissected giving the plant a delicate, almost fern-like appearance. Prior to bloom, young growth may be mistaken for the native bleeding heart (Dicentra formosa). Stems are forked and tend to be rather brittle, especially at the swollen joints or nodes. Leaves and stems are hairy and light green, turning to red in the fall or when grown in bright light conditions. Herb Robert has small (about 1/2 inch across), showy bright pink to magenta flowers with 5 petals. They are held above the foliage, often in pairs. Herb Robert reproduces only by seed. These are produced in a capsule with the elongated beak characteristic of many species in the geranium family. At capsule maturity, tiny (less than 1/8 inch) brown seeds are forcefully ejected, travelling up to 20 feet from the parent plant or hitching a ride on passing animals or people. Seeds may germinate in the fall, with the new plant overwintering as a small rosette, or may lie dormant through the winter and germinate in the spring. They may be viable for as long as 5 years. Roots are fibrous and easily pulled. Herb Robert is also known as "stinky Bob" due to the pungent odor produced by the crushed leaves. SPECIAL INFORMATION: In WASHINGTON and OREGON, it is designated as a Class 'B' noxious weed. Management of this species may be required by law in your county. In addition, it is on the Oregon noxious weed quarantine list, which prohibits sale, purchase, and transport of plants, seeds, and plant parts. Consult your local Noxious Weed Control Board for more information.
Habitat
Herb Robert is very adaptable, growing in a wide range of soils and light conditions. Moist forests, grasslands, and dry rocky outcrops can be colonized by this plant, and it can be a significant garden pest as well. It prefers partly shady to shady and moist sites such as gardens and woodlands, but it can easily tolerate sunny, drier areas as well. It is found from sea level to about 4,000-foot elevations throughout many areas in western Washington. Herb Robert is capable of spreading into undisturbed areas and displacing native plants.

Management Options

Non-Chemical Management
  • Apply organic mulches, such as bark, compost, grass clippings, straw, and other materials, in a layer from two to several inches thick for effective weed management.
  • Inorganic mulches, such as plastic, commercial "weed barrier" fabrics and other materials such as roofing paper, is an effective weed management option. Cover inorganic mulches with a thin layer of soil or organic mulch.
  • Hand-pulling or hoeing is an excellent control method for small infestations. Grasp plants firmly near the base to ensure removal of the roots. Bag plants that are in bloom or have set seed; dispose of these plants in the trash (do not compost).
  • Mowing, roto-tilling or use of string trimmers will help control plants that have not begun to bloom or set seed. Do not use these methods on plants that have begun to flower or seed as they will further disperse the seeds.
  • Because seeds germinate throughout much of the year, monitor the site regularly for new seedlings.
  • Composting is acceptable for non-flowering and non-seeding plants, but backyard composts often are not hot enough to kill seeds. Herb Robert has been known to spread from compost piles.
Select non-chemical management options as your first choice!

Chemical Management
IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides

Apply according to label directions. Glyphosate products should be applied as spot treatments only! NOTE: Some ingredients listed here are only available in combination. Read the label carefully on combination products to make sure the product is suitable for your specific situation.

Landscape areas
  • trifluralin
  • products containing triclopyr
  • products containing 2,4-D
  • glyphosate
  • products containing dicamba
Turf areas
  • products containing 2,4-D
  • products containing dicamba
  • triclopyr
Bare ground areas
  • glyphosate
  • products containing 2,4-D
  • products containing dicamba
  • triclopyr
  • products containing dicamba
Images
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Caption: close-up of flower/plant
Photo by: King County Noxious Weed Control Program
Caption: Herb Robert stem hairs
Photo by: T. W. Miller
Caption: Herb Robert
Photo by: T.W. Miller
Caption: Herb Robert leaves
Photo by: T.W. Miller
Caption: Herb Robert young fruits
Photo by: T.W. Miller
Caption: Herb Robert leaf
Photo by: T.W. Miller
Caption: Geranium flowers (front)
Photo by: T.W. Miller
Caption: Herb Robert cotyledons
Photo by: T.W. Miller