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The Dirt | Weed of the Month March 2025 | Herb Robert (Geranium robertianum)

Our weed of the month for March is Herb Robert (Geranium robertianum). This little annual plant goes by a variety of rather fun common names, which include but are not limited to: Stinky Bob, Robert’s geranium, Red Robin, and more. So much variety!

A small purple flower surrounded by green leaves and some brown seed pods.
Close up of herb robert leaves and flower.
Green foliage, ferns, and grass in a forest undergrowth setting.
Herb Robert leaves
Green leafy plants growing on the forest floor amid twigs and dried leaves.
Herb Robert leaves
A small purple flower surrounded by green leaves and some brown seed pods.
Green foliage, ferns, and grass in a forest undergrowth setting.
Green leafy plants growing on the forest floor amid twigs and dried leaves.

 

Infographic on Herb Robert, a noxious weed, describing its features, problems, and management methods.
Weed of the month graphic.

This species is commonly found in moist, partially shaded areas such as forest understories or along riparian areas. It can handle a variety of light intensities and often pops up opportunistically when other species are removed during restoration activities. Every aspect of the plant is noticeable. Beginning in late April-early May, you’d likely notice this species by its small, five-petaled pink flowers. If not flowering, it can also be identified by its bright red stems on a mature plant and its deeply dissected leaves in a 3-way arrangement like the letter ‘T’.

Though this species is charismatic, it can also be problematic. As mentioned, it’s an opportunistic invader that competes with native germinant plants in early spring. You may have even seen it in similar environments with another common weedy geranium species shining geranium, which together have potential to cover large swaths of forest or riparian areas if left unmanaged.

All and all, if you have this species on your property, keep an eye on its spread. If it’s a small patch that you’d like to remove, this species has a shallow root system that is easily hand-pulled. If you have a larger infestation that requires chemical control, consult with a local pesticide applicator or visit the PNW Weed Handbook for guidance on this species.