Herb-Robert

Geranium robertianum

WA B desig. Geraniaceae
Data Maturity Baseline

Washington State Classification

Class B — Control Required (Designated)

It poses a threat to the forest understory and to plant diversity in the forests of Western Washington.

Required control in Region 1 (all western Washington counties)

Quick Reference

Type
annual herb
Origin
Europe, Asia and Northern Africa

Identification

Growth Habit

Herb Robert is both a winter and a spring annual or bienennial. It is a low growing plant that is hairy and shallowly rooted. Plants have a pungent odor when crushed.

Leaves

The leaves are deeply dissected and light green. The foliage turns red in late fall.

Flowers

Flowers have 5 pink petals.

Fruit & Seeds

The fruit is a capsule. Seeds are brown and about 2 mm in length.

Impact

It poses a threat to the forest understory and to plant diversity in the forests of Western Washington. It is capable of growing under a full, closed canopy. Where it occurs, there appears to be fewer native herbaceous species.

Ecology & Spread

Habitat

Herb Robert is highly adaptable. It is found in moist forests with canopy closure and on dry rocky outcrops. It is also becoming a significant garden pest in some areas. It is found from sea level to mid-mountainous areas in both the Cascades and Olympics. Please click here to see a county level distribution map of herb-Robert in Washington.

Spread Mechanisms

seed

Reproduction

Herb Robert reproduces by seeds. The seeds are ejected in response to drying of the capsule. Reports indicate that they may be ejected as far as 15 to 20 feet from the mother plant.

Regional Notes — Puget Sound

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Control Methods

Mechanical

  • Herb Robert has a very shallow, weak root system which makes hand pulling easy. Aim to pull plants before they go to seed as seeds can survive in the soil for a number of years. Frequently monitor sites and control as needed. Plants can be mowed or cut back with a string trimmer or weed whacker early in the season before they flower.

Cultural / Prevention

  • 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