Weed of the Month | Herb-Robert (Geranium Robertianum)

Herb-Robert is not related to the container and bedding plant we commonly call geranium. That is a southern African native, known botanically as Pelargonium. True Geraniums are also known as cranesbills and can be found growing naturally in Europe, Asia, North Africa and in the case of herb-Robert, also in northeastern North America.

All cranesbills have 5-petalled flowers. Herb-Robert’s are small, pink and prolific from April until frost. They often cover plants that grow 12-18 inches tall and spread even wider. Leaves are lacey and finely dissected and stems are often reddish and noticeably hairy. Extremely adaptable, herb-Robert is at home in moist, fertile woodlands and hedge rows, as well as in gravelly and rocky soils. Though it prefers partial shade, it will also grow in sun, where stems and leaves become even redder than usual. Leaves and stems also turn bright red at the end of the season.

An annual or biennial, herb-Robert produces seeds as prolifically as it does flowers. Dried capsules can eject seeds 15 feet or more, so herb-Robert can easily cover large areas, outcompete other plants and become weedy. Though native in northeastern North America, it is not in the northwest, where it has been introduced and declared a noxious weed, as it threatens the diversity of the native forest understory.

Herb-Robert can take over beds and borders and naturalistic plantings, too, but fortunately pulls easily from the ground. Smothering large areas with cardboard and mulch also works as a means of control. At Bridge Gardens, we weed it from certain areas, where we don’t want it to make a foothold and spread, but let it proliferate in others, like at the base of shrubs or under trees. Though eradication is achievable after a number of years, if you’re obsessed and diligent enough, it may be best to opt for control.