This persistent weed is said by some to have been introduced by the Romans possibly to use as a herb to treat gout or arthritis (hence its other name: Goutweed). Stace is not sure whether any P. agraria is native or not. It is an umbellifer which produces seed after displaying an ordinary white umbel but it spreads using rhizomes any bit of which can give rise to a new plant. It was present in the garden when we arrived decades ago we have failed to get rid of it - it always returns. It has populated these island so well that only small parts of the Cairngorms and the North-West highlands haven't yet been invaded. Even Japanese Knotweed (Fallopia japonica) hasn't spread as thoroughly throughout the UK.