Definition of "willowherb"

Never had they noticed the roses so vivid, the willow-herb so riotous, the meadow-sweet so odorous and pervading.

1908 October, Kenneth Grahame, “The Piper at the Gates”, in The Wind in the Willows, New York, N.Y.: Charles Scribner’s Sons, page 153

And willows, willow-herb, and grass, / And meadowsweet, and haycocks dry, / No whit less still and lonely fair / Than the high cloudlets in the sky.

1917, Edward Thomas, “Adlestrop”, in Poems, London: Selwyn & Blount, page 40

Passengers on the Northern Line may often have noticed the flowering shrubs that glow from the banks of the cutting between Hendon and Brent. There are many such pleasant stretches of track. Their flowers range from the rather formal blossoming of outer London to the wilder flowering of the country, where willow-herb and broom, traveller's joy and campion, go rioting over the chalky banks of the Metropolitan Line.

1950 July, “Traveller's Joy”, in Railway Magazine, page ii (advertisement)