The phenomenon of weed resistance was first observed in 1968 in Washington State (USA). Common groundsel (Senecio vulgaris) was discovered to be resistant to the triazine herbicides after multiple exposure to simazine and atrazine in a nursery crop situation.

The Beginning of Weed Resistance<BR> 1968 (Washington) <BR> nursery crops <BR> common groundsel <BR> atrazine <BR> simazine <BR>Photo: The Missouri Flora Web-Site<BR>