1. (

• ragwort ,
• tansy_ragwort ,
• ragweed ,
• benweed ,
• Senecio_jacobaea

) widespread European weed having yellow daisylike flowers; sometimes an obnoxious weed and toxic to cattle if consumed in quantity ;

2. (

• ragweed ,
• ambrosia ,
• bitterweed ,

) any of numerous chiefly North American weedy plants constituting the genus Ambrosia that produce highly allergenic pollen responsible for much hay fever and asthma ;