ragweed

n. species of weed from the daisy family (produces pollen that often causes allergic reactions)

Ragweeds (Ambrosia), also called bitterweeds and bloodweeds, are a genus of flowering plants from the sunflower family (Asteraceae).The scientific name of this genus is sometimes claimed to be derived from the Ancient Greek term for the perfumed nourishment of the gods, ambrosia (ἀμβροσία) which would be quite ironical since the genus is best known for one fact: its pollen produces severe and widespread allergies. However, the generic name is actually cognate with the name of the divine dish, both being derived from ambrotos (άμβροτος), "immortal". In the case of the plants, this aptly refers to their tenaciousness, which makes it hard to rid an area of them if they occur as invasive weeds.
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Noun
1. widespread European weed having yellow daisylike flowers; sometimes an obnoxious weed and toxic to cattle if consumed in quantity
(synonym) ragwort, tansy ragwort, benweed, Senecio jacobaea
(hypernym) weed
(member-holonym) Senecio, genus Senecio
2. any of numerous chiefly North American weedy plants constituting the genus Ambrosia that produce highly allergenic pollen responsible for much hay fever and asthma
(synonym) ambrosia, bitterweed
(hypernym) weed
(hyponym) common ragweed, Ambrosia artemisiifolia
(member-holonym) genus Ambrosia

(n.)
A common American composite weed (Ambrosia artemisiaefolia) with finely divided leaves; hogweed.
  

Any of a number of weedy composite herbs that produce a pollen that is a frequent cause of allergies. Of all allergy sufferers in the United States, 75% are allergic to ragweed.