Ragweed might be the most allergenic plant out there. Seventy-five percent of people with pollen allergies are allergic to ragweed. To make matters worse, the plant is extremely common -- there are 17 types of ragweed in North America. Ragweed releases huge numbers of pollen grains into the air. If you live in the United States and you want to escape it, head New England or West Coast.
- Appearance: It grows no more than 2 feet high, with pointy leaves and clusters of greenish-yellow blooms. The flowers open downward to release pollen into the wind.
- Peak time: It pollinates in summer and fall.
- Location: You'll find it growing along roadsides, in fields, along riverbanks, in vacant lots and in rural areas. It likes to grow where soil is disturbed -- by water streams, salting of roads or cultivation. It's easily overgrown by other plants, but if it finds disturbed soil, it will take root and thrive.