Ragweed is a common plant that causes misery for millions of people every late summer and fall. While the plant itself looks unremarkable, with rough leaves and small greenish flowers, it produces enormous amounts of pollen (the fine powder plants release to reproduce). A single ragweed plant can release a billion pollen grains in a season, and these tiny particles float through the air for miles.

For people with allergies, breathing in ragweed pollen triggers sneezing, itchy eyes, runny noses, and general misery. This is why so many people feel terrible in August and September, even though they're perfectly fine the rest of the year.

Ragweed grows easily in disturbed soil, like empty lots, roadsides, and construction sites. It's native to North America but has spread to other continents, where it causes the same allergic problems. Some cities and towns have programs to eliminate ragweed before it releases pollen, which helps reduce the annual hay fever season that affects so many people.