Ragweed is a pest and so is its pollen. The plant is one of the biggest culprits behind hay fever, and not just in North America. Ragweed has invaded every continent except Antarctica and is spreading to new areas all the time. During peak season the ragweed pollen count may reach 250 grains per cubic meter (35ft3) of air. And it can take as little as 10 grains to trigger an allergic reaction.