Ragweed is easy to overlook because the flowers are tiny and light green, which blend in with the plant's green leaves. There are basic two types of ragweed; common ragweed and giant ragweed.
This weed comes up when soil is disturbed. When a plot of land is cleared, ragweed is one of the first plants to sprout up. Its seeds can still germinate for decades.
Ragweed pollen is lightweight and spread by the wind. Once you learn to identify ragweed, you'll see it growing in fields, ditches, vacant lots, alleys, along sidewalks and railroad embankments, and other places.
Goldenrod blooms the same time as ragweed, but has bright yellow flowers. So when people start having allergy symptoms in the fall, they often think goldenrod is causing it because their blooms are so bright and visible. But goldenrod's pollen is heavy and spread by insects, so it's not to blame for fall allergies.