Here's the culprit that's making your eyes itch - Bursage. The male and female flowers are separate but on the same plant. The top flowers are the males and the pollen blows away in yellow clouds. The bottom flowers are the females. These have been pollinated and have produced the bur-like fruit that gets in your socks in in your dog's fur. However, they do have a role in nature providing ground cover and are major nurse plants for many cactus species including saguaros. That's probably more than you want to know about bursage.
Photo: Doris Evans 03 30 2008