The weed that causes 75 percent of all hay fever is ragweed which has numerous species. One ragweed plant is estimated to produce up to 1 billion pollen grains. Other weeds that cause allergic reactions are cocklebur, lamb's quarters, plantain, pigweed, tumbleweed or Russian thistle and sagebrush. Trees pollinate in late winter and spring. Ash, beech, birch, cedar, cottonwood, box, elder, elm, hickory, maple and oak pollen can trigger allergies.
Grasses pollinate in late spring and summer.
Those that cause allergic reactions include Kentucky bluegrass, timothy, Johnson, Bermuda, redtop, orchard, rye and sweet vernal grasses. Much pollen is released early in the morning, shortly after dawn. This results in high counts near the source plants. Pollen travels best on warm, dry, breezy days and peaks in urban areas midday. Pollen counts are lowest during chilly, wet periods.