Pollen is produced by almost all trees, weeds and grasses. All true pollen comes from plant sources and is different than mold and dust. Some common trees with high pollen production include birch, alder, cedar, hazelnut and hornbeam. A longer, hotter growing season tends to increase pollen production in plants.
Grasses known to produce pollen include ryegrass and timothy while weeds such as ragweed, nettle, mugwort and goosefoot produce pollen. Weather that is hot and dry helps foster pollen particles in the air, and winds dislodge the particles from all three primary sources, increasing pollen counts. Bright colored and sweet smelling flowers produce pollen as well, but it is typically not airborne, since those plants attract bees and birds to the pollen as a means of pollinating other plants and do not rely on wind.