Common plant allergens include Spring: Tree pollens (eg, oak, elm, maple, alder, birch, juniper, olive) Summer: Grass pollens (eg, Bermuda, timothy, sweet vernal, orchard, Johnson) and weed pollens (eg, Russian thistle, English plantain) Fall: Other weed pollens (eg, ragweed) Causes also differ by region, and seasonal allergic rhinitis is occasionally caused by airborne fungal (mold) spores.