Cedar and pine can begin to pollinate as early as December. Starting in January, many other trees begin to pollinate, such as hazelnut, juniper, cypress and spruce. The next group begins in February: alder, birch and elm. The last group to pollinate in March/April includes oak, sweet gum, sycamore, cottonwood, maple, ash, hickory-pecan, beech, locust and walnut. The next pollinators are grasses. Usually the grass season is at its peak from Memorial Day (end of May) to Fourth of July, but depending on the climate can shift a little earlier or a little later. The Willamette Valley has very high grass pollen due to nearly 500,000 acres of land used for commercial production of grass seed. The last group of pollinators is weeds. Typically, these pollinate toward the end of summer and into fall, until first frost. People who have allergies to perennial allergens such as feathers, animal dander, dust mites or mold may have symptoms year-round. Symptoms may be worse during colder weather with increased indoor exposure.