Allegy prevalence; Timothy hay/grass
Arapahoe County Colorado trees and shrubs
Littleton, CO is located in Arapahoe County. Many of the suburban communities surrounding Denver, including those in Arapahoe County, are ideal locations for horse properties and other domesticated animals (goats, chickens, llama/alpaca, rabbits, etc.) Timothy hay is a cool season grass that is very winter hardy so it is readily available and affordable for horse owners.
Kentucky bluegrass is the most widely used lawn grass in the Metro area, however, perennial ryegrass is also a popular cool season turfgrass lawn found in the area.
Oaks are also common in Colorado, and are increasing in popularity with homeowners and municipalities. Those include Gambel oak, bur oak, English oak, Wavyleaf oak and Crimson Spire oak.
There is no way to know how much timothy hay, perennial ryegrass or number of oak trees is found in any given area. Local news/weather outlets, as well as, health care networks and websites such as pollen.com post allergen forecast levels based on tree, ragweed, mold and grass pollen levels.
Consider talking with an Allergist / Immunologist for research-based information on regions that are more favorable for your particular allergies.
The Littleton area is an urbanized part of S Denver, so there's not a lot of grass hay grown in the immediate area. Grass hay grown is mostly smooth brome, orchardgrass and tall fescue. I'm not an allergist but I'd assume that it is the grass pollen that you're allergic to, not just the grass itself. I'd also assume that you have other grass allergies, not just to the pollen of timothy grass and perennial ryegrass.
As Donnetta noted, Kentucky bluegrass and perennial ryegrass are used here primarily as turfgrasses, so they are being mowed frequently enough that flowering stems that produce pollen are regularly cut before pollen can be produced.
As for oak trees, Donnetta mentioned several of the landscape oaks grown here. They produce male flowers (pollen) in springtime at about the time of leafing. Oak and several other types of trees produce large quantities of very small, lightweight pollen, so that it can be dispersed easily by the wind rather than by insects. Wind-pollinated trees (oak, ash, cottonwood, pine, juniper and many others) tend to be more of a problem for allergy sufferers than trees which have showy flowers, fewer and larger pollen grains; these being pollinated by insects (apple, apricot, plum, cherry, peach and many others)