Is it allergy season in Colorado?

Colorado's allergy story is altitude. The mile-high city sits high enough that the pollen season is genuinely shorter than comparable cities at sea level — but that compression means multiple allergens peak simultaneously rather than sequentially. Juniper begins in February as a quiet winter surprise. Then cottonwood, ash, and elm arrive in April, followed by oak in May. By late June the tree season is largely over and grass takes the summer. The Eastern Plains adds intense ragweed and Russian thistle (kochia) through fall.

Colorado's climate zones differ more dramatically than most states. The Eastern Plains are flat, agricultural, and windy — similar to Kansas. The Front Range is urban and semi-arid. The foothills add mountain species. And above 8,000 feet, the season compresses to just a few summer months.

Denver / Front Range Urban Corridor

Juniper in winter, cottonwood in spring, compressed but intense

From Fort Collins through Denver to Pueblo, the Front Range corridor shares a tight seasonal calendar. Juniper begins in February — a winter allergy surprise for many. Cottonwood and ash overlap in April–May in a concentrated peak. Then grass runs through summer, followed by ragweed. Denver's dry, windy days push counts to Very High levels during the April–May window.

Juniper · Feb–MarCottonwood · Apr–MayGrass · Jun–AugRagweed · Aug–Oct

Boulder / Northern Front Range

Foothills add mountain species alongside Front Range allergens

Boulder sits where the Front Range meets the foothills, adding native mountain vegetation to the typical Front Range allergen mix. Cottonwood is especially prominent along Boulder Creek in late April–May. The university city sees intense allergy seasons in spring — oak and ash arrive slightly later than in Denver due to higher elevation.

Cottonwood · Apr–MayOak · May–JunGrass · Jun–AugRagweed · Aug–Sep

Colorado Springs / Southern Front Range

Higher elevation delays arrival; Ponderosa pine adds a dimension

Colorado Springs sits 6,035 feet — a few hundred feet above Denver — which delays some species by a week. Ponderosa pine in the adjacent mountains adds a tree pollen dimension not common in central Denver. The drier southern climate makes kochia (a Russian thistle relative) a bigger weed allergen in late summer on the plains east of the city.

Juniper · Feb–MarCottonwood · Apr–MayKochia · Aug–OctGrass · Jun–Aug

High Country / Mountain Towns

Short season, real summer break — genuine relief Oct–May

Aspen, Vail, Steamboat Springs, and mountain resort towns operate on a very different calendar. The season starts in June, driven by grass and wildflowers, and is essentially finished by September. The thin air at 8,000–10,000 feet means lower absolute pollen counts than the Front Range. Mountain residents get 7–8 months of near-zero pollen.

Grass · Jun–AugWildflowers · Jul–AugRagweed · Aug–Sep (mild)Oct–May: near-zero pollen

Colorado's compressed season means shorter suffering but also shorter recovery. Each row shows trees (blue), grasses (green), and weeds (amber) for one region across 12 months.

Oral Allergy Syndrome (also called pollen-food allergy syndrome) causes tingling or mild itching in the mouth when you eat certain raw foods during the relevant pollen season. The proteins in the food are similar enough to the pollen protein that your immune system cross-reacts. Cooking usually deactivates the offending protein, so the same food cooked may cause no reaction.

Oak & Birch pollen

Stone fruits & apples

Apples, peaches, cherries, plums, pears, almonds, hazelnuts, carrots, celery

Peak risk April–June in CO — later than eastern US due to elevation. Cottonwood is related to willow and can also cross-react with foods in broadly sensitized individuals.

Grass pollen

Tomatoes, potatoes & melons

Tomatoes, potatoes, kiwi, watermelon, cantaloupe, oranges (in some cases)

Peak risk June–August in Denver; later at altitude. Timothy and brome grass dominate Colorado's grass pollen season.

Ragweed / Kochia

Melons & bananas

Watermelon, cantaloupe, honeydew, banana, zucchini, cucumber, sunflower seeds

Peak risk August–October. Colorado's eastern plains add kochia (Russian thistle family) as a significant weed allergen alongside ragweed.

Not medical advice. If you suspect OAS, speak with an allergist — it can sometimes progress, and symptoms that extend beyond the mouth should be evaluated.