FleaDates › Flea season › Colorado
Mountain West · CO
When does flea season start in Colorado?
Flea season in Colorado typically runs mid-April to late October, with the heaviest pressure May–Sep. Outside that window outdoor fleas slow down — they don't disappear.
The Colorado flea picture
Colorado's fleas run on a mountain schedule. Averages hovering right around 30 degrees in December and January keep midwinter genuinely still outdoors, activity then climbs through May, holds at its peak from June into September, and drops off quickly after mid-October. Denver and Colorado Springs sit high and dry while Grand Junction bakes hotter on the Western Slope, and none of that matters to fleas already established in a heated basement — which is why year-round prevention is still the standard advice along the Front Range.
One thing that's true in every state: indoor fleas are year-round everywhere. A heated house in January is a fine flea nursery, which is why the calendar above describes outdoor pressure — not whether fleas are possible.
Month-by-month flea pressure in Colorado
| Month | Typical pressure | Average temperature |
|---|---|---|
| January | Quiet | 30.4°F |
| February | Quiet | 33.8°F |
| March | Low | 42.6°F |
| April | Low | 49.1°F |
| May | High | 58.8°F |
| June | Peak | 69.5°F |
| July | Peak | 75.6°F |
| August | Peak | 73.1°F |
| September | Peak | 65°F |
| October | Moderate | 51.7°F |
| November | Quiet | 39.5°F |
| December | Quiet | 30.4°F |
Estimates from NOAA U.S. Climate Normals (1991–2020), averaged across 3 weather stations near Colorado's larger cities, with activity thresholds from published veterinary and entomology research. Real years run early or late with the weather.
Flea season near you
Climate shifts even within Colorado — check the city closest to you:
Ticks are a different calendar. In Colorado tick activity typically runs late February to late November — see the Colorado tick season page.
Frequently asked questions
When does flea season start in Colorado?
Around mid-April in a typical year, once average temperatures hold around 50°F — the point where fleas begin reproducing in earnest outdoors. Warm springs pull that earlier, which is a good argument for not waiting.
When does flea season end in Colorado?
Outdoor flea pressure typically fades around late October, when sustained cold shuts down reproduction outside. Indoors is another story — heated homes stay flea-friendly all winter.
Does my dog need flea prevention in winter in Colorado?
Yes — veterinary organizations recommend year-round prevention everywhere, Colorado included. Fleas overwinter indoors on pets and in bedding regardless of the weather outside, and a warm snap can restart outdoor activity early. The season calendar tells you when to be extra vigilant, not when to stop.
What about ticks in Colorado?
Ticks run on their own, earlier calendar — in Colorado that's late February to late November. See the Colorado tick season page for the month-by-month picture and the species involved.