When to Plant Onion in Colorado

Growing Onion in Colorado: Complete Guide

Onion (Allium cepa) is a warm-season crop that needs a head start indoors before Colorado's last frost. Most Colorado gardeners begin seeds indoors around Tuesday, February 23, 2027 and move transplants outside around Tuesday, May 4, 2027, after the risk of frost has passed.

Because Colorado stretches across 7 USDA hardiness zones (4a, 4b, 5a, 5b, 6a, 6b, 7a), planting windows can shift by as much as six to eight weeks from one end of the state to the other. Gardeners in the warmer southern zones can start earlier, while those in cooler northern zones should wait until the zone table above shows their last frost has passed.

With 100 days to maturity, Onion needs the indoor head start that Colorado's growing season requires. Starting indoors ensures you have strong transplants ready the moment outdoor conditions are right.

Onion requires full sun (6+ hours) and moderate moisture. Mountain-state gardens contend with short seasons and intense UV at elevation — cold nights can persist well into June at higher altitudes.

For the most precise planting dates tailored to your exact location, enter your ZIP code on our planting calendar or use the zone table above to find the schedule for your area of Colorado.