When to Plant Basil in Colorado
The king of herbs. Basil and tomatoes are best friends in the garden and in the kitchen.
The Short Answer
Colorado Frost Dates
Your planting dates depend on which part of Colorado you're in. Here are the frost date ranges by region:
| Region | Zones | Last Frost (Spring) | First Frost (Fall) |
|---|---|---|---|
| Front Range (Denver) | 5a, 5b, 6a | May 1 - May 15 | Sep 25 - Oct 10 |
| Western Slope | 4b, 5a, 5b, 6a | May 10 - Jun 1 | Sep 15 - Oct 5 |
| Mountain Regions | 3a, 3b, 4a, 4b | Jun 1 - Jun 20 | Aug 25 - Sep 15 |
Basil Planting Schedule for Colorado
Front Range (Denver) (Zones 5a, 5b, 6a)
Average last frost: May 1 - May 15 · Average first frost: Sep 25 - Oct 10
Western Slope (Zones 4b, 5a, 5b, 6a)
Average last frost: May 10 - Jun 1 · Average first frost: Sep 15 - Oct 5
Mountain Regions (Zones 3a, 3b, 4a, 4b)
Average last frost: Jun 1 - Jun 20 · Average first frost: Aug 25 - Sep 15
Growing Basil in Colorado
Basil in Colorado's Climate
Basil needs warmth and dies at the first hint of frost. Start seeds indoors 6 weeks before your last frost and don't transplant until nights stay above 55°F — often 2-3 weeks after your official last frost date. Your growing window may be only 10-14 weeks, so make them count: pinch flower buds aggressively and harvest often. Consider growing in containers you can move indoors during unexpected cold snaps.
Soil Considerations for Colorado
Alkaline clay soils common along Front Range. Rocky, thin soils in mountains. Amend heavily with compost and sulfur to lower pH. Make sure soil has warmed to at least 60°F before planting basil outside.
Colorado Climate & Growing Season
High altitude means intense sun but cool nights. Low humidity. Hail risk in late spring. Short but intense growing season at elevation. Basil cannot tolerate any frost, so wait until all frost danger has passed before transplanting outside. Watch local forecasts carefully in spring.
Growing season length varies across Colorado: Front Range (Denver) (5a, 5b, 6a) has a last frost around May 1 - May 15, while Mountain Regions (3a, 3b, 4a, 4b) sees frost end around Jun 1 - Jun 20. This difference matters for basil — transplant timing shifts by several weeks across the state.
Growing Tips
Pinch off flower buds to keep leaves producing. Harvest from the top down, cutting just above a leaf pair. Succession plant for season-long supply.
Companion Planting
Plant basil alongside these companions for better growth:
Keep basil away from:
The Bottom Line
Last reviewed: March 29, 2026