When to plant basil in Colorado
Colorado’s average last spring frost is May 5, and the first fall frost typically arrives around October 1 — about 149 frost-free days (USDA zones 3b, 4b, 5a, 5b, 6a, 7a). Basil needs 30–60 days to mature, which fits comfortably within Colorado's growing season.
Colorado covers a wide range of climates, so your exact frost dates may differ from the state average by two weeks or more. For dates tuned to your town, use the calendar generator.
When to plant basil in Colorado: the short answer
For a spring planting, plant out May 12 – May 29, with seeds started indoors Mar 24 – Apr 7. Wait until the soil is at least 65°F and frost is over.
Succession sow again in Colorado: Jun 9 · Jun 30 · Jul 21 · Aug 11
Windows are ~1–2 week bands around each date. Frost dates are a statewide average from NOAA/NCEI normals; your town may differ by two weeks or more. Calendar year 2026.
How to grow basil in Colorado, step by step
- Start seeds indoors (Mar 24 – Apr 7). Sow seeds ⅛ inch deep in seed-starting mix under bright light. They take 5-10 days to sprout, fastest in soil around 80°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. It needs at least six to eight hours of direct sun a day and loose soil at least 10 inches deep, with a pH between 6.0 and 7.0. Dig in compost before planting.
- Plant out (May 12 – May 29). Space plants about 10 inches apart in rows 12 inches apart. It is killed by frost, so nothing goes outside until the danger has passed.
- Water and feed. Give it steady, even moisture, about an inch of water a week from rain or the hose.
- Watch for pests and problems. Pests: aphids, Japanese beetle, slugs. Diseases and disorders: basil downy mildew, fusarium wilt, damping off.
- Harvest (from about Jun 18 – Jul 18). Basil takes 30-60 days to reach harvest. Cold soil stalls it - wait for warm nights. Pinch above a leaf pair every couple of weeks and remove flower buds to keep leaves coming. Downy mildew-resistant varieties are worth seeking out.
Figures come from our crop data; dates use Colorado’s average frost dates. More in the full basil growing guide.
Basil planting dates by USDA zone in Colorado
Colorado spans USDA zones 3b, 4b, 5a, 5b, 6a, 7a. Colder zones, usually higher or further inland, have a later last spring frost and an earlier first fall frost than the state average, so the basil dates shift with them.
| Zone | Last frost | First frost | Season | Start indoors | Plant basil |
|---|---|---|---|---|---|
| Zone 3 | May 15 | Sep 15 | 120 days | Apr 3 – Apr 17 | May 22 – Jun 8 |
| Zone 4 | May 10 | Sep 25 | 135 days | Mar 29 – Apr 12 | May 17 – Jun 3 |
| Zone 5 | Apr 30 | Oct 5 | 160 days | Mar 19 – Apr 2 | May 7 – May 24 |
| Zone 6 | Apr 15 | Oct 15 | 185 days | Mar 4 – Mar 18 | Apr 22 – May 9 |
| Zone 7 | Apr 5 | Oct 25 | 200 days | Feb 22 – Mar 8 | Apr 12 – Apr 29 |
When to plant basil in nearby states
Questions gardeners in Colorado ask
When is the best time to plant basil in Colorado?
Plant basil transplants outdoors in Colorado between May 12 and May 29. That timing is set by Colorado's average last spring frost of May 5; Colorado spans several climates, so cooler and higher parts of the state run later.
When should I start basil seeds indoors in Colorado?
Start seeds indoors between March 24 and April 7, about 5 weeks before the last frost, so the seedlings are ready to plant out from May 12.
What is the latest I can plant basil in Colorado?
About September 1. Basil needs at least 30 days to mature, and frost around October 1 ends the season, so later plantings risk being cut short.
When will basil planted in Colorado be ready to harvest?
Planted on time, basil is usually ready from about June 18 to July 18, 30-60 days after planting.
When is the last frost in Colorado?
On average around May 5, with the first fall frost around October 1 - about 149 frost-free days. Your town may differ by two weeks or more.