When to plant carrots in zone 8b

Sow outdoors between Mar 3 and Mar 17. That is from 2 wk before to on the zone 8b's median last frost, Mar 171. Across most of zone 8b that date falls between Feb 21 and Apr 1.

Why a range: zone 8b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Mar 6 to Apr 12, so a single zone date can be weeks off for you. The rule comes from Colorado State University Extension, written for CO and applied here to zone 8b's frost dates.

Last reviewed: 2026-11-02
Stations: 652
ZIP codes: 2,535

Photo: Markus Spiske / Pexels

These dates use the typical last frost for the whole zone. Your ZIP code gives a closer answer.

Sow outdoorsColorado StateSow outdoors across most of the zone: Feb 21 to Apr 1Sow outdoors at the zone median: Mar 3 to Mar 17 (Colorado State)

Start indoors
Plant outdoors
Range across the zone
Frost-free season (median)
Spring frost risk (90% to 10%)
Fall frost risk (10% to 90%)

Figure 1. Carrots in zone 8b. Dark bars: at the zone's median frost dates. Pale bars: across most of the zone's ZIP codes. Hatched blue: where the zone's last and first frosts fall.

Sow outdoors: Mar 3range Feb 21 – Apr 1

What each source says, applied to zone 8b

The same frost dates give different answers depending on whose rule you follow. These are all the published rules we hold for carrots, worked out at zone 8b's median frost dates.

Table 1. Carrots rules from every source, at zone 8b's median last frost (Mar 17) and first frost (Nov 13).
Source	What	Rule	Zone 8b median
Colorado State1Used above	Sow outdoors	From 2 wk before to on the last frost	Mar 3 – Mar 17
Utah State3	Sow outdoors	2 weeks before last frost	Mar 3
Oregon State, eastern OR4	Sow outdoors	4 weeks before last frost	Feb 18
Texas A&M (E-502)5	Sow outdoors	4-6 weeks before last frost	Feb 4 – Feb 18
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Aug 7 – Aug 21

Frost across zone 8b

Last spring frostmedian Mar 17Full range across the zone: Feb 11 to Jun 3Most of the zone (10th to 90th percentile): Mar 6 to Apr 1Zone median: Mar 17
First fall frostmedian Nov 13Full range across the zone: Sep 14 to Dec 13Most of the zone (10th to 90th percentile): Nov 4 to Nov 23Zone median: Nov 13

Frost-free season (median)

Figure 2. Frost dates across zone 8b, from 652 NOAA stations matched to 2,535 ZIP codes.

Planting carrots with others

Onions Tested in a study Intercropped onions reduced carrot fly (Psila rosae) attack on carrots, probably because onion volatiles disrupt host finding; onion thrips on the onions were also reduced.6 6 6 7 8 8
Marigold Tested in a study Claimed: French marigold protects carrots from carrot fly. Tested: mixing carrots with French marigold (Tagetes patula) did not reduce carrot fly.6
Lettuce Extension guidance Deep-taprooted carrots paired with shallow, wide-rooted lettuce use different soil layers, so their roots compete less.7 8 8

Sources

Colorado State University Extension, Vegetable Planting Guide (CMG GardenNotes #720), reviewed September 2024. Retrieved 2026-11-02.
NOAA National Centers for Environmental Information, U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), 2021. Retrieved 2026-11-02.
Utah State University Extension, Suggested Vegetable Planting Dates for Utah, January 2018; revised May 2025. Retrieved 2026-11-02.
Oregon State University Extension Service, Eastern Oregon seed-starting schedule, Revised April 2025. Retrieved 2026-11-02.
Texas AgriLife Extension Service, The Texas A&M System, Texas Home Vegetable Gardening Guide (E-502), 2/09. Retrieved 2026-11-02.
Entomologia Experimentalis et Applicata 36(2):159-167 (John Wiley & Sons); abstract read via FAO AGRIS, Effect of mixed cropping on some insect pests of carrots and onions, 1984. Retrieved 2026-11-02.
Washington State University Extension, Gardening with Companion Plants (Home Garden Series), EM128E, December 2023. Retrieved 2026-11-02.
University of California Agriculture and Natural Resources (UCCE Master Gardener Program), Companion Planting Chart: Companion planting in the vegetable garden, File dated 2023-09 in URL path; no date printed. Retrieved 2026-11-02.