When to plant carrots in zone 8a

Sow outdoors between Mar 15 and Mar 29. That is from 2 wk before to on the zone 8a's median last frost, Mar 291. Across most of zone 8a that date falls between Mar 6 and Apr 7.

Why a range: zone 8a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Mar 20 to Apr 72, 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 8a's frost dates.

Last reviewed: 2026-11-02
Stations: 650
ZIP codes: 2,528

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: Mar 6 to Apr 7Sow outdoors at the zone median: Mar 15 to Mar 29 (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 8a. 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 15range Mar 6 – Apr 7

What each source says, applied to zone 8a

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 8a's median frost dates.

Table 1. Carrots rules from every source, at zone 8a's median last frost (Mar 29) and first frost (Nov 6).
Source	What	Rule	Zone 8a median
Colorado State1Used above	Sow outdoors	From 2 wk before to on the last frost	Mar 15 – Mar 29
Utah State3	Sow outdoors	2 weeks before last frost	Mar 15
Oregon State, eastern OR4	Sow outdoors	4 weeks before last frost	Mar 1
Texas A&M (E-502)5	Sow outdoors	4-6 weeks before last frost	Feb 16 – Mar 1
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Jul 31 – Aug 14

Frost across zone 8a

Last spring frostmedian Mar 29Full range across the zone: Feb 28 to Jun 8Most of the zone (10th to 90th percentile): Mar 20 to Apr 7Zone median: Mar 29
First fall frostmedian Nov 6Full range across the zone: Sep 22 to Nov 26Most of the zone (10th to 90th percentile): Oct 31 to Nov 14Zone median: Nov 6

Frost-free season (median)

Figure 2. Frost dates across zone 8a, from 650 NOAA stations matched to 2,528 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.