When to plant carrots in zone 4a

Sow outdoors between Apr 28 and May 12. That is from 2 wk before to on the zone 4a's median last frost, May 121. Across most of zone 4a that date falls between Apr 21 and May 24.

Why a range: zone 4a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from May 5 to May 242, 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 4a's frost dates.

Last reviewed: 2026-11-02
Stations: 276
ZIP codes: 720

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: Apr 21 to May 24Sow outdoors at the zone median: Apr 28 to May 12 (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 4a. 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: Apr 28range Apr 21 – May 24

What each source says, applied to zone 4a

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 4a's median frost dates.

Table 1. Carrots rules from every source, at zone 4a's median last frost (May 12) and first frost (Sep 28).
Source	What	Rule	Zone 4a median
Colorado State1Used above	Sow outdoors	From 2 wk before to on the last frost	Apr 28 – May 12
Utah State3	Sow outdoors	2 weeks before last frost	Apr 28
Oregon State, eastern OR4	Sow outdoors	4 weeks before last frost	Apr 14
Texas A&M (E-502)5	Sow outdoors	4-6 weeks before last frost	Mar 31 – Apr 14
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Jun 22 – Jul 6

Frost across zone 4a

Last spring frostmedian May 12Full range across the zone: May 2 to Jun 14Most of the zone (10th to 90th percentile): May 5 to May 24Zone median: May 12
First fall frostmedian Sep 28Full range across the zone: Sep 1 to Oct 9Most of the zone (10th to 90th percentile): Sep 20 to Oct 4Zone median: Sep 28

Frost-free season (median)

Figure 2. Frost dates across zone 4a, from 276 NOAA stations matched to 720 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.