When to plant carrots in zone 4b

Sow outdoors between Apr 24 and May 8. That is from 2 wk before to on the zone 4b's median last frost, May 81. Across most of zone 4b that date falls between Apr 17 and May 25.

Why a range: zone 4b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from May 1 to May 252, 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 4b's frost dates.

Last reviewed: 2026-11-02
Stations: 443
ZIP codes: 1,127

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 17 to May 25Sow outdoors at the zone median: Apr 24 to May 8 (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 4b. 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 24range Apr 17 – May 25

What each source says, applied to zone 4b

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 4b's median frost dates.

Table 1. Carrots rules from every source, at zone 4b's median last frost (May 8) and first frost (Oct 1).
Source	What	Rule	Zone 4b median
Colorado State1Used above	Sow outdoors	From 2 wk before to on the last frost	Apr 24 – May 8
Utah State3	Sow outdoors	2 weeks before last frost	Apr 24
Oregon State, eastern OR4	Sow outdoors	4 weeks before last frost	Apr 10
Texas A&M (E-502)5	Sow outdoors	4-6 weeks before last frost	Mar 27 – Apr 10
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Jun 25 – Jul 9

Frost across zone 4b

Last spring frostmedian May 8Full range across the zone: Apr 22 to Jun 18Most of the zone (10th to 90th percentile): May 1 to May 25Zone median: May 8
First fall frostmedian Oct 1Full range across the zone: Sep 1 to Oct 20Most of the zone (10th to 90th percentile): Sep 22 to Oct 8Zone median: Oct 1

Frost-free season (median)

Figure 2. Frost dates across zone 4b, from 443 NOAA stations matched to 1,127 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.