When to plant potatoes 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: Pixabay / 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. Potatoes 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
First harvest, roughly: Aug 31125 days from planting1

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 potatoes, worked out at zone 4a's median frost dates.

Table 1. Potatoes 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
Texas A&M (E-502)4	Sow outdoors	4-6 weeks before last frost	Mar 31 – Apr 14
Texas A&M (E-502)4	Plant for fall	14-16 weeks before first frost	Jun 8 – Jun 22

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 potatoes with others

Tomatoes Extension guidance Shared late blight: the pathogen overwinters mainly on infected potato tubers and spreads by wind to nearby tomatoes.5 5 6 6

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.
Texas AgriLife Extension Service, The Texas A&M System, Texas Home Vegetable Gardening Guide (E-502), 2/09. Retrieved 2026-11-02.
Washington State University Extension (HortSense), Tomato: Late blight (HortSense fact sheet), reviewed Last review date: 2026-08-28. 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.