When to plant potatoes in zone 9a

Sow outdoors between Feb 17 and Mar 2. That is from 2 wk before to on the zone 9a's median last frost, Mar 21. Across most of zone 9a that date falls between Feb 2 and Mar 18.

Why a range: zone 9a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Feb 16 to Mar 182, 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 9a's frost dates.

Last reviewed: 2026-11-02
Stations: 432
ZIP codes: 1,671

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: Feb 2 to Mar 18Sow outdoors at the zone median: Feb 17 to Mar 2 (Colorado State)
Plant for fallTexas A&M (E-502)Plant for fall across most of the zone: Jul 27 to Sep 2Plant for fall at the zone median: Aug 6 to Aug 20 (Texas A&M (E-502))

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 9a. 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: Feb 17range Feb 2 – Mar 18
First harvest, roughly: Jun 21125 days from planting1
Fall planting: Aug 614-16 weeks before first frost

What each source says, applied to zone 9a

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 9a's median frost dates.

Table 1. Potatoes rules from every source, at zone 9a's median last frost (Mar 2) and first frost (Nov 26).
Source	What	Rule	Zone 9a median
Colorado State1Used above	Sow outdoors	From 2 wk before to on the last frost	Feb 17 – Mar 2
Utah State3	Sow outdoors	2 weeks before last frost	Feb 17
Texas A&M (E-502)4	Sow outdoors	4-6 weeks before last frost	Jan 20 – Feb 3
Texas A&M (E-502)4Used above	Plant for fall	14-16 weeks before first frost	Aug 6 – Aug 20

Frost across zone 9a

Last spring frostmedian Mar 2Full range across the zone: Jan 6 to Jun 3Most of the zone (10th to 90th percentile): Feb 16 to Mar 18Zone median: Mar 2
First fall frostmedian Nov 26Full range across the zone: Sep 14 to Feb 1Most of the zone (10th to 90th percentile): Nov 16 to Dec 9Zone median: Nov 26

Frost-free season (median)

Figure 2. Frost dates across zone 9a, from 432 NOAA stations matched to 1,671 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.