When to plant tomatoes in zone 9a

Transplant outdoors around Mar 16. That is 2 weeks after zone 9a's median last frost, Mar 21. Across most of zone 9a that date falls between Mar 1 and Apr 1.

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: Thể Phạm / Pexels

These dates use the typical last frost for the whole zone. Your ZIP code gives a closer answer.

Start indoorsOregon State, central ORStart indoors across most of the zone: Dec 22 to Jan 22Start indoors at the zone median: Jan 6 (Oregon State, central OR)
Transplant outdoorsColorado StateTransplant outdoors across most of the zone: Mar 1 to Apr 1Transplant outdoors at the zone median: Mar 16 (Colorado State)
Plant for fallTexas A&M (E-502)Plant for fall across most of the zone: Aug 10 to Sep 16Plant for fall at the zone median: Aug 20 to Sep 3 (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. Tomatoes 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.

Start seeds indoors: Jan 68 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Mar 16range Mar 1 – Apr 1
First harvest, roughly: May 2065 days from transplant1
Fall planting: Aug 2012-14 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 tomatoes, worked out at zone 9a's median frost dates.

Table 1. Tomatoes 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	Transplant outdoors	2 weeks after last frost	Mar 16
Utah State3	Transplant outdoors	2 weeks after last frost	Mar 16
Oregon State, central OR4	Transplant outdoors	On the last frost	Mar 2
Oregon State, central OR4Used above	Start indoors	8 weeks before last frost	Jan 6
Texas A&M (E-502)5	Transplant outdoors	1-8 weeks after last frost	Mar 9 – Apr 27
Texas A&M (E-502)5Used above	Plant for fall	12-14 weeks before first frost	Aug 20 – Sep 3

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 tomatoes with others

Basil Tested in a study Claims: basil raises combined yield, improves tomato flavour and masks tomatoes from thrips and hornworms. Garden-scale trials found no consistent flavour effect and mixed yield results.6 6 6 7 7 8 8 9 10
Marigold Tested in a study French marigold grown alongside tomato slowed glasshouse whitefly build-up (airborne limonene). Marigold grown before tomato reduced root-knot nematode galling in the following tomato crop.9 11 11 12 12 12 10
Potatoes Extension guidance Shared late blight: the pathogen overwinters mainly on infected potato tubers and spreads by wind to nearby tomatoes.13 13 10 10