When to plant tomatoes in zone 6a

Transplant outdoors around May 12. That is 2 weeks after zone 6a's median last frost, Apr 281. Across most of zone 6a that date falls between Apr 30 and May 24.

Why a range: zone 6a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Apr 16 to May 102, 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 6a's frost dates.

Last reviewed: 2026-11-02
Stations: 924
ZIP codes: 4,375

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: Feb 20 to Mar 15Start indoors at the zone median: Mar 3 (Oregon State, central OR)
Transplant outdoorsColorado StateTransplant outdoors across most of the zone: Apr 30 to May 24Transplant outdoors at the zone median: 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. Tomatoes in zone 6a. 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: Mar 38 weeks before last frost (Oregon State, central OR)
Transplant outdoors: May 12range Apr 30 – May 24
First harvest, roughly: Jul 1665 days from transplant1

What each source says, applied to zone 6a

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 6a's median frost dates.

Table 1. Tomatoes rules from every source, at zone 6a's median last frost (Apr 28) and first frost (Oct 17).
Source	What	Rule	Zone 6a median
Colorado State1Used above	Transplant outdoors	2 weeks after last frost	May 12
Utah State3	Transplant outdoors	2 weeks after last frost	May 12
Oregon State, central OR4	Transplant outdoors	On the last frost	Apr 28
Oregon State, central OR4Used above	Start indoors	8 weeks before last frost	Mar 3
Texas A&M (E-502)5	Transplant outdoors	1-8 weeks after last frost	May 5 – Jun 23
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Jul 11 – Jul 25

Frost across zone 6a

Last spring frostmedian Apr 28Full range across the zone: Apr 4 to Jun 21Most of the zone (10th to 90th percentile): Apr 16 to May 10Zone median: Apr 28
First fall frostmedian Oct 17Full range across the zone: Sep 6 to Nov 12Most of the zone (10th to 90th percentile): Oct 5 to Oct 24Zone median: Oct 17

Frost-free season (median)

Figure 2. Frost dates across zone 6a, from 924 NOAA stations matched to 4,375 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