When to plant tomatoes in zone 6b

Transplant outdoors around May 6. That is 2 weeks after zone 6b's median last frost, Apr 221. Across most of zone 6b that date falls between Apr 24 and May 17.

Why a range: zone 6b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Apr 10 to May 32, 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 6b's frost dates.

Last reviewed: 2026-11-02
Stations: 899
ZIP codes: 4,372

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 14 to Mar 8Start indoors at the zone median: Feb 26 (Oregon State, central OR)
Transplant outdoorsColorado StateTransplant outdoors across most of the zone: Apr 24 to May 17Transplant outdoors at the zone median: May 6 (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 6b. 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: Feb 268 weeks before last frost (Oregon State, central OR)
Transplant outdoors: May 6range Apr 24 – May 17
First harvest, roughly: Jul 1065 days from transplant1

What each source says, applied to zone 6b

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 6b's median frost dates.

Table 1. Tomatoes rules from every source, at zone 6b's median last frost (Apr 22) and first frost (Oct 22).
Source	What	Rule	Zone 6b median
Colorado State1Used above	Transplant outdoors	2 weeks after last frost	May 6
Utah State3	Transplant outdoors	2 weeks after last frost	May 6
Oregon State, central OR4	Transplant outdoors	On the last frost	Apr 22
Oregon State, central OR4Used above	Start indoors	8 weeks before last frost	Feb 26
Texas A&M (E-502)5	Transplant outdoors	1-8 weeks after last frost	Apr 29 – Jun 17
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Jul 16 – Jul 30

Frost across zone 6b

Last spring frostmedian Apr 22Full range across the zone: Mar 31 to Jun 21Most of the zone (10th to 90th percentile): Apr 10 to May 3Zone median: Apr 22
First fall frostmedian Oct 22Full range across the zone: Sep 4 to Nov 12Most of the zone (10th to 90th percentile): Oct 10 to Oct 29Zone median: Oct 22

Frost-free season (median)

Figure 2. Frost dates across zone 6b, from 899 NOAA stations matched to 4,372 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