When to plant tomatoes in zone 8b

Transplant outdoors around Mar 31. That is 2 weeks after zone 8b's median last frost, Mar 171. Across most of zone 8b that date falls between Mar 20 and Apr 15.

Why a range: zone 8b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Mar 6 to Apr 12, 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 8b's frost dates.

Last reviewed: 2026-11-02
Stations: 652
ZIP codes: 2,535

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: Jan 10 to Feb 5Start indoors at the zone median: Jan 21 (Oregon State, central OR)
Transplant outdoorsColorado StateTransplant outdoors across most of the zone: Mar 20 to Apr 15Transplant outdoors at the zone median: Mar 31 (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 8b. 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 218 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Mar 31range Mar 20 – Apr 15
First harvest, roughly: Jun 465 days from transplant1

What each source says, applied to zone 8b

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 8b's median frost dates.

Table 1. Tomatoes rules from every source, at zone 8b's median last frost (Mar 17) and first frost (Nov 13).
Source	What	Rule	Zone 8b median
Colorado State1Used above	Transplant outdoors	2 weeks after last frost	Mar 31
Utah State3	Transplant outdoors	2 weeks after last frost	Mar 31
Oregon State, central OR4	Transplant outdoors	On the last frost	Mar 17
Oregon State, central OR4Used above	Start indoors	8 weeks before last frost	Jan 21
Texas A&M (E-502)5	Transplant outdoors	1-8 weeks after last frost	Mar 24 – May 12
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Aug 7 – Aug 21

Frost across zone 8b

Last spring frostmedian Mar 17Full range across the zone: Feb 11 to Jun 3Most of the zone (10th to 90th percentile): Mar 6 to Apr 1Zone median: Mar 17
First fall frostmedian Nov 13Full range across the zone: Sep 14 to Dec 13Most of the zone (10th to 90th percentile): Nov 4 to Nov 23Zone median: Nov 13

Frost-free season (median)

Figure 2. Frost dates across zone 8b, from 652 NOAA stations matched to 2,535 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