When to plant tomatoes in zone 4b

Transplant outdoors around May 22. That is 2 weeks after zone 4b's median last frost, May 81. Across most of zone 4b that date falls between May 15 and Jun 8.

Why a range: zone 4b is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from May 1 to May 252, 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 4b's frost dates.

Last reviewed: 2026-11-02
Stations: 443
ZIP codes: 1,127

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: Mar 6 to Mar 30Start indoors at the zone median: Mar 13 (Oregon State, central OR)
Transplant outdoorsColorado StateTransplant outdoors across most of the zone: May 15 to Jun 8Transplant outdoors at the zone median: May 22 (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 4b. 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 138 weeks before last frost (Oregon State, central OR)
Transplant outdoors: May 22range May 15 – Jun 8
First harvest, roughly: Jul 2665 days from transplant1

What each source says, applied to zone 4b

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 4b's median frost dates.

Table 1. Tomatoes rules from every source, at zone 4b's median last frost (May 8) and first frost (Oct 1).
Source	What	Rule	Zone 4b median
Colorado State1Used above	Transplant outdoors	2 weeks after last frost	May 22
Utah State3	Transplant outdoors	2 weeks after last frost	May 22
Oregon State, central OR4	Transplant outdoors	On the last frost	May 8
Oregon State, central OR4Used above	Start indoors	8 weeks before last frost	Mar 13
Texas A&M (E-502)5	Transplant outdoors	1-8 weeks after last frost	May 15 – Jul 3
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Jun 25 – Jul 9

Frost across zone 4b

Last spring frostmedian May 8Full range across the zone: Apr 22 to Jun 18Most of the zone (10th to 90th percentile): May 1 to May 25Zone median: May 8
First fall frostmedian Oct 1Full range across the zone: Sep 1 to Oct 20Most of the zone (10th to 90th percentile): Sep 22 to Oct 8Zone median: Oct 1

Frost-free season (median)

Figure 2. Frost dates across zone 4b, from 443 NOAA stations matched to 1,127 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