When to plant tomatoes in zone 5a

Transplant outdoors around May 17. That is 2 weeks after zone 5a's median last frost, May 31. Across most of zone 5a that date falls between May 10 and Jun 3.

Why a range: zone 5a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Apr 26 to May 202, 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 5a's frost dates.

Last reviewed: 2026-11-02
Stations: 655
ZIP codes: 2,089

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 1 to Mar 25Start indoors at the zone median: Mar 8 (Oregon State, central OR)
Transplant outdoorsColorado StateTransplant outdoors across most of the zone: May 10 to Jun 3Transplant outdoors at the zone median: May 17 (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 5a. 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 88 weeks before last frost (Oregon State, central OR)
Transplant outdoors: May 17range May 10 – Jun 3
First harvest, roughly: Jul 2165 days from transplant1

What each source says, applied to zone 5a

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 5a's median frost dates.

Table 1. Tomatoes rules from every source, at zone 5a's median last frost (May 3) and first frost (Oct 5).
Source	What	Rule	Zone 5a median
Colorado State1Used above	Transplant outdoors	2 weeks after last frost	May 17
Utah State3	Transplant outdoors	2 weeks after last frost	May 17
Oregon State, central OR4	Transplant outdoors	On the last frost	May 3
Oregon State, central OR4Used above	Start indoors	8 weeks before last frost	Mar 8
Texas A&M (E-502)5	Transplant outdoors	1-8 weeks after last frost	May 10 – Jun 28
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Jun 29 – Jul 13

Frost across zone 5a

Last spring frostmedian May 3Full range across the zone: Apr 13 to Jun 20Most of the zone (10th to 90th percentile): Apr 26 to May 20Zone median: May 3
First fall frostmedian Oct 5Full range across the zone: Sep 1 to Oct 28Most of the zone (10th to 90th percentile): Sep 26 to Oct 13Zone median: Oct 5

Frost-free season (median)

Figure 2. Frost dates across zone 5a, from 655 NOAA stations matched to 2,089 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