When to plant tomatoes in zone 4a

Transplant outdoors around May 26. That is 2 weeks after zone 4a's median last frost, May 121. Across most of zone 4a that date falls between May 19 and Jun 7.

Why a range: zone 4a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from May 5 to May 242, 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 4a's frost dates.

Last reviewed: 2026-11-02
Stations: 276
ZIP codes: 720

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 10 to Mar 29Start indoors at the zone median: Mar 17 (Oregon State, central OR)
Transplant outdoorsColorado StateTransplant outdoors across most of the zone: May 19 to Jun 7Transplant outdoors at the zone median: May 26 (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 4a. 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 178 weeks before last frost (Oregon State, central OR)
Transplant outdoors: May 26range May 19 – Jun 7
First harvest, roughly: Jul 3065 days from transplant1

What each source says, applied to zone 4a

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 4a's median frost dates.

Table 1. Tomatoes rules from every source, at zone 4a's median last frost (May 12) and first frost (Sep 28).
Source	What	Rule	Zone 4a median
Colorado State1Used above	Transplant outdoors	2 weeks after last frost	May 26
Utah State3	Transplant outdoors	2 weeks after last frost	May 26
Oregon State, central OR4	Transplant outdoors	On the last frost	May 12
Oregon State, central OR4Used above	Start indoors	8 weeks before last frost	Mar 17
Texas A&M (E-502)5	Transplant outdoors	1-8 weeks after last frost	May 19 – Jul 7
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Jun 22 – Jul 6

Frost across zone 4a

Last spring frostmedian May 12Full range across the zone: May 2 to Jun 14Most of the zone (10th to 90th percentile): May 5 to May 24Zone median: May 12
First fall frostmedian Sep 28Full range across the zone: Sep 1 to Oct 9Most of the zone (10th to 90th percentile): Sep 20 to Oct 4Zone median: Sep 28

Frost-free season (median)

Figure 2. Frost dates across zone 4a, from 276 NOAA stations matched to 720 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