When to plant tomatoes in zone 8a

Transplant outdoors around Apr 12. That is 2 weeks after zone 8a's median last frost, Mar 291. Across most of zone 8a that date falls between Apr 3 and Apr 21.

Why a range: zone 8a is defined by winter lows, not frost dates. Its ZIP codes have median last frosts from Mar 20 to Apr 72, 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 8a's frost dates.

Last reviewed: 2026-11-02
Stations: 650
ZIP codes: 2,528

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 24 to Feb 11Start indoors at the zone median: Feb 2 (Oregon State, central OR)
Transplant outdoorsColorado StateTransplant outdoors across most of the zone: Apr 3 to Apr 21Transplant outdoors at the zone median: Apr 12 (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 8a. 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 28 weeks before last frost (Oregon State, central OR)
Transplant outdoors: Apr 12range Apr 3 – Apr 21
First harvest, roughly: Jun 1665 days from transplant1

What each source says, applied to zone 8a

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 8a's median frost dates.

Table 1. Tomatoes rules from every source, at zone 8a's median last frost (Mar 29) and first frost (Nov 6).
Source	What	Rule	Zone 8a median
Colorado State1Used above	Transplant outdoors	2 weeks after last frost	Apr 12
Utah State3	Transplant outdoors	2 weeks after last frost	Apr 12
Oregon State, central OR4	Transplant outdoors	On the last frost	Mar 29
Oregon State, central OR4Used above	Start indoors	8 weeks before last frost	Feb 2
Texas A&M (E-502)5	Transplant outdoors	1-8 weeks after last frost	Apr 5 – May 24
Texas A&M (E-502)5	Plant for fall	12-14 weeks before first frost	Jul 31 – Aug 14

Frost across zone 8a

Last spring frostmedian Mar 29Full range across the zone: Feb 28 to Jun 8Most of the zone (10th to 90th percentile): Mar 20 to Apr 7Zone median: Mar 29
First fall frostmedian Nov 6Full range across the zone: Sep 22 to Nov 26Most of the zone (10th to 90th percentile): Oct 31 to Nov 14Zone median: Nov 6

Frost-free season (median)

Figure 2. Frost dates across zone 8a, from 650 NOAA stations matched to 2,528 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