When to plant zucchini 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 Oregon State University Extension Service.

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

Photo: Gu Bra / 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: Apr 24 to May 18Start indoors at the zone median: May 1 (Oregon State, central OR)
Transplant outdoorsOregon State, central ORTransplant outdoors across most of the zone: May 15 to Jun 8Transplant outdoors at the zone median: May 22 (Oregon State, central OR)
Sow outdoorsColorado StateSow outdoors across most of the zone: May 1 to May 25Sow outdoors at the zone median: May 8 (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. Zucchini 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: May 11 weeks before last frost (Oregon State, central OR)
Transplant outdoors: May 22range May 15 – Jun 8

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 zucchini, worked out at zone 4b's median frost dates.

Table 1. Zucchini 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 State3Used above	Sow outdoors	On the last frost	May 8
Utah State4	Sow outdoors	On the last frost	May 8
Oregon State, central OR1Used above	Transplant outdoors	2 weeks after last frost	May 22
Oregon State, central OR1Used above	Start indoors	1 weeks before last frost	May 1
Texas A&M (E-502)5	Sow outdoors	1-4 weeks after last frost	May 15 – Jun 5
Texas A&M (E-502)5	Plant for fall	12-15 weeks before first frost	Jun 18 – 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 zucchini with others

Marigold Tested in a study Tradition lists marigold as a squash companion (no reason given). Field tests of squash (Cucurbita pepo) intercropped with French marigold found no consistent nematode effect.6 7 7 8 8
Blue Hubbard squash Tested in a study A border of Blue Hubbard squash draws striped cucumber beetles away from the summer squash it encircles; the border is sprayed when beetles arrive.9 9 9 10 10 9

Sources

Oregon State University Extension Service, When to start seeds indoors in central Oregon, reviewed 2024. Retrieved 2026-11-02.
NOAA National Centers for Environmental Information, U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), 2021. Retrieved 2026-11-02.
Colorado State University Extension, Vegetable Planting Guide (CMG GardenNotes #720), reviewed September 2024. Retrieved 2026-11-02.
Utah State University Extension, Suggested Vegetable Planting Dates for Utah, January 2018; revised May 2025. Retrieved 2026-11-02.
Texas AgriLife Extension Service, The Texas A&M System, Texas Home Vegetable Gardening Guide (E-502), 2/09. Retrieved 2026-11-02.
University of California Agriculture and Natural Resources (UCCE Master Gardener Program), Companion Planting Chart: Companion planting in the vegetable garden, File dated 2023-09 in URL path; no date printed. Retrieved 2026-11-02.
Journal of Nematology 25(4):666-673 (Society of Nematologists); FLVC journal host, Effects of Mixed Cropping on a Soil Nematode Community in Honduras, 1993. Retrieved 2026-11-02.
University of Florida, IFAS Extension (Entomology and Nematology Department), Marigolds (Tagetes spp.) for Nematode Management (ENY-056), reviewed Reviewed November 2019; page footer "Reviewed: 01/2024". Retrieved 2026-11-02.
University of Vermont Extension, Vermont Vegetable and Berry Program, Perimeter Trap Cropping: A Novel Approach to Insect Pest Control, July 2004. Retrieved 2026-11-02.
Journal of Extension 42(5), Article 13, Demonstrating a Perimeter Trap Crop Approach to Pest Management on Summer Squash in New England, October 2004. Retrieved 2026-11-02.