PlantAlong

When to plant zucchini 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 Oregon State University Extension Service.

Last reviewed
Stations
276
ZIP codes
720
Photo: Gu Bra / Pexels
These dates use the typical last frost for the whole zone. Your ZIP code gives a closer answer.
  1. Start indoorsOregon State, central OR
  2. Transplant outdoorsOregon State, central OR
  3. Sow outdoorsColorado 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 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
May 51 weeks before last frost (Oregon State, central OR)
Transplant outdoors
May 26range May 19 – Jun 7

Five digits. The calendar is computed on your device.

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

Table 1. Zucchini rules from every source, at zone 4a's median last frost (May 12) and first frost (Sep 28).
SourceWhatRuleZone 4a median
Colorado State3Used aboveSow outdoorsOn the last frostMay 12
Utah State4Sow outdoorsOn the last frostMay 12
Oregon State, central OR1Used aboveTransplant outdoors2 weeks after last frostMay 26
Oregon State, central OR1Used aboveStart indoors1 weeks before last frostMay 5
Texas A&M (E-502)5Sow outdoors1-4 weeks after last frostMay 19 – Jun 9
Texas A&M (E-502)5Plant for fall12-15 weeks before first frostJun 15 – Jul 6

Frost across zone 4a

  1. Last spring frostmedian May 12
  2. First fall frostmedian Sep 28
  • Frost-free season (median)
Figure 2. Frost dates across zone 4a, from 276 NOAA stations matched to 720 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.67788
  • 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.99910109

Sources

  1. Oregon State University Extension Service, When to start seeds indoors in central Oregon, reviewed 2024. Retrieved 2026-11-02.
  2. NOAA National Centers for Environmental Information, U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), 2021. Retrieved 2026-11-02.
  3. Colorado State University Extension, Vegetable Planting Guide (CMG GardenNotes #720), reviewed September 2024. Retrieved 2026-11-02.
  4. Utah State University Extension, Suggested Vegetable Planting Dates for Utah, January 2018; revised May 2025. Retrieved 2026-11-02.
  5. Texas AgriLife Extension Service, The Texas A&M System, Texas Home Vegetable Gardening Guide (E-502), 2/09. Retrieved 2026-11-02.
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. 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.