When to Plant Onion

Enter your ZIP code to see the onion planting windows from the frost dates at the nearest weather station. The rules below come from university extension publications and count from the average last or first frost.

Your area

Risk level

How much frost risk you'll accept. Each level uses the frost dates with this chance of a frost after the last-frost date or before the first-frost date: Conservative 10%, Typical 30%, Aggressive 50%.

Planting year

The year the frost dates and windows are placed on.

Onion planting windows

Onion planting rules

Each rule counts days or weeks from the average last spring frost or first fall frost in its source. YepPlant applies the same count to the 32°F frost dates at each risk level: Conservative (10%), Typical (30%), and Aggressive (50%). A range in a rule becomes a range in the window. Where sources differ or don't give a single value, the value is YepPlant's reading of the sources, and it stays within what they say.

Onion planting rules, from university extension publications
Rule	Value	Sources
Frost threshold	32°F frost dates	—
Planting material	Seedlings or sets	UMD-ON, COR-ON, CSU, UMD-PYepPlant's reading of these sources
Transplant or plant sets	28 to 14 days before the last spring frost	CSU, UNH, COR-ONYepPlant's reading of these sources
Start seeds indoors	8 to 12 weeks before transplanting	NCSU, UMD-S, UNH, UMD-ONYepPlant's reading of these sources
Bought seedlings or sets	Planting dates also apply to bought seedlings or sets	UMD-ON, COR-ONYepPlant's reading of these sources
Sow outdoors	Not available	—
Days to maturity	Not available	—
Fall planting	Not available	—
Crop rotation	Plant this crop's family in the same spot at most once every 3 years	NCSU, COR-ON

Not available means no rule from the sources is used for it, so no window depends on it. Rows without a source follow the rules shared by all crops, described on the methods and sources page.

These windows are for planting onion seedlings or sets in spring, the crop that long-day and intermediate varieties suit. Onions form bulbs in response to day length, and varieties are grouped as long-day, intermediate, or short-day. In the Deep South, short-day varieties are also grown over the winter. This page doesn't show planting dates for that winter crop, because the sources don't give a rule for it based on frost dates.

Sources

University extension publications, with the date each was retrieved. The frost dates come from NOAA's 1991–2020 Climate Normals at the weather station nearest your ZIP code, and the station is found from the Census Bureau's ZIP Code Tabulation Areas.

CSU: Colorado State University Extension, GardenNotes #720: Vegetable Planting Guide: Used for: Planting material, Transplant or plant sets. Retrieved October 13, 2026.
USU: Utah State University Extension: Suggested Vegetable Planting Dates for Utah: Consulted, but no rule on this page comes from it. Retrieved October 13, 2026.
UMD-P: University of Maryland Extension: Vegetables for Spring and Fall Planting: Used for: Planting material. Retrieved October 13, 2026.
NCSU: NC State Extension, Extension Gardener Handbook Chapter 16: Vegetable Gardening: Used for: Start seeds indoors, Crop rotation. Retrieved October 13, 2026.
UMD-S: University of Maryland Extension HGIC: Starting Vegetable Seeds Indoors: Used for: Start seeds indoors. Retrieved October 13, 2026.
PUR: Purdue Extension, HO-66-W: The Fall Vegetable Garden: Consulted, but no rule on this page comes from it. Retrieved October 13, 2026.
UNL: Nebraska Extension, NebGuide G1343: Fall Vegetable Gardening: Consulted, but no rule on this page comes from it. Retrieved October 13, 2026.
OSU: Oregon State University Extension: Plant cold-hardy vegetables now for a spring harvest: Consulted, but no rule on this page comes from it. Retrieved October 13, 2026.
UNH: UNH Cooperative Extension: Growing Vegetables: When to Plant your Vegetable Garden: Used for: Transplant or plant sets, Start seeds indoors. Retrieved October 13, 2026.
COR-ON: Cornell University, Explore Cornell - Home Gardening, Vegetable Growing Guides: Onions: Used for: Planting material, Transplant or plant sets, Bought seedlings or sets, Crop rotation. Retrieved October 13, 2026.
UMD-ON: University of Maryland Extension HGIC: Growing Onions in a Home Garden: Used for: Planting material, Start seeds indoors, Bought seedlings or sets. Retrieved October 13, 2026.

Data sources

NOAA U.S. Climate Normals, frost dates, 1991–2020 normals, version 1.0.1: Files: us-climate-normals_1991-2020_v1.0.1_annualseasonal_multivariate_by-station_c20230404.tar.gz, inventory_30yr.txt from NOAA NCEI. Retrieved October 10, 2026.
U.S. Census Bureau ZIP Code Tabulation Areas, 2026 Gazetteer Files and 2020 ZCTA to County Relationship File: Files: 2026_Gaz_zcta_national.zip, tab20_zcta520_county20_natl.txt, national_state2020.txt from the U.S. Census Bureau. Retrieved October 10, 2026.