Utah planting calendar

When to plant onions in Utah — sow, transplant & harvest dates

Utah is mostly USDA zone 6b (range 4a-9a). Dates below are derived from onions's frost tolerance and Utah's frost window — not generic national averages.

Onions planting timetable for Utah

Stage	When in Utah	Anchor
Start seeds indoors	mid-February (February 14)	10 weeks before the last frost (late April (Wasatch Front))
Transplant outside	late March (March 28)	28 days before the last frost (late April (Wasatch Front))
First harvest (estimate)	mid-July (July 16)	~110 days from transplant

Dates are state-wide averages for the dominant zone. Local microclimates — elevation, urban heat, coastal moderation — can shift the window by 1-2 weeks. Use the frost-date calculator for a date tuned to your town.

Why Utah's climate shifts the onions dates

Utah's last spring frost averages late April (Wasatch Front) and first fall frost mid-October (Wasatch Front), which sets the whole planting clock. Utah ranges from alpine mountains to warm southern desert. Elevation and aridity drive plant choice; the Wasatch Front has the main growing belt. Sow early — onions bolt once daytime temperatures hold above 24 °C, so the earlier they go in, the longer the harvest.

Onions are day-length sensitive: long-day varieties (zones 1–6) begin bulbing when days exceed 14 hours, short-day types (zones 7–10) bulb at 10–12 hours, and intermediate-day varieties span zones 5–6. Start seeds indoors 10–12 weeks before the last spring frost and transplant out 4–6 weeks before it — young onion seedlings tolerate frost down to about -6 °C once hardened off. In zones 8–10 a second planting from sets in autumn is common, overwintering for an early-summer harvest.

Frost-risk note

Don't plant before late April (Wasatch Front) — a hard freeze can still set young plants back. In the high Uinta and central mountains (zone 4a) the safe date runs a week or two later.

Regional variation within Utah

the high Uinta and central mountains (zone 4a) runs roughly 1-2 weeks behind the state average; the southwest Dixie around St. George (zone 9a) can plant 1-2 weeks earlier.

Salt Lake City — USDA zone 7a
Provo — USDA zone 7a
St. George — USDA zone 8b
Ogden — USDA zone 7a
Logan — USDA zone 6a

What else to plant in Utah around then

The same early window suits peas, lettuce, spinach, and onion sets.

Quick-grow guide

Sun: Full sun — 6+ hours direct.
Soil temperature for germination: 10-35 °C (50-95 °F).
Spacing: 4-6 inches (10-15 cm) between plants.
Days to harvest: ~110 days from planting out.

Frequently asked questions

When is the best time to plant onions in Utah?

In Utah (mostly USDA zone 6b), sow onions indoors around mid-February, transplant outdoors late March (before the last frost, late April), and harvest from mid-July. Onions are cold-hardy — they tolerate frost and actively prefer cool weather, so they go in well before the last spring frost and bolt in summer heat.

What USDA zone is Utah?

Most of Utah sits in USDA hardiness zone 6b, with the state spanning roughly 4a-9a from the high Uinta and central mountains (zone 4a) to the southwest Dixie around St. George (zone 9a). The last spring frost averages late April (Wasatch Front) and the first fall frost mid-October (Wasatch Front).

Can you grow onions in Utah?

Yes. Utah's dominant zone 6b supports onions — the key is timing. Onions are cold-hardy — they tolerate frost and actively prefer cool weather, so they go in well before the last spring frost and bolt in summer heat.

Does the planting date change across Utah?

the high Uinta and central mountains (zone 4a) runs roughly 1-2 weeks behind the state average; the southwest Dixie around St. George (zone 9a) can plant 1-2 weeks earlier.

What else can I plant in Utah around the same time?

The same early window suits peas, lettuce, spinach, and onion sets.

Source and methodology

State zone spans from the USDA Plant Hardiness Zone Map (2023); frost-date averages from NOAA Climate Data Online. Hot-state two-season timing cross-checked against the UF/IFAS Florida Gardening Calendar and the University of Arizona Cooperative Extension planting calendar. Curated by the Growli editorial team.