New Hampshire planting calendar

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

New Hampshire is mostly USDA zone 5b (range 3b-6a). Dates below are derived from onions's frost tolerance and New Hampshire's frost window — not generic national averages.

Onions planting timetable for New Hampshire

Stage	When in New Hampshire	Anchor
Start seeds indoors	early March (March 6)	10 weeks before the last frost (mid-May)
Transplant outside	mid-April (April 17)	28 days before the last frost (mid-May)
First harvest (estimate)	early August (August 5)	~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 New Hampshire's climate shifts the onions dates

New Hampshire's last spring frost averages mid-May and first fall frost late September, which sets the whole planting clock. New Hampshire is a cold New England state with a brief but reliable summer, milder along the small seacoast and colder in the mountains. 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 mid-May — a hard freeze can still set young plants back. In the White Mountains and far north (zone 3b) the safe date runs a week or two later.

Regional variation within New Hampshire

the White Mountains and far north (zone 3b) runs roughly 1-2 weeks behind the state average; the short Atlantic seacoast near Portsmouth (zone 6a) can plant 1-2 weeks earlier.

Manchester — USDA zone 5b
Concord — USDA zone 5b
Nashua — USDA zone 5b
Portsmouth — USDA zone 6a

What else to plant in New Hampshire 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 New Hampshire?

In New Hampshire (mostly USDA zone 5b), sow onions indoors around early March, transplant outdoors mid-April (before the last frost, mid-May), and harvest from early August. 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 New Hampshire?

Most of New Hampshire sits in USDA hardiness zone 5b, with the state spanning roughly 3b-6a from the White Mountains and far north (zone 3b) to the short Atlantic seacoast near Portsmouth (zone 6a). The last spring frost averages mid-May and the first fall frost late September.

Can you grow onions in New Hampshire?

Yes. New Hampshire's dominant zone 5b 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 New Hampshire?

the White Mountains and far north (zone 3b) runs roughly 1-2 weeks behind the state average; the short Atlantic seacoast near Portsmouth (zone 6a) can plant 1-2 weeks earlier.

What else can I plant in New Hampshire 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.