When to plant onions in New Hampshire
New Hampshire’s average last spring frost is May 13, and the first fall frost typically arrives around September 25 — about 135 frost-free days (USDA zones 3b, 4a, 4b, 5a, 5b, 6a). Onion needs 90–120 days to mature, which fits comfortably within New Hampshire's growing season.
When to plant onions in New Hampshire: the short answer
For a spring planting, plant out Apr 15 – May 2, with seeds started indoors Feb 25 – Mar 11. Seeds germinate in soil from 40 to 90°F.
Windows are ~1–2 week bands around each date. Frost dates are a statewide average from NOAA/NCEI normals; your town may differ by two weeks or more. Calendar year 2026.
How to grow onions in New Hampshire, step by step
- Start seeds indoors (Feb 25 – Mar 11). Sow seeds ½ inch deep in seed-starting mix under bright light. They take 7-14 days to sprout, fastest in soil around 70°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. They need at least six to eight hours of direct sun a day and loose soil at least 12 inches deep, with a pH between 6.0 and 7.0. Dig in compost before planting.
- Plant out (Apr 15 – May 2). Space plants about 4 inches apart in rows 12 inches apart. It handles frost well, so it can go out before the last frost date. As a cool-season crop it does best before summer heat arrives in New Hampshire.
- Water and feed. Give it steady, even moisture, about an inch of water a week from rain or the hose.
- Watch for pests and problems. Pests: onion maggot, onion thrips. Diseases and disorders: downy mildew, white rot, neck rot, purple blotch.
- Harvest (from about Jul 21 – Aug 20). Onion takes 90-120 days to reach harvest. Match the day-length type to your latitude: long-day onions for the north, short-day for the south, day-neutral anywhere. Bulbing is triggered by day length, not calendar date.
Figures come from our crop data; dates use New Hampshire’s average frost dates. More in the full onion growing guide.
Onion planting dates by USDA zone in New Hampshire
New Hampshire spans USDA zones 3b, 4a, 4b, 5a, 5b, 6a. Colder zones, usually higher or further inland, have a later last spring frost and an earlier first fall frost than the state average, so the onion dates shift with them.
When to plant onions in nearby states
| State | Last frost | Plant out |
|---|---|---|
| New Hampshire (this page) | May 13 | Apr 15 – May 2 |
| Maine | May 14 | Apr 16 – May 3 |
| Vermont | May 14 | Apr 16 – May 3 |
| Massachusetts | April 24 | Mar 27 – Apr 13 |
| Rhode Island | April 25 | Mar 28 – Apr 14 |
| Connecticut | April 25 | Mar 28 – Apr 14 |
| New York | May 1 | Apr 3 – Apr 20 |
Questions gardeners in New Hampshire ask
When is the best time to plant onions in New Hampshire?
Plant onion transplants outdoors in New Hampshire between April 15 and May 2. That timing is set by New Hampshire's average last spring frost of May 13.
When should I start onion seeds indoors in New Hampshire?
Start seeds indoors between February 25 and March 11, about 10 weeks before the last frost, so the seedlings are ready to plant out from April 15.
When will onions planted in New Hampshire be ready to harvest?
Planted on time, onions are usually ready from about July 21 to August 20, 90-120 days after transplanting.
When is the last frost in New Hampshire?
On average around May 13, with the first fall frost around September 25 - about 135 frost-free days. Your town may differ by two weeks or more.