When to plant onions in Connecticut
Connecticut’s average last spring frost is April 25, and the first fall frost typically arrives around October 15 — about 173 frost-free days (USDA zones 5b, 6a, 6b, 7a). Onion needs 90–120 days to mature, which fits comfortably within Connecticut's growing season.
When to plant onions in Connecticut: the short answer
For a spring planting, plant out Mar 28 – Apr 14, with seeds started indoors Feb 7 – Feb 21. 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 Connecticut, step by step
- Start seeds indoors (Feb 7 – Feb 21). 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 (Mar 28 – Apr 14). 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 Connecticut.
- 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 3 – Aug 2). 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 Connecticut’s average frost dates. More in the full onion growing guide.
Onion planting dates by USDA zone in Connecticut
Connecticut spans USDA zones 5b, 6a, 6b, 7a. 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 |
|---|---|---|
| Connecticut (this page) | April 25 | Mar 28 – Apr 14 |
| Maine | May 14 | Apr 16 – May 3 |
| New Hampshire | May 13 | Apr 15 – May 2 |
| Vermont | May 14 | Apr 16 – May 3 |
| Massachusetts | April 24 | Mar 27 – Apr 13 |
| Rhode Island | April 25 | Mar 28 – Apr 14 |
| New York | May 1 | Apr 3 – Apr 20 |
Questions gardeners in Connecticut ask
When is the best time to plant onions in Connecticut?
Plant onion transplants outdoors in Connecticut between March 28 and April 14. That timing is set by Connecticut's average last spring frost of April 25.
When should I start onion seeds indoors in Connecticut?
Start seeds indoors between February 7 and February 21, about 10 weeks before the last frost, so the seedlings are ready to plant out from March 28.
When will onions planted in Connecticut be ready to harvest?
Planted on time, onions are usually ready from about July 3 to August 2, 90-120 days after transplanting.
When is the last frost in Connecticut?
On average around April 25, with the first fall frost around October 15 - about 173 frost-free days. Your town may differ by two weeks or more.