When to plant onions in Delaware
Delaware’s average last spring frost is April 14, and the first fall frost typically arrives around October 22 — about 191 frost-free days (USDA zones 7a, 7b). Onion needs 90–120 days to mature, which fits comfortably within Delaware's growing season.
When to plant onions in Delaware: the short answer
For a spring planting, plant out Mar 17 – Apr 3, with seeds started indoors Jan 27 – Feb 10. 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 Delaware, step by step
- Start seeds indoors (Jan 27 – Feb 10). 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 17 – Apr 3). 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 Delaware.
- 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 Jun 22 – Jul 22). 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 Delaware’s average frost dates. More in the full onion growing guide.
When to plant onions in nearby states
| State | Last frost | Plant out |
|---|---|---|
| Delaware (this page) | April 14 | Mar 17 – Apr 3 |
| New Jersey | April 20 | Mar 23 – Apr 9 |
| Pennsylvania | May 1 | Apr 3 – Apr 20 |
| Maryland | April 14 | Mar 17 – Apr 3 |
| District of Columbia | April 10 | Mar 13 – Mar 30 |
| Virginia | April 15 | Mar 18 – Apr 4 |
| West Virginia | May 5 | Apr 7 – Apr 24 |
Questions gardeners in Delaware ask
When is the best time to plant onions in Delaware?
Plant onion transplants outdoors in Delaware between March 17 and April 3. That timing is set by Delaware's average last spring frost of April 14.
When should I start onion seeds indoors in Delaware?
Start seeds indoors between January 27 and February 10, about 10 weeks before the last frost, so the seedlings are ready to plant out from March 17.
When will onions planted in Delaware be ready to harvest?
Planted on time, onions are usually ready from about June 22 to July 22, 90-120 days after transplanting.
When is the last frost in Delaware?
On average around April 14, with the first fall frost around October 22 - about 191 frost-free days. Your town may differ by two weeks or more.