When to Plant Onions in Seattle

Can Onions Mature in Seattle?

Growing degree days measure how much useful warmth typically accumulates during the season. For onions, this helps estimate whether local heat accumulation is usually enough for the crop to reach maturity on time.

From the usual planting window, Seattle typically provides about 3442 growing degree days for onions. With a typical crop target of 1300, that leaves a heat margin of +2142. That large heat margin means season length is usually not the limiting issue here. The more useful question is how gardeners use that room to improve sizing, finish quality, and harvest timing.

When Is It Too Late to Plant?

If planting later than usual, this table shows how much growing degree day heat is still available from each point in the season. For onions, it is most useful for judging how much freedom you still have to plant for quality, finish, and harvest goals as the season moves along.

How Different Onion Varieties Affect Results

In Seattle, most onion varieties are usually realistic choices. Gardeners can often choose across the maturity range without giving up much day-to-day reliability.

Varieties that often fit well here include:

• Walla Walla — large and popular, but still best when started early enough to build size
• Copra — a dependable storage onion with good all-around practicality
• Redwing — a strong red storage type where the season is reasonably supportive
• Patterson — a solid keeping onion that wants enough runway to size up well
• Ailsa Craig — more exposed in shorter seasons because it benefits from a longer finishing run

Main risk: The most common issue here is not climate but management: uneven growth, delayed planting, or harvesting outside the best quality window.

How Frost Affects Planting Dates for Onions in Seattle

Seattle usually has about 249 frost-free days, with a typical last spring frost around March 13 and a typical first fall frost around November 17.

Onions are generally lightly frost tolerant and temperatures below about 28°F ( -2 °C) can slow growth or damage plants.

Onions are usually tolerant enough of cool conditions that frost dates act more like planning markers than hard limits. In practice, timing and steady early growth matter more than avoiding every light frost.

Setbacks here usually come from practical decisions rather than from season length: planting later than ideal, uneven growth, poor moisture management, or harvesting outside the best eating window.

In Seattle, onions already have plenty of seasonal room when planted around February 20. In practical terms, the best spots are usually south-facing walls, sheltered gardens, raised beds, and sunnier urban lots. Cooler spots like low spots, exposed sites, and shadier yards are more likely to stay cooler and be less forgiving. For onions, warmer local sites usually help the crop get established earlier and grow a little more steadily.