Climate-based onion planting guide for Mount Vernon, Washington
When to Plant Onions in Mount Vernon: Timing and Maturity Guide
Onions are usually a comfortable fit in Mount Vernon. The season is generally supportive enough that consistency, sizing, and harvest goals matter more than season pressure.
Typical Planting Window
Use the planting dates below for onions in Mount Vernon.
Gardeners usually start indoors around January 6 and plant outdoors from about March 3. Most varieties need about 95–110 days to reach maturity once they are in the garden.
Onions are usually a comfortable fit in Mount Vernon. Gardeners usually get the best results when they use that margin to improve finish quality and uniformity.
Even here, the climate does not guarantee an even finish. The better results still come from steady growth, consistent sizing, and harvesting when the crop is actually ready.
Best local strategy: Plant in the normal window and use the extra margin to focus on steady growth, plant health, and finishing cleanly.
Can Onions Mature in Mount Vernon?
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, Mount Vernon typically provides about 2725 growing degree days for onions. With a typical crop target of 1300, that leaves a heat margin of +1425. 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.
GDD Checkpoints for Mount Vernon
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.
| Checkpoint | Remaining GDD | Heat margin | Fit vs typical target |
|---|---|---|---|
| Apr 15 | 2652 | +1352 | Comfortable |
| May 1 | 2561 | +1261 | Comfortable |
| May 15 | 2438 | +1138 | Comfortable |
| Jun 1 | 2229 | +929 | Comfortable |
| Jun 15 | 2030 | +730 | Comfortable |
| Jul 1 | 1788 | +488 | Comfortable |
Best Onion Varieties for Mount Vernon
Most onion varieties can succeed in Mount Vernon in a typical year. That gives gardeners room to choose for the kind of harvest they want, not just for minimum maturity speed.
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
| Variety class | Typical days to maturity | Typical GDD need | Local fit |
|---|---|---|---|
| Very early | 90–95 | 1100 | Good fit |
| Early | 95–105 | 1200 | Good fit |
| Mid-season | 105–115 | 1300 | Good fit |
| Late | 115–120 | 1400 | Good fit |
Main risk: The usual setbacks here come from management choices rather than from the season itself.
How Frost Affects Onions in Mount Vernon
Mount Vernon usually has about 224 frost-free days, with a typical last spring frost around March 24 and a typical first fall frost around November 3.
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.
The most common problems here are not climatic ones. Gardeners usually lose ground through timing, uneven growth, or letting the crop move past its best stage.
In Mount Vernon, onions usually have a solid seasonal margin when planted around March 3. Local gardens do not all warm and cool at the same pace. The warmest garden spots are usually south-facing walls, sheltered gardens, raised beds, and sunnier urban lots. Cooler spots like low spots, exposed sites, and shadier yards tend to warm up later and usually provide less heat. For onions, warmer garden spots usually improve early growth and can make timing a little more forgiving.
Related crops
Related crops worth comparing for the same city:
For a broader local overview, see the Mount Vernon planting guide. You can also use the Growing Degree Day Planner to test planting dates and crop timing.