Practical planning tools for short growing seasons.
Climate-based onion planting guide for Jackson, Wyoming
When to Plant Onions in Jackson
In Jackson, onions can work, but the local season leaves limited room for delay or slower choices.
Typical Planting Window
Borderline in this climate
Use the planting dates below for onions in Jackson.
Start indoors
April 4
Typical planting windowMay 30 – June 13
MethodTransplant
Typical days to maturity95–110
Onions are usually started indoors around April 4 and planted outdoors during the normal local window of May 30 to June 13.
Most varieties need about 95–110 days to reach maturity once they are in the garden.
Gardeners can still grow onions in Jackson, but success usually depends on treating earliness and warm placement as part of the plan rather than as nice bonuses.
Within Wyoming, Jackson usually reaches planting time for onions a little later than many comparable locations.
Best local strategy:
Use the earliest practical timing, favor quicker varieties, and avoid cooler exposed sites.
Can Onions Mature in Jackson?
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.
Available GDD (base 45)1237
Typical crop GDD target1300
Heat margin-63
From the usual planting window, Jackson typically provides about 1237 growing degree days for onions. With a typical crop target of 1300, that leaves a heat margin of -63. That narrow heat margin means small delays or slower varieties can quickly reduce the odds of timely maturity.
When Is It Too Late to Plant?
When planting later than usual, this table shows how much growing degree day heat is still available from each point in the season. As planting gets pushed back, the remaining heat drops and the crop becomes less likely to mature on time.
Checkpoint
Remaining GDD
Heat margin
Fit vs typical target
Apr 15
1530
+230
Comfortable
May 15
1521
+221
Comfortable
Jun 1
1442
+142
Usually fits
Jun 15
1329
+29
Tight fit
Jul 1
1147
-153
Usually short
How Different Onion Varieties Affect Results
In Jackson, very early onion varieties are usually the most dependable choices, while early and mid-season types sit closer to the line when planting is delayed or the season is less forgiving.
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
Best Onion Varieties for Jackson
Very early onion varieties are usually the strongest all-around match in Jackson. The local season can support onions, so the main choice is usually about bulb size, sweetness, color, and keeping quality.
June 20
local season starts
August 31
frost pressure returns
Less heat used1237 GDD available
Hover or tap the dots to see which recommended varieties use that much local heat.
For Jackson, start with Walla Walla for onions when you want large sweet onions.
Look at Copra when you specifically want dependable storage onions.
Compare each variety’s heat need and maturity timing against the local frost-free window before choosing what to grow.
Recommended starting point
Walla WallaVery early
1100 GDD needed1237 available before frost
June 20August 31
Tight fit
Why this fit?
Local season fit:
Walla Walla leaves about 137 GDD cushion against the normal Jackson crop heat estimate.
Best for: large sweet onions.
A large, popular onion that can be rewarding, but still needs an early enough start to build size.
Tradeoff: Needs an early enough start to build size.
Also realistic
CopraEarly
1200 GDD needed1237 available before frost
June 20August 31
Tight fit
Why this fit?
Local season fit:
Copra leaves about 37 GDD cushion against the normal Jackson crop heat estimate.
Best for: dependable storage onions.
A practical storage onion with good all-around usefulness when started early.
Tradeoff: Still needs enough season to size up.
GDD comparisons are a planning shortcut, not a guarantee. Soil, watering, sowing depth, pests, transplant quality, and harvest goals still affect the final result.
Varieties that didn’t make the cut
These varieties are not the main picks for Jackson because they either run past the normal season or leave too little margin before frost.
ailsa craigLate
Needs1400 GDD
Jackson gives1237 GDD
Gap
163 GDD short
1237 GDD available before frost163 more GDD needed
June 20August 31
Runs past season
Why not a main pick?
Local season fit:
ailsa craig usually needs about 163 more GDD than Jackson provides before frost.
Best for: large exhibition onions.
A large onion that is more exposed in shorter seasons because it benefits from a longer finishing run.
Tradeoff: More exposed in shorter seasons.
pattersonMid-season
Needs1300 GDD
Jackson gives1237 GDD
Gap
63 GDD short
1237 GDD available before frost63 more GDD needed
June 20August 31
Runs past season
Why not a main pick?
Local season fit:
patterson usually needs about 63 more GDD than Jackson provides before frost.
Best for: long-keeping onions.
A solid keeping onion that wants enough runway to size up well before the season closes.
Tradeoff: Needs enough runway before the season closes.
redwingMid-season
Needs1300 GDD
Jackson gives1237 GDD
Gap
63 GDD short
1237 GDD available before frost63 more GDD needed
June 20August 31
Runs past season
Why not a main pick?
Local season fit:
redwing usually needs about 63 more GDD than Jackson provides before frost.
Best for: red storage onions.
A strong red onion that makes sense where the season is supportive enough for good bulb sizing.
Tradeoff: Needs a supportive season for good bulb sizing.
Variety class
Typical days to maturity
Typical GDD need
Local fit
Very early
90–95
1100
Workable
Early
95–105
1200
Tight
Mid-season
105–115
1300
Tight
Late
115–120
1400
Poor fit
Main risk: Delays in planting or slower onion varieties can quickly push maturity past fall frost.
How Frost Affects Planting Dates for Onions in Jackson
Jackson usually has about 72 frost-free days, with a typical last spring frost around June 20 and a typical first fall frost around August 31.
Typical last spring frostJune 20
Typical first fall frostAugust 31
Typical frost-free days72
Minimum safe temperature28°F /
-2
°C
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 problem is running short on season. Late planting, slower varieties, and cooler exposed sites can turn a possible crop into a disappointing one.
Onions are closer to the limits of the local season in Jackson before fall frost around August 31, so microclimate plays a bigger role here than it does for easier crops. Local gardens do not all warm and cool at the same pace. 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.
Protect onions with strong starts and steady moisture
The useful setup is about strong early growth, steady moisture, and getting the crop to a clean finish.
Soil and planting setup
For storage crops, the best gains usually come from strong early growth and a clean finish.
