When to Plant Onions in Anchorage

Can Onions Mature in Anchorage?

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, Anchorage typically provides about 1396 growing degree days for onions. With a typical crop target of 1300, that leaves a heat margin of +96. That heat margin usually gives the crop enough room to finish, but not so much that delays stop mattering. Timing and variety choice still affect how comfortably the crop fits.

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. It is most useful for judging how much flexibility you still have before the crop starts losing margin.

How Different Onion Varieties Affect Results

In Anchorage, very early to mid-season onion varieties are usually the best fit in a typical year. Slower choices can still work when gardeners want their specific qualities and do not give away margin through delay.

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

Main risk: The usual risk here is losing time early, since delayed planting or cool starts can slow maturity for longer-season onion varieties.

How Frost Affects Planting Dates for Onions in Anchorage

Anchorage usually has about 151 frost-free days, with a typical last spring frost around May 1 and a typical first fall frost around September 29.

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 usual trouble comes from delayed planting or from choosing slower varieties when the local season would reward simpler, faster choices.

In Anchorage, onions usually have enough season to work well, but site warmth still affects how comfortably they finish before the usual fall frost around September 29. 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.