When to Plant Onions in Great Falls

Can Onions Mature in Great Falls?

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, Great Falls typically provides about 2185 growing degree days for onions. With a typical crop target of 1300, that leaves a heat margin of +885. That heat margin usually gives the crop a dependable buffer, so gardeners have some flexibility in planting date and variety choice without pushing the crop close to the edge.

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

The season in Great Falls usually supports most onion varieties comfortably, which means the more useful decision is what kind of crop you want rather than simply how fast it finishes.

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: When this crop underperforms in Great Falls, the culprit is usually timing or variety choice rather than the climate itself.

How Frost Affects Planting Dates for Onions in Great Falls

Great Falls usually has about 148 frost-free days, with a typical last spring frost around May 6 and a typical first fall frost around October 1.

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.

When this crop underperforms in Great Falls, the culprit is usually timing or variety choice rather than the climate itself.

In Great Falls, the local season usually gives onions plenty of breathing room when planting happens around April 15. For a better local margin, gardeners usually do best in south-facing walls, sheltered gardens, raised beds, and sunnier urban lots. Cooler spots like low spots, exposed sites, and shadier yards often make timing tighter. For onions, the best local sites often help the crop get moving earlier and make timing a little more forgiving.