When to Plant Onions in Thompson

Can Onions Mature in Thompson?

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, Thompson typically provides about 1214 growing degree days for onions. With a typical crop target of 1300, that leaves a heat margin of -86. 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.

How Different Onion Varieties Affect Results

In Thompson, 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

Main risk: This is close enough that any delay in planting, or any extra days to maturity, can be the difference between finishing and falling short before frost.

How Frost Affects Planting Dates for Onions in Thompson

Thompson usually has about 75 frost-free days, with a typical last spring frost around June 14 and a typical first fall frost around August 28.

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.

In Thompson, the seasonal margin for onions is tighter before the usual fall frost around August 28, which makes local site warmth more important than it is for easier crops. Season length is often limited by late spring and an early-closing fall window, especially for warm-season crops. The warmest garden spots are usually south-facing walls, raised beds, sheltered backyards, and urban heat pockets. Cooler spots like open windy yards, low frost pockets, and exposed sites that lose heat quickly 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.