When to Plant Onions in Amherst

Can Onions Mature in Amherst?

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, Amherst typically provides about 1962 growing degree days for onions. With a typical crop target of 1300, that leaves a heat margin of +662. 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

Most onion varieties can succeed in Amherst 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

Main risk: The most common problems here are practical ones: planting too late, losing momentum early, or choosing varieties that ask for more season than necessary.

How Frost Affects Planting Dates for Onions in Amherst

Amherst usually has about 129 frost-free days, with a typical last spring frost around May 27 and a typical first fall frost around October 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 setbacks here are practical: planting too late, losing momentum early, or choosing varieties that ask for more season than necessary.

In Amherst, onions usually have a solid seasonal margin when planted around May 6. 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.