Zone 3 · Alliums Growing Guide

When to Plant Onion
in Zone 3

Plant from seeds (start very early), sets, or transplants. Choose varieties for your day-length: long-day onions for zones 5+, short-day for zones 7 and south. Day-length matters more than anything.

Plant outdoors
May 1 – May 20
Start indoors
Feb 15 – Mar 1
Last Frost
~May 15
Days to harvest
90–120 days
Difficulty
Medium
Start indoors — Zone 3
Harvest window: Aug – Sep

Planting Calendar

In Zone 3, Onion should be planted outdoors May 1 – May 20, after the average last frost date of approximately May 15. Start seeds indoors around Feb 15 – Mar 1 for a head start.

Start from seed indoors 10–12 weeks early. Long-day varieties only. Choose fast-maturing types.

Care Guide

Sunlight
Full sun · 6–8 hrs/day
Watering
Moderate
Spacing
4–6 inches
Soil
Loose, well-draining, fertile
Days to maturity
90–120 days
Soil pH
6.0–7.0
plant tip · Zone 3

Start from seed indoors 10–12 weeks early. Long-day varieties only. Choose fast-maturing types.

Feeding Schedule

How often
monthly
Feed type
high nitrogen then high potassium
Key timing
switch to high-potassium 6 weeks before harvest

Feed nitrogen monthly until bulbs start to form, then switch to high-potassium feed to encourage bulb sizing.

Companion Planting for Onion

What you plant next to onion makes a real difference — some plants actively help, others compete or attract pests.

Grows well with
  • Carrots
  • Brassicas
  • Tomatoes
  • Beets
Keep away from
  • Beans
  • Peas
  • Asparagus

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Common questions

In Zone 3, start onion seeds indoors around Feb 15 – Mar 1, then transplant outdoors May 1 – May 20 after your last frost around May 15. Start from seed indoors 10–12 weeks early. Long-day varieties only. Choose fast-maturing types.
Zone 3 has an average last spring frost of approximately May 15 and a first fall frost around Sep 15. These dates vary by specific location — use your ZIP code in the plant app for precision based on the nearest NOAA weather station.
Good companion crops for onion include Carrots, Brassicas, Tomatoes, Beets. Avoid planting near Beans, Peas, Asparagus, which can compete for nutrients or attract pests.
Onion typically takes 90–120 days from transplant or direct sow to harvest in Zone 3. Expected harvest window: Aug – Sep.

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Onion in other zones
Crop Guides