Zone 6 · Alliums Growing Guide

When to Plant Onion
in Zone 6

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
Mar 15 – Apr 20
Start indoors
Jan 5 – Jan 20
Last Frost
~Apr 1
Days to harvest
90–120 days
Difficulty
Medium
Growing — Zone 6
Harvest window: Jun – Aug

Planting Calendar

In Zone 6, Onion should be planted outdoors Mar 15 – Apr 20, after the average last frost date of approximately Apr 1. Start seeds indoors around Jan 5 – Jan 20 for a head start.

Transition zone — intermediate-day varieties work well. Sets can be planted early March.

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 6

Transition zone — intermediate-day varieties work well. Sets can be planted early March.

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 6, start onion seeds indoors around Jan 5 – Jan 20, then transplant outdoors Mar 15 – Apr 20 after your last frost around Apr 1. Transition zone — intermediate-day varieties work well. Sets can be planted early March.
Zone 6 has an average last spring frost of approximately Apr 1 and a first fall frost around Oct 31. 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 6. Expected harvest window: Jun – Aug.

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Onion in other zones
Crop Guides