Zone 10 · Alliums Growing Guide

When to Plant Onion
in Zone 10

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
Oct 15 – Jan 15
Start indoors
Direct sow
Last Frost
~Jan 15
Days to harvest
90–120 days
Difficulty
Medium
Late season — Zone 10
Harvest window: Feb – Apr

Planting Calendar

In Zone 10, Onion should be planted outdoors Oct 15 – Jan 15, after the average last frost date of approximately Jan 15.

Short-day only. Fall and winter planting season.

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 10

Short-day only. Fall and winter planting season.

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 10, onion is a fall-planted crop. Plant outdoors Oct 15 – Jan 15 once temperatures cool in autumn. It overwinters and is ready to harvest around Feb – Apr. No last frost consideration — plant before the ground freezes.
Zone 10 has an average last spring frost of approximately Jan 15 and a first fall frost around Dec 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 10. Expected harvest window: Feb – Apr.

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Onion in other zones
Crop Guides