Last edited on 28 Oct 2026. View history

red onion Red Onion

Allium cepa WD

A variety of onion

How to sow red onion: Direct seed indoors or plant sets (small bulbs) outside.

Sun requirement for red onion: Plant in Full Sun

Red onions are onion cultivars that have purplish red skin and white flesh tinged with red. They are members of the Allium family along with garlic and leeks. They have a sweet to mild flavor and can be eaten raw (in salads and guacamole), grilled, sautéed, or baked. Make sure to choose a variety suited to your day length.

6
plantings
2
harvests
1
seed packet
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Red onion facts

Lifespan

Annual living and reproducing in a single year or less
42 weeks median lifespan

First harvest expected

40 weeks after planting

Height × spread

60cm × 13cm

Row Spacing

10cm

Grown for

bulb harvested 1 time
What are growing degree days?

A plant grows with warmth, not with days. A growing degree day is one day's worth of useful warmth: how many degrees the day's average temperature is above the lowest temperature the crop grows at. Add them up from planting and you know how far along the crop is, whatever the calendar says.

Say that lowest temperature is 10°C. A day averaging 17°C gives 7 growing degree days. A day averaging 8°C gives none, as it is too cold for the crop to grow. That is why the same lettuce takes about 50 days in a summer and 90 in an autumn: it needs the same total warmth either way.

For red onion, the FAO EcoCrop database gives a lowest growing temperature of 4°C and a shortest growing cycle of 85 days at its best temperature, about 1233 growing degree days to a first harvest.

Where you've told us where your garden is, we use this and the weather there to predict when your red onion will be ready to harvest. We don't use it for how long a plant lives or when it finishes: we go by what members have logged for that.

Growing red onions

  1. Sow seeds in trays 8 weeks before planting out
  2. Dig compost into the bed 1 week before planting out
  3. Water regularly at planting out every week
  4. Weed around the plants 2 weeks after planting out every 2 weeks
  5. Feed with a nitrogen fertiliser 4 weeks after planting out every month
  6. Stop watering as the tops fall over 1 week before first harvest
  7. Lift the bulbs at first harvest
  8. Dry the bulbs in an airy place 1 week after first harvest

How members grow red onions

Sunniness

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Planted from

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Harvested for

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Varieties of red onion

Image of California red onion

California red onion

Allium cepa
Image of red Burgundy Onion

red Burgundy Onion

Photos

Red onion plantings

red onion
red onion
by shiny
20201025_183252
20201025_183252
by cas3y
DSC_6843
DSC_6843
by shiny
red onion
red onion
by shiny
2020-03-23_08-25-32
2020-03-23_08-25-32
by shiny
More photos

Video

Where red onions grow

Only plantings by members who have set their locations are shown.

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Red onion harvests

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Nutritional data

A summary of nutritional data per 100g for Onion, mature, red skinned, raw.

Energy 113.0 kJ
Protein 0.8 g
Carbohydrate 5.5 g
- Sugars 5.5 g
Fibre 1.4 g
Sodium 5.0 mg
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Ecological requirements

Data from the FAO EcoCrop database for Allium cepa L.

Family / Taxonomy Liliopsida
Liliidae
Asparagales
Alliaceae
Optimal temperature 12 - 25 °C
Absolute temperature 4 - 30 °C
Optimal rainfall 350 - 600 mm/year
Absolute rainfall 300 - 2800 mm/year
Optimal soil pH 6.0 - 7.0
Absolute soil pH 4.3 - 8.3
Crop cycle 85 - 175 days
Max altitude 2000 m
Life form herb
Life span biennial
Habit erect
Optimal soil depth medium (50-150 cm)
Optimal soil texture medium, organic
Optimal soil fertility moderate
Optimal soil drainage well (dry spells)
Optimal soil salinity low (<4 dS/m)
Photoperiod short day (<12 hours), neutral day (12-14 hours), long day (>14 hours)
Optimal light clear skies

Licensed under CC-BY-4.0 .

Food and Agriculture Organization of the United Nations (2025) ECOCROP, the crop ecological requirements database. Version 1. https://data.apps.fao.org/catalog/dataset/ecocrop. Accessed through the CGIAR Climate Action Data Hub, https://cgiar-climate-data-hub.github.io/catalog/ecocrop_1/.

Learn more about red onions

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