Red onion

Red onion: plant 10 cm (3.9 in) apart in the row and 30 cm (12 in) between rows, 16 per square. Good neighbours: Carrot.

Key facts

Spacing in the row	10 cm (3.9 in)
Between rows	30 cm (12 in)
In a 30 cm (1 ft) square	16 per square
Sun	Full sun, 6–8 hours a day
Root depth	25 cm (10 in)
Height when grown	40 cm (16 in)
Crop rotation group	Roots and onions
Support	None needed

Sowing and germination

How it is started	From bulbs or sets, started in trays
Seeds per hole	1
Plants kept after thinning	1
Seeds per metre of row	10
Sowing depth	1.5 cm (0.6 in)
Soil temperature	from 0 °C, best 18–29 °C
Days to emerge	4–7 days
Light while germinating	Light makes no difference
Before sowing	nothing needed
When to plant	Indoors 8 weeks before the last frost; outdoors before the last frost; soil at least 7 °C

From seedling to plant

After emergence	13–20 °C; take the cover off as soon as they show
Pricking out	sown where it grows
Takes transplanting	Easily
Hardening off	5–7 days
Life cycle	Biennial, harvested in its first year
Cold tolerance	Takes a light frost → hardy, takes hard frost
Soil	Ordinary garden soil, free draining, pH 6–7
In a pot	8 L, 25 cm (9.8 in)
Watering	Water when the soil is dry two fingers down; while bulbs form
Feeding	Moderate; at planting, while growing
Support and training	None needed

Harvest and planning

Days to first harvest	100–130 days, from planting
Ready when	The tops bend over and dry
Harvest window	All at once, 2–3 weeks
Typical yield	3–5 kg per m²
Botanical family	Amaryllidaceae
Watch out for	Onion fly, downy mildew

Good neighbours

Carrot

Why, with the sources →

Plan Red onion in your garden

Open the free planner, drop Red onion onto a bed and see how many plants fit.

Start Planning Your Garden

Sowing and harvest

Plant out	Mar–Apr
Harvest	Jul–Aug

My dates for this crop

My country Last spring frost where I live

Give the date once and this page, the sowing calendar and the planner all follow it.

Months for a temperate climate: about a month later in the north and a month earlier in the south. Give your own last frost date for exact dates.

What to plant next to Red onion →

From the same family

Plants of the Amaryllidaceae family share diseases and pests, so they follow each other in the rotation only after a break.

What to plant after Red onion

Crops that still have time to grow on the same spot once it is lifted, from another rotation group.

Sources

The values on this page marked as checked come from these sources: spacing, sowing depth, germination, soil, timing and safety notes, each against the source named here.

Onions, Garlic and Shallots (426-411) — Virginia Cooperative Extension, Virginia Tech for: between rows, spacing in the row; checked 2026-10-30
Soil Temperature Conditions for Vegetable Seed Germination (GN 154, after J.F. Harrington, UC Davis) — UC Cooperative Extension, UC ANR for: days to emerge, soil temperature; checked 2026-10-30
Onions and related crops: growing tips — UMass Amherst Extension for: between rows; checked 2026-10-30
People Foods to Avoid Feeding Your Pets — ASPCA Animal Poison Control Center for: safety; checked 2026-10-28

More in Onion family

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