Plant care
Onions (bulb onion) care
Allium cepa
Also called bulb onion, common onion, salad onion (green type).
Light
Onions is a sun-lover and needs the brightest spot in the home to thrive. 6+ hours of direct sun. Indoors that almost always means a south or west-facing windowsill in the northern hemisphere. Plants moved abruptly from low light to direct sun will scorch — acclimate them over 7-10 days by giving a little more sun each day.
Watering
Outdoor onions crops want 2-3 cm per week, more during bulb sizing. The single best habit is a finger-test before watering — push a finger 3-4 cm into the soil. If it comes back damp, wait a day. If it comes back dust-dry, water deeply at the base of the plant. Consistent moisture during the spring growth phase produces big bulbs. Reduce water in the last few weeks before harvest.
Soil and pot
Onions grows best in rich, well-drained loam. Compost-rich; pH 6.0-7.0. Heavy clays produce misshapen bulbs. A pot with a working drainage hole is non-negotiable for this species — even free-draining mix will turn soggy in a closed planter. If you love the look of a decorative pot without a hole, use it as a cachepot around an inner nursery pot you can lift out to water.
Humidity and temperature
Onions sits happiest at around 40-70% (outdoor) humidity and 10-24°C (50-75°F). Outdoor humidity rarely matters. If you keep the room above 10 year-round and avoid placing the plant near a cold draught, a hot radiator, or an air-conditioning vent, you have already handled the two biggest indoor stressors.
Fertilising
Feed onions sparingly. A balanced feed at planting; side-dress with nitrogen once leaves are growing strongly. Skip fertiliser entirely on a stressed, recently-repotted, or actively wilting plant — fertiliser salts make damage worse, not better. Wait for a round of healthy new growth before resuming a feeding rhythm.
Common problems
Below are the issues we see most often on onions in the Growli community. Each is annotated with the most common cause so you know where to start.
- Bolting to flower — Cold spell after planting triggers premature flowering; use heat-treated sets.
- Small bulbs — Wrong day-length variety, planted too late, or under-watered during bulb formation.
- Soft rot at the neck — Wet conditions at harvest; dry bulbs thoroughly before storage.
- Downy mildew — Wet leaves and humid air; rotate planting site and improve drainage.
- Onion thrips — Silvery streaks on leaves; tolerable in small numbers, treat with horticultural soap if heavy.
Companion plants
Onions pairs well with Carrot, Tomato, Lettuce, and Strawberry. These are species with similar light and water needs, so you can grow them in the same bed or container without conflict.
Propagation
Sow seed in late winter under cover, or plant sets in spring. Heat-treated sets resist bolting. Propagation is the cheapest, most satisfying way to expand a collection — and it doubles as insurance against losing a mature plant to an accident. Take a backup cutting once the parent is established and healthy.
Toxicity to pets
Onions is toxic to pets. ASPCA lists Allium cepa as toxic to cats, dogs, and especially horses due to N-propyl disulfide, which causes haemolytic anaemia. Both raw and cooked onion are toxic. Garlic, leeks, and chives carry the same risk. If you keep cats, dogs, or curious children in the house, weigh placement carefully — a high shelf or a hanging planter is enough for casual safety. For severe ingestion incidents, call your local vet and the ASPCA Animal Poison Control Center (in the US, 888-426-4435).
Pet-safety status is sourced from the ASPCA Toxic and Non-Toxic Plant List, which catalogues the most-asked-about plants for cats, dogs, and horses.
Onions care — frequently asked questions
What is the common name for Allium cepa?
Allium cepa is most commonly called Onions, but it is also known as bulb onion, common onion, salad onion (green type). The names refer to the same species, so care instructions for Onions apply identically to anything sold as bulb onion.
How much light does onions need?
Onions grows best in direct sun (at least 4-6 hours). 6+ hours of direct sun.
How often should I water onions?
Water onions 2-3 cm per week, more during bulb sizing. Consistent moisture during the spring growth phase produces big bulbs. Reduce water in the last few weeks before harvest. The finger-test (or lifting the pot to feel its weight) beats a fixed weekly calendar because pot size, light, and season all change how fast the soil dries.
Is onions toxic to cats and dogs?
Onions is toxic to pets. ASPCA lists Allium cepa as toxic to cats, dogs, and especially horses due to N-propyl disulfide, which causes haemolytic anaemia. Both raw and cooked onion are toxic. Garlic, leeks, and chives carry the same risk.
What USDA hardiness zone does onions grow in?
Onions is rated for USDA zone Grown as an annual in zones 3-11 and RHS hardiness H5 (overwintered types). Outside that range, grow it as a container plant that overwinters indoors before the first hard frost.
Onions deep-dive guides
Every aspect of onions care, each with its own calibrated guide:
- Onions watering schedule
- Onions light requirements
- Best soil mix for onions
- Onions fertilizing guide
- When to repot onions
- How to propagate onions
- Onions growth rate & size
- Onions cold hardiness
- Onions temperature & humidity
- Is onions toxic to cats & dogs?
Related guides
Onions is also known as bulb onion, common onion, and salad onion (green type).