Plant onions about 2 weeks before your last frost, 4 inches apart in rows 12 inches apart, and harvest in 90 to 120 days when the tops fall over. Good companions are carrots, beets and lettuce; keep peas and beans away. Day length, not soil, is what decides bulb size.
Day length decides your crop
This is the thing most onion advice leaves out, and it is why most onion crops disappoint.
Onions grow leaves first and then, at a trigger, stop making leaves and start filling a bulb. The trigger is day length, not temperature or age. Every leaf the plant has made by that moment becomes one ring of the bulb — so a plant with few leaves when the trigger arrives makes a small onion, permanently, however well you treat it afterwards.
Varieties are sorted by the day length that triggers them:
Short-day onions bulb at about 10 to 12 hours of daylight. Grown in the north, where days lengthen past that very early, they bulb while the plant is still tiny and give small onions.
Long-day onions need about 14 to 16 hours. Grown in the far south, where days never reach that, they may never bulb properly at all.
Day-neutral or intermediate varieties bulb across a wider band and are the safe choice if you are unsure.
The practical rule: the further north you are, the longer the day length your variety should need. Buying whatever the shop has is the single commonest cause of a disappointing onion crop, and no amount of feeding corrects it.
Onion fertilizer: what and when
Onions want nitrogen early and none late, and the timing follows directly from how they grow.
Through the leafy stage, nitrogen is building the leaves that become bulb rings, so this is when feeding pays. Side-dress every few weeks from establishment until bulbing starts.
Once bulbing begins — when you see the base start to swell — stop the nitrogen. Feeding past that point gives soft, thick-necked bulbs that rot in storage rather than bigger ones.
Phosphorus at planting helps the young root system, and potassium supports storage quality. What the three numbers on fertilizer mean covers reading a bag properly, and the fertilizer calculator converts an analysis into an amount for your bed. For organic sources and how fast each releases, how to add nitrogen to soil has the comparison.
Onion companion plants
Good neighbours, and why.
Carrots are the classic pairing, and the claimed mechanism runs both ways: onion scent is said to confuse carrot root fly, and carrot scent to confuse onion fly. The evidence is mixed, but the pairing works practically regardless — both are narrow, upright crops that interleave well in a row without shading each other.
Beets and lettuce are simple space companions. Onions cast almost no shade and occupy a narrow footprint, so low leafy crops use the ground between rows productively.
Bad neighbours, and why.
Peas and beans. The traditional exclusion, widely reported and not firmly explained. It is worth following on the balance of experience even though the mechanism is unclear — whether companion planting actually works covers how to weigh claims like this one.
Asparagus. A clearer case, and it is competition rather than chemistry: both are long-season crops in the same root zone, and an asparagus bed must not be disturbed by lifting onions.
Spacing and when to plant
4 inches between plants, 12 inches between rows.
That is close, and deliberately so: onions have narrow, upright foliage and tolerate crowding. Spacing them wider does not give bigger bulbs — day length does that. Use the plant spacing calculator to work out how many a bed takes.
Plant about 2 weeks before your last frost. Onions are hardy and want a long leafy run before the day length trigger arrives, so early planting means more leaves and a bigger bulb.
Autumn planting is the other approach: overwintering varieties go in in autumn, sit through the cold, and bulb much earlier the following summer. They do not store as well as spring-planted onions but they fill the early summer gap.
Sets or seed
Sets are small immature bulbs from the previous year. They are easier, faster by a month or more, and more forgiving of cold soil. They are also more prone to bolting if the set is large or gets a cold check.
Seed gives a wider variety choice and better storage, at the cost of a longer season and an indoor start about 12 weeks before the last frost.
Most gardeners should use sets, and choose small ones — sets much over half an inch across bolt more readily.
Growing in containers
Onions grow well in containers because they root shallowly. Allow 8 inches of depth and space plants at the same 4 inches. A wide, shallow trough suits them better than a deep pot.
Spring onions are even easier and can be grown at 1 inch spacing in a window box.
Watering
About an inch a week, steadily, through the leafy stage. Onions are shallow-rooted and dry out faster than they look.
Stop watering once the tops start to yellow and fall. The bulb is finishing and wet soil at that stage causes rot in storage.
Harvest and curing
The crop tells you when. When about half the tops have flopped over naturally, stop watering and lift within a week or two. Do not bend the tops over yourself — the old advice to do so shortens storage life.
Lift on a dry day and cure them: spread in a single layer somewhere warm, dry and airy for two to three weeks until the necks are papery and completely dry. A properly cured onion with a tight neck stores for months; one lifted and bagged straight away does not.
Regrowing green onions
The white root ends of spring onions regrow readily. Stand them in an inch of water on a windowsill and shoots appear within days. Change the water every couple of days.
Expect two or three useful cuttings before the flavour thins. Potting them into soil rather than leaving them in water keeps them productive considerably longer.
Common problems
Small bulbs. Day length mismatch, or too few leaves before bulbing. Choose the right variety type and plant early.
Bolting. A flower stalk means the bulb stops growing and will not store. Caused by oversized sets or a cold check after planting. Use small sets, and use any bolted onion immediately.
Thick necks that rot in storage. Too much nitrogen late, or lifting before the tops fell.
Onion fly maggots. Row cover at planting is the practical defence; rotation helps over time.
Onions interleave well with carrots and should be kept out of any bed holding peas.