Day length decides everything
Onion varieties are classified by the day length that triggers bulbing: long-day (14–16 hours, bred for northern latitudes), intermediate (12–14 hours) and short-day (10–12 hours, bred for southern latitudes).
Plant a long-day onion indoors in winter under 12 hours of light and it will grow leaves indefinitely without ever forming a bulb. Plant a short-day variety under 16 hours and it may bulb before it has enough leaf area to size up, giving you a small onion.
The practical answer for an indoor grower: match the variety to the light schedule you can actually provide, and be prepared to run lights for 14–16 hours if you want onions. This is the one crop where the light timer matters more than the reservoir.
Garlic, and why it is different
Garlic is propagated from cloves, not from true seed, and commercial garlic rarely produces viable seed at all. You plant a clove, it grows a plant, the bulb splits into new cloves — that is the whole cycle.
The complication is vernalisation: garlic needs several weeks of cold (below about 10 °C (50 °F)) to initiate proper bulb formation. Without it you get a single round bulb that never divides. In practice, garlic belongs in soil outdoors over winter, not in a warm indoor reservoir.
If you want an allium in a hydro system, choose onions from seed, leeks, or chives. Chives are the most obliging of the lot — perennial, cut-and-come-again, and forgiving of a small pot.
Feeding: light and steady
Alliums are not heavy feeders. Keep EC low, around 1.2–1.6, and pH 6.0–6.5. Excess nitrogen gives soft, thick-necked onions that store badly and bolt in their first year.
For bulb onions, reduce nitrogen and let the EC drift down as bulbing begins. For leeks and chives, which are grown for leaf, a steady moderate feed throughout is right.
Alliums are shallow-rooted and in a net pot that means the root mass is compact. Do not let the water level drop far — they have less buffering than a tomato.
Saving seed from alliums
Onions and leeks are biennial: grow the bulb or plant in year one, overwinter it, and it flowers in year two. The flower head is a globe of small white or purple flowers on a tall hollow stem, and it is spectacular.
They are insect-pollinated and cross readily — with other varieties of the same species, and onions will also cross with shallots. Isolation of 800 metres to a mile is the textbook figure; in practice, grow one variety for seed per year, or bag the flower head and hand-pollinate.
Chives and garlic chives are the exception: they set viable seed readily in the same season and self-sow enthusiastically if you let them.
What goes wrong
No bulb forms. Day length mismatch, nearly always. Nothing else produces this symptom.
Bolting in year one. A cold snap after planting, or a plant that was too large when transplanted. Onion sets and seedlings bolt if they experience a chill above a certain size.
Thick neck that will not dry down. Too much nitrogen late in the season, or harvesting before the tops have fallen naturally.
White rot. A soil-borne fungus; rare in hydro, and one of the genuine advantages of growing this way.