How to Grow Onions from Sets
Bear in mind that each set (mini onion bulb) will swell and grow into a bulb up to the size of a tennis ball, so you need to leave plenty of room between each one. Plant them with 10cm (4inches) or more between each bulb and when growing in rows leave at least 30cm (12inches) between the rows.
Gently push each mini onion bulb into the prepared soil until just the very tip is protruding, leave at least 10cm (4inches) between each bulb. Plant them in blocks or rows for convenience.
Weed between the onions and keep the soil moist until they have established. Onions are shallow rooted, so take care when weeding.
In mid to late summer when the leaves have started to yellow and die off, place a fork under the swollen bulbs and lift them out so that the sun can ripen the base of the bulbs. When the foliage has died off completely string them up for winter use.