When you receive your onion sets unpack them and put them into a cool, light, well-ventilated and frost free place, away from direct sunlight until you are ready to plant them.
Heat-treated sets (which have had their flowering potential suppressed, so are bolt resistant) should not be planted before late March or April. Otherwise plant between February and April, as soon as the soil is sufficiently dry and warm; in practice this is usually late winter or early spring or sandy soils, and mid-spring for clay-based oils.
Prepare the soil by digging over and incorporating some general-purpose fertilizer, work the soil down to a fine tilth as if preparing a seed bed.
Onion sets are planted into a shallow drill (groove) in the soil, created with a string line and a draw hoe, or by laying a plank across the bed and running a trowel along its edge. The drill should be about the same depth as the onion sets. Remove any loose papery skins before planting the onion sets. Push the sets into the soil at the base of the drill, with their pointed tips upwards.
Spacing can be anything from 5cm (2 in) to 15cm (6in) apart. You can plant 6" (15cm) apart each way or spaced at 4" (10cm) in rows 8"-12" (20-30cm) apart. Closer spacing produces smaller bulbs but there is no point going over 6" (15cm) apart unless you are trying to grow giants. Fill in the drill with soil by running the edge of the rake along its edgeto draw soil over the sets, or use the trowel in a similar fashion. Use the trowel to firm in the sets. When they are planted, the tips at least should still be protruding from the soil surface.
(It has been found that large sets are more prone to bolting so do not discard small sets in the pack in favour of them.)
In the spring there is rarely the need to water newly planted sets. Keep weeds checked as dense weed growth will seriously affect yield. Water if the weather is dry (not otherwise) and feed occasionally.
Onions form a bulb when the temperature and the number of daylight hours hit the right combination for them which triggers their clock. Until that happens, onions use the daylight to produce a good deal of top growth before they form
bulbs (and the more top growth, the bigger the bulb). When the day reaches the right number of hours for that variety of onion, the onion will stop forming top growth, and form a bulb instead. The size of the bulb that eventually forms depends on the size of the "stalks", and the number of them. There will be 1 ring in the onion for every stalk that formed, and the larger the stalk, the larger each ring will be. Bulb formation will pause though during dry, very hot or very cold weather.
Break off any flower stems which appear. Mulching is useful for cutting down watering and for suppressing weeds. Stop watering once the onions have swollen and pull back the covering earth or mulch to expose the bulb surface to the sun.
When the bulb is mature the foliage turns yellow and topples over. Leave them for two weeks and then carefully lift with a fork on a dry day. Onions which are not for immediate use must be dried. Spread out the bulbs on sacking or in trays; outdoors if the weather is warm and sunny or indoors if the weather is wet.
Drying will take 7 to 21 days, depending on the size of the bulbs and air temperature. Inspect the bulbs carefully: all soft, spotted and thick-necked onions should be set aside for kitchen use or freezing. The rest can be stored. Store in trays, net bags etc; anything where the air can circulate. Choose a cool and well-lit place to store them where they will keep until late spring.
This article is reproduced from information in the leaflet supplied with onion sets from D T Brown Seeds. Their website can be found here