Using Onions sets is easier for the novice gardener than growing from seed also there are many experienced growers who also prefer growing onions from sets because they are far less fuss. In general, they are easier because there is no transplanting or thinning required and they are more disease resistant, also because the sets are miniature bulbs they have more stored energy than a seed and so they can get off to a quicker start. However, many varieties are not available as sets, and seed grown onions do seem to store better and longer once harvested.
Onion sets are small onions that have had their growth arrested the previous autumn and in general, come from sub-tropical climates ready for planting out in temperate climates in mid-spring.
Japanese onion sets are planted during September or October, you may even get away with planting through to December (this is being based in the UK). Japanese onions are ready to harvest a couple of months before spring-sown onions and should provide a crop of onions to fill the gap between the last of the store onions from the previous year and the start of the new autumn crop. To grow Japanese onions follow the same instructions as for maincrop onions but the spacing between the plants can be smaller.
How to Space out Onions
Plant out the bulbs in rows 20-30cm (8”-12”) apart with 10-15cm (4”-6”) between them. The closer the spacing the smaller the bulb remembering that smaller bulbs tend to have more flavour. Unless you are growing for the giant classes at exhibitions etc. there is no point in spacing more than 15cm (6”) apart. In my personal experience, the smaller sets in the pack are less likely to bolt than the bigger ones.
Many of the old gardening books will tell you to firm the ground before planting and then to push the bulb into the ground so that the “neck” at the top of the set is just visible above ground. However doing this, especially on heavy clay-based soil, can damage the bulb and kill it. It is therefore, better to plant in a drill (a shallow furrow) and then to backfill the soil slightly firming it around the bulbs.
It used to be thought that sets had a tendency to rise out of the ground and to “travel” a short distance from where they had been planted, however, this is much more likely to be caused by birds picking at the necks thinking that they are worms, therefore providing some form of protection such as netting or placing cloches over the sets is needed. Gently replace any that have been disturbed.
The other big problem with sets is bolting. Bolting is the onion plant suddenly sending up a strong thick central stalk that carries a seed head. These stalks should be removed as soon as possible and these onions should be the first to be used as they will not store well. The heat treatment of sets has gone some way to preventing this problem, but these sets do cost more.