Bulbing onions – and by association, shallots – are not only versatile, but are also indispensable to any cook who cares. They are clear-cut candidates for the Desert Island vegetable, and if dried and stored properly, will be on hand for eating right though the winter and into the spring. What more could you ask for?

Many vegetable gardeners prefer growing onions and shallots from sets rather than from seed. They claim that sets are easier to use than seeds, and to some extent this is true. Once a system is devised, however, propagating from seed is almost as easy as growing from sets. Besides, seeds are cheaper than sets, and the choice of varieties available as seeds – for onions, anyway – far exceeds those available as sets. All things considered, seeds win over sets, hands down.

Direct sowing into the garden

Both onion and shallot seeds can be sown outdoors, directly into the garden, from March to early April, though shallots can be sown as late as mid April. For the earliest sowings, warm up the soil by covering it with cloches or a sheet of clear polythene. The soil should be fertile, well-drained and worked into a fine tilth. The seeds should be sown about 10mm deep. They are slow to germinate and seedling growth is weak, so be sure to keep weeds under control.

• Onions: Sow seeds thinly in rows about 30cm apart. Once the seedlings are up, thin to leave about 6cm between them. This gives medium-sized bulbs ideal for the kitchen. A wider spacing, up to 10cm, will produce larger bulbs.

• Shallots: To produce single bulbs shallots are grown at higher populations than onions, so leave 15cm between rows and 2.5cm between plants within a row. A wider spacing between individual plants encourages the bulbs to ‘split’ into clusters, which is not what is wanted from shallots grown from seed.

Growing from transplants

Gardeners with a greenhouse or tunnel can produce onions and shallots from their own-grown transplants. This is an effective and relatively easy option that saves on seed and reduces the need for weeding.

The requirements for transplant production are relatively modest: modular trays divided into cells (those about 35–45mm across will do) and multi-purpose compost. Fill the trays with the compost and sow about 6 seeds per cell. If the seed is of good quality, this should give about 4 or 5 seedlings in each cell. Sow seeds at the following times:

• For onions: from the end of February to the beginning of April. Sowings can also be made as early as January, though extra heating might be necessary – aim for about 10–15ºC to help germination.

• For shallots: between the middle of March to the middle of April.

Both onion and shallot seed don’t germinate well at high temperatures, and later sowings under cover may overheat during hot, sunny days. If this happens, move the trays outside where it is cooler. Because of the small amount of compost in each of the cells, nutrient levels will run down before the seedlings are ready for transplanting. A shot of liquid fertiliser four weeks or so after sowing will keep growth moving along.

Onions and shallots are ready for transplanting when they are about 15cm tall – depending on the weather, this could take up to 8 weeks. Before transplanting, harden off the plants by moving the trays outdoors during the day for 4 or 5 days.

To transplant, carefully remove each clump of compost and the attendant seedlings from the cells, and plant directly into well-prepared ground. Don’t separate the individual plants – they will find their own space as they grow to full size. When transplanting onions, leave 30cm between rows and 25–30cm between clumps in a row. Shallots should be transplanted about 15–20cm apart each way between clumps.

In lieu of modular trays, seed can be scattered into compost-filled pots. When the plants are big enough, break off clumps of 4 or 5 plants and transplant them in the same way as those grown in cells.

To help with weed control, onions and shallots can be transplanted through holes cut into a black plastic mulch. The holes have to be weeded only 2 or 3 times before the bulbs get big enough to force out the weeds.

Harvesting, drying and storing

Onions and shallots are ready to harvest when the leaves on the majority of plants have fallen over. At this point – usually in August or September – the bulbs should be lifted from the soil with a fork and dried.

Drying can be done outdoors if it is warm and the bulbs are protected from rain and dew. Lay them out for a week or more on a sheet of plastic or upturned boxes, turning occasionally to expose all the surfaces to the sun. Alternatively, the onions can be left in a greenhouse or tunnel, where it is more likely to be drier and warmer. Drying is complete when the necks have shrivelled, the roots have shrunken and turned brown, and the leaves crinkle like paper. Occasionally, a few thick-necked bulbs will not dry out completely, and these should be eaten immediately since they won’t keep.

Store onions and shallots under cover – a shed or garage will do, especially if it is draughty. The bulbs can be plaited and hung up, or laid out in a double layer on a slatted tray. They have a low freezing point and so will tolerate some frost. New leaves sprouting from a bulb spells the beginning of the end, and bulbs should be eaten as quickly as possible once this has started. Storage life varies from variety to variety: Long Red Florence has one of the shortest, and won’t last much into December, while shallots can keep into the early summer.

Pests and diseases

Onions and shallots can be attacked by numerous pests and diseases, though in reality most gardeners are unlikely to have serious problems with most of them. For some of the more common ones, see the table.

Chemical controls are available to help produce a problem-free crop, but there are also preventative strategies that can be effectively employed. For example, crop rotation is important, and members of the allium family (leeks, onions and shallots) should not be grown on the same piece of ground for more than one year in four (or longer, if possible). Good soil drainage should also be promoted, either by adding organic matter or growing on raised beds.

Problem Symptom Preventative measure
Onion fly

Maggots bore into underground part of seedlings

• Cover with fleece or fine net

• Use transplants

Downy mildew Pale oval blotches on leaves; leaves die from the tips downwards. Infected bulbs will not store

• Use wide row spacings to promote ventilation

• Destroy infected plants

• Do not replant in same spot for five to six years

White rot Stunted plants; yellow and wilted leaves; white mould on stems and bulbs

• Remove and destroy infected plants (which must not be composted)

• Do not replant in same spot for 18 years

Neck rot

Attacks onions in store. Grey mould on neck, with brown rot spreading through bulb

• Make sure bulbs are undamaged and thoroughly dry before storing

• Remove diseased bulbs from store