How to grow onions

Onions are part of the allium group of plants that includes leeks, garlic, spring onions, shallots, red and white onions and the dramatic ornamental alliums for the flower garden too.

Onions can be grown from seeds or sets. Sets are small, immature bulbs that are dormant when you buy them but spring into life once planted. I usually grow from sets but if you have a preference for a certain variety you may need to get seeds.

Grow onions in a sunny bed. They prefer a well drained soil so raised beds are helpful if you have heavy soil. They will also benefit from the addition of well rotted manure or garden compost a month or two before planting. A general purpose fertiliser can also be added according to the makers instruction.

Keep the bed well weeded as onions don’t like to compete for nutrition.

Water if the weather is very dry.

Cut off any flower spikes as soon as you see them. If they are allowed to go to seed the bulbs will stop growing and become tough.

Sow onion seeds

Onions seeds can be sown from January to March. I sow in 6 x 4 cell trays of multipurpose compost. The seeds are quite small but I find them just about big enough to handle individually. I sow three or four per cell and thin down to one per cell when they’re beginning to grow. You can prick out and transplant rather than discard but as the seeds are quite inexpensive and there are plenty in a pack I don’t do this. Place the tray on a warm sunny window sill or a propagator to encourage germination. Keep the soil moist but do not over water. Once germinated remove from the propagator or they will grow too quickly and become weak and leggy. Once established harden the young plants by leaving them outside in a sheltered spot for a few days before planting out in March or April. Space the plants four to six inches apart with twelve inches between rows. Wider spacing should produce larger onions.

Alternatively seeds can be sow direct into a prepared seed bed. Sow thinly, and thin out to the spacings above. Thinnings can be transplanted if desired.

Grow onions from sets

Maincrop onion sets should be planted in March or April. They can be planted direct or started in pots of multipurpose compost. This is my preferred method because the sets often get pulled up by birds. Planting them out once they have an established root system makes it more difficult for the birds to do this. Alternatively, netting can be used to protect them.

Make sure you plant onion sets the right way up with the pointed end upwards and the rounded end in the soil. You should be able to see a tiny root on the rounded end. Plant with care, making a hole with a dibber or trowel to avoid damage. Discard sets that have sprouted as they are likely to go to seed. The tips of the sets should be just about level with the surface of the soil. Spacing, as above, should be four to six inches between plants and twelve inches between rows.

They will be ready to harvest in July or August.

Overwintering onions sets, often known as Japanese onions, should be planted in late autumn or early winter and left to stand over winter. I usually plant my overwintering onion sets in modules in October and plant out in November. Otherwise they should be cared for as maincrop onion sets. Overwintering onions will be ready to crop in June or July, about a month or so before main crop varieties.

Harvest onions

Onions are ready to harvest when the foliage begins to go yellow and fall over. At this point the bulbs stop growing. On a dry, sunny day ease them gently from the soil with a hand fork being careful not to damage the ‘plate’ (the point where the roots join the bulb) as damage can allow disease to enter and rotting to occur. Leave to dry on the ground if possible, otherwise dry in a well ventilated shed. They can also be dried in a greenhouse but caution is required as they may overheat and begin to cook and soften and will not store well. Once the foliage has completely dried they can be trimmed and stored. Check regularly and discard any damaged or diseased onions.

Onion problems

Onion White Rot is a common problem. This is a fungal disease that causes the onions foliage to wilt and a mildew-like growth around the roots of the onions. The bulbs will not keep. There is no known treatment for this disease. It is a soil bourne disease so try not to allow it to spread on tools or boots. It can stay in the soil for many years so don’t plant in the same place again for as long as possible.

Onion Downy Mildew is another fungal disease that affects the leaves and bulbs of onions and tends to stop development giving lower yields. Good spacing and regular weeding to allow good air circulation can reduce the likelihood of the disease which likes damp conditions. Affected foliage can be removed.

Onion Rust or Leek Rust is another fungal disease. It causes yellow spots to appear on the leaves and affects yield. Remove affected foliage and the plants may survive.

Do not compost any diseased material, it should be burned or taken to a municipal recycling centre where the heat created by the volume of composted material will kill the fungus.