How to Grow
Onions and shallots are easy to grow from seeds, plants or sets – small bulbs. Seeds offer the greatest choice of varieties and flexible sowing times but can be more prone to pests and diseases and take longer to grow. Sets and plants are simpler to bring on and less likely to have health issues, and mature earlier. At the same time they offer less of a choice when it comes to varieties. Sets are more likely to bolt -produce flowers instead of bulbs. Shallots grown from sets split to form clumps of bulbs.
Seeds, sets or plants?
Onions and Shallots are raised either from seed, plants or from sets (small bulbs), each has pros and cons:
Onion & shallot sets:
- Advantages: easier to grow, less prone to pests and diseases, matures earlier
- Disadvantages: less choice of variety, more prone to bolting
Onion & shallot seeds:
- Advantages: available for all varieties, less prone to bolting, more flexible sowing times
- Disadvantages: more labour, longer growing season, more susceptible to pests and diseases
Onion & shallot plants:
- Advantages: easier to grow, less prone to pests and diseases and have had a head-start on arrival, meaning a shorter growing season for you
- Disadvantages: Less choice of variety and more expensive
How to grow onions
Where to grow onions and shallots
Plant shallots and onions in an open, sunny site in fertile soil that is free draining and contains enough organic matter to retain moisture. Make sure onions, shallots or garlic haven’t been grown there in the previous three years.
How to prepare the soil for onions and shallots
Plan ahead by digging well-rotted compost into the soil a few months before planting in order to increase soil fertility and structure and improve moisture retention. Onion fertiliser can also be used to provide all the nutrients needed for healthy growth. Apply lime to acid soils in autumn and winter to create a better growing environment. Remove weeds.
When to grow onions and shallots
Plant sets from September to November or in March and April. Put in plants in April. Shallot and onion seeds can be sown in August and September or from January to March. Sow spring onions from March to July or August to October for winter-hardy ones.
How to sow or plant onions and shallots
Sets: push into the soil with the pointed end upwards. Leave 5 to 10cm (2 to 4 inches) between onions and 15 to 20cm (6 to 8 inches) between shallots. Space rows of onions at between 25cm to 30cm (10 inches to one foot) apart and shallots at 30 to 45cm (1ft to 18 inches).
Plants: use a dibber to create a hole for the plants and make sure they sit in the soil no deeper or shallower than they were in their cell tray. Water in.
Seeds: sow indoors in a seed tray in late winter or earlier spring for an earlier start. Prick out and space about 5cm once they get to 1cm in height and reach the ‘crookneck’ stage. Allow young plants to ‘harden off’ – acclimatise to the weather – when they develop two or three leaves and then transfer to eventual growing position in mid to late spring. Alternatively, once the weather is warmer, sow direct in soil that’s been worked to a fine texture. Sow thinly in rows of about 2cm in depth and 30cm apart. Thin seedlings as they grow.
When to water onions and shallots
Water during prolonged dry spells until mid-summer. Be careful not to get water on leaves, which can encourage diseases.
How to protect onions and shallots
Cover emerging onions and shallots with horticultural fleece to stop them from being uprooted by birds. Remove fleece once they’re established. Remove weeds by hand to avoid slicing bulbs with a hoe.
How and when to harvest onions and shallots
Lift shallots and onions with a fork once the leaves have gone yellow, clean off soil and leave in a cool, light and dry place for a couple of weeks to dry the outer skins. Store in a cool place.
Lift autumn-planted shallots and onions in June or July and spring-planted ones from August to September.
Harvest spring onions May to September or winter-hardy ones from February to May.
What to watch for
Onion fly can cause yellowing leaves on established plants, which should be lifted and burned.
Bulb rot causes plants to turn yellow, with leaf tips dying off. Affected plants will have missing roots.
Black blisters on leaves are caused by the fungal disease smut, which leads to plants becoming stunted and distorted.
Yellow shrivelled leaves and soft bulbs is caused by the disease known as shanking.
Onion white rot can cause foliage to wither during dry weather, with the base developing a white fluffy mould.