Site and soil
Choose an open, sunny site and well-drained soil. High humidity around the foliage and wet soils make the crop more prone to disease attack. Onions do not thrive on acid soils (below pH 6.5). Reduce acidity by applying lime in autumn and winter.
Onions have a limited root system therefore improving the soil with plentiful organic matter is invaluable. Apply two bucketfuls of well-rotted manure or other organic matter such as garden compost every square metre (yard). This will help add nutrients, improve the soil structure and hold moisture. Avoid using fresh manure.
Little fertiliser is required at planting, apply 35g per sq m (1oz per sq yd) of Growmore or twice as much organic fertiliser such as dried poultry manure pellets. If you cannot apply organic matter – use more fertiliser, up to twice as much if the soil is poor.
Sowing and planting
There are two main ways to raise onions and shallots;
- For convenience most gardeners choose to grow them from sets (immature onion or shallot, grown for planting)
- Although more challenging, it is also possible to raise the crop from seed, the advantage being that seed-grown plants can be less susceptible to bolting
To ensure a good crop, the plant has to be strongly growing by late spring as the lengthening days trigger the formation of bulbs – the more leaf at this time, the better the bulb.
Sow onion seeds in modules in mid- to late winter and keep in the greenhouse at 10-16°C (50-60°F). Sow shallot seeds in late winter. Harden off before planting out in spring.
Although one plant per module is effective, growing three to four plants per module saves space. Sow five to six seeds per module and thin out if necessary to three or four plants. When multi-seeded modules are planted out, the onions form a clump of bulbs.
Once the soil is dryer and beginning to warm up, start sowing directly.
Sow onion seeds from late winter until mid-spring 1.3cm (½in) deep in rows 20cm (8in) apart. Thin out first to 5cm (2in) and later to 10cm (4in). Closer planting will result in more bulbs and a heavier crop, but smaller bulbs.
Sow shallots a little bit later from early- to mid-spring. Unlike shallot sets, seed-grown plants will only produce a single bulb. Thin out as required to 2.5-7.5cm (1-3in) apart.
Growing onions from sets is generally easier and the crop will mature earlier. As onion sets grow they form a larger bulb. Shallot sets split, forming a clump of bulbs.
- Sets are also less likely to be affected by disease
- However, they are more prone to bolting (when a flower is produced instead of a bulb)
- To reduce the risk of sets bolting, plant the heat-treated sets offered for cultivars prone to bolting
Sets are generally planted in spring:
- Plant onion sets in early to mid-spring 5-10cm (2-4in) apart leaving 25-30cm (10in-1ft) between the rows
- Plant about 2cm (¾in) deep, in drills or merely push the sets into loose earth. Only the tips of sets should show
- Cover with fleece to prevent birds uprooting the sets. Remove the fleece once established in the soil
- Plant shallot sets 15-20cm (6-8in) apart in rows 30-45cm (1ft-18in) apart. Only the tips of the bulbs should show
Some onion and shallot cultivars are suitable for autumn planting:
- These types are less sensitive to cold exposure that induces bolting (flowering)
- Aim to plant onions in September and shallots in October
- Use the same spacing for spring-planted sets or module grown plants
- Autumn planting is not suitable for heavy soils prone to waterlogging as the crop is more likely to succumb to disease
As the foliage casts little shade the crop can be easily swamped by weeds, which would negatively affect the plants’ growth and subsequent yields. Hand weed regularly. Hoeing can be tricky as the foliage and top of the bulb can be easily damaged. Alternatively, consider planting through black plastic sheeting to suppress weeds.
Water every 14 days during prolonged spells of dry weather until mid-summer. Watering of spring-planted or spring-sown crops after mid-summer can reduce the storage quality.
Try to avoid overhead irrigation that encourages fungal diseases.
Over-wintered onions and shallots greatly benefit from an application of 35g per sq m (1oz per sq yd) of nitrogen-rich fertiliser such as sulphate of ammonia in late winter. This not only enhances growth but can also suppress premature flowering. Alternatively, use dry poultry manure.