Onions remain one of the most popular crops throughout the world and this author has been growing them regularly each year with generally great success compared to some vegetables. They are also a valuable base for most savoury dishes and there is increasing evidence for their health benefits, especially in combating heart disease and digestive issues.
One of the most popular and probably easiest methods to obtain a good crop of onions is to grow them from sets rather than seeds. Onions require good drainage on an open site in very fertile soil. I prepare the ground in September but sets are started off as late as November. A good dressing of manure, compost with a general purpose fertiliser make for the ideal feed on soil which may well have been used for growing other crops. If the soil is light, then it not be firmed down but I prefer to have it flat and reasonably compact.
Over-wintering/Japanese Onion sets
I plant my sets from September to October when the clocks return to their winter settings so that they grow well over this period. Otherwise, when the conditions have returned to more benign settings, then plant sets of other varieties from mid-April to mid-May.
Onions can be planted to within a few centimeters of each other depending on the size desired. The closer the planting, the smaller the grown bulb so I choose my distances based on the size of the onion set. It is important to prevent the onion bed from drying out especially during hot summer weather which is around June/July when they grow really well. Exhibition onions need to be planted no nearer than 30 cm apart, but for smaller varieties this can be reduced to 18cm.
After planting out any bulbs, cover with protective netting or horticultural fleece to reduce bird attacks, insects and pest damage.
Onion seed is best sown on the surface of John Innes Sowing Compost, in trays in the greenhouse or propagator from December to early April. Lightly cover with a sprinkling of vermiculite or some more compost. Germination takes about two weeks at a temperature of 13 to 14°C. The compost needs to be kept moist but never overwatered.
Where smaller onions are required, outdoor sowings can be tried in April. Sow thinly in shallow drills 1.5cm deep, with rows spaced about 30cm apart.
Following germination, the seedlings reach the ‘crook’ stage where they are about 4cm tall. These are transplanted like most seedlings into John Innes No2 compost usually in individual modules. Watering is only done when needed usually when the compost is partially dry.
Avoid Botrytis which devastates seedlings by ensuring they are well ventilated although the temperature needs to be kept constant.
The young plants can be transferred to a cold frame from mid- to late-March but ensure the crops are well hardened off before planting outdoors in April. Leave 30 to 45cm between the plants and 45cm between the rows.
Thin, outdoor sowings to 15-30cm apart.
Onions can be fed with a small amount of fertiliser, early in their growing season. Too much feed causes the onions to form thick necks with poor keeping quality. Mulching around the growing bulbs helps to conserve soil moisture and suppress weeds.
Any plants producing flower spikes need to have these removed as this implies bolting which is highly undesirable.
Harvest in August to September
Onions store better when harvested with little growth left on the plant. The exhibition types are usually lifted 10 to 14 days prior to any showing. Dead and broken skin should be removed and then left to dry either in the Sun or in a warm dry area. Store all the onions in a cool but frost-free shed or garage. Cut the tops but leave enough for handling.
Spring Planting Small Onions From Seed: ‘Red Baron’, ‘Ailsa Craig’ ‘Golden Bear’, ‘Rijnsburger’.
-Ailsa Craig – large globed variety with a mild flavour and golden, straw-coloured skins. Traditional favourite for exhibition purposes.
cv. Hybound. A relatively modern and high yielding early Rijnsbuger variety producing attractive brown bulbs. This onion has a straw-coloured, attractive outer skin with very good outer skin protection. Use for storing.
cv. Snowball. Sets are available from mid-December to March.
Spring Planting Small Onions From Sets: ‘Rumba’, ‘Santero’, ‘Red Baron’ ‘Centurion’.
Autumn Planting Small Onions From Sets: ‘Radar’, ‘Red Baron’, ‘Shakespeare’, ‘Senshyu Yellow’, ‘Electric’.