Know your onions!

'Spring onions' are further sub divided into those which form swollen bulbs if left to maturity and those remaining pencil thin (like slender leeks).

Bulb onions (Allium cepa) form leaves to swell the bulbs through the longer daylength periods of Spring up to mid Summer and then commence the ripening process in readiness for harvesting by late Summer.

All 'spring onions' (also Allium cepa) can be sown over a longer period of Spring to late Summer.

The ' non bulbing pencil varieties (Allium fistulosum) are generally less pungent, and a few Allium cepa x fistulosum 'spring onions' have attributes of both types but eventually form a swollen bulb. Most are silver/white skinned but there are modern introductions with several skin thicknesses of red.

Why Use Onion Sets?

Most gardeners plant 'sets' to produce their onion crop as they are much easier to handle, can be planted anytime from late February to end of April, are much quicker to establish, eliminates the need to thin out, and often prove more reliable. Choose smaller sets, 17/21mm grade is best but 14/21mm is commonly available. Larger sets over 24mm are much more prone to bolting. Commercially purchased sets are likely to have been fungicidal dipped to minimise the serious neck rot fungal disease and other pathogens, but the seed catalogues normally sell untreated sets. Choice of set varieties used to be basically Stuttgarter or Sturon but in recent years many selections and F1 varieties are obtainable. From seed the two most popular bulb varieties are Ailsa Craig and Bedfordshire Champion.

How to sow Onions as seed Sow seed sparingly direct outdoors 13mm ( in) deep in drills about 38cm (15in) apart during March or April if the soil is not too wet or cold. Later sowings through April will produce smaller bulbs as the growing plants have less daylength to produce sufficient leaves to swell the bulbs. Covering the designated area with cloches or polythene two weeks beforehand to dry and warm the soil will be beneficial. Many gardeners prefer to 'multi sow' in 5cm ( 2in) pots or propapacks of multipurpose compost under glass during March, thinning to four seedlings per unit, before planting out a complete unit 20cm (8in) apart from late May into prepared soil. This method gives average 250g (8oz ) bulbs. The wider the planting distance the larger and heavier the bulbs may grow, allowing 20cm (8in) for the largest bulb potential. The distance between the rows is not critical but room is needed to hoe and hand weed around the developing bulbs. Weeds can cause staining and inhibits bulb growth if not controlled. Weeding is less problematical with 'spring onions' as the weeds are pulled up when harvesting. 'Spring onions' are best sown liberally and unthinned in broad drills 15 - 20cm ( 6 - 8 in) wide, at 13mm ( in) depth. Sow successionally from March to August for pullings to late October and onwards if weather remains mild.

Onion sets are planted out direct Allow 10cm (4in) between the sets in the row and 38cm (15in) between rows. Do not push sets into the soil as this may damage the fine roots. Use a trowel to plant each set sufficiently deep to just cover them. Sprinkling fine soil to cover the 'tip' will deter birds from pulling them out, any that are can be replanted, as they are quick to establish. A useful tip if the soil remains too wet to work on is to plant individual sets into 5cm (2in) pots or used plastic drinking cups of multi purpose compost. Plant out some three weeks later once the initial flush of leaves show. Bolting can be a problem from sets, particularly red skinned Red Baron types that have not been heat treated. Heat Treatment arrests the growing point and 'cures' (darkens the skin) and reduces the moisture content. The process takes over 15 weeks, therefore availability is not before mid March. When planted into warmer soils they are quick to establish and grow away.

Use the leaves too!

Whether from bulb or spring onions many gardeners enjoy the green leaves as a chives substitute, often planting a few extra sets close to each other just for the leaves, adding to stir fries, soups, egg and potato dishes.

Onions do best in well prepared soil rich in well rotted manure or compost. Yields can be increased by giving a high nitrogen feed to encourage extra leaf growth before the plants begin to bulb in early summer. Keep watered during growth but only until mid July otherwise ripening can be delayed and the bulb quality affected. A useful tip when pulling 'spring onions' if the soil is dry - give a thorough soaking beforehand and then pulling is much easier.

Bulb onions need lots of sunshine on their backs to ripen completely. Most varieties have good storing capabilities if well ripened. Never bend the tops over as this can allow infections to enter. The tops will die back naturally except those 'bull necked' ones which are best eaten first as they will not store well. Choosing a sunny day carefully tease under the bulbs with your garden fork to break the roots. Leave for a couple of weeks and then if the skins are rustle dry and the weather remains dry lift completely and lay on the soil to 'cure' and harden the skin. If wet knock off the worst of the soil and lay bulbs out on the greenhouse staging or shed. Store only sound bulbs in used tights, onion nets, or strings or in slatted trays allowing for good air circulation which is important, in a dry, frost free place.

Bulb onions suffer from five major pests and diseases These are Onion Fly, Neck rot, White Rot, Eelworm and Downy mildew- but 'spring onions' tend to be problem free unless in White Rot infected soil and occasional mildew fro later sowings. Onion Fly shows as yellowing wilting leaves and white maggots can be seen at the base of the bulbs on lifting, usually worst in dry soils during July. Neck Rot is a fungal infection noticeable on those stored bulbs that have not fully ripened, a common problem during cooler, wetter summers. White Rot shows as a fluffy mould and is a serious fungal problem as it remains active in the soil over many years. There are no resistant varieties. Eelworm (nematodes) are microscopic soil living pests which distort and stunt growth (similarly with potatoes although there are eelworm resistant potato varieties) but no resistant onion varieties. Downy mildew (grey mould) is a serious problem in cooler, wet summers, spray with dithane when first noticed. Downy Mildew resistant varieties are under commercial assessment and showing some promise. White Rot control by digging in volumes of onion waste which triggers the fungus into activity and then it dies off once the onion waste has decomposed is also under trials evaluations with promising results.

For your complete onion programme Spring planted bulbs from sets ( not heat treated). Planted Late February to mid April. In order of maturity from early July onwards. Maincrop maturing varieties tend to store for longer than earliest ripening varieties.