by John Curtis

To grow or not to grow them

Drawing by Amanda Rofe

The principal advantage of planting onion sets in the autumn is that your onions are ready to harvest around a month before spring-sown onion sets are. Also, because they are harvested around a month earlier, you can potentially immediately follow them with another crop that would be too late to plant a month later. Sweet corn and winter squash are examples.

For a good yield, onions need moist soil when they are bulking up (onion-speak for when the growth of the bulbs accelerates). With luck this will be provided by rain, but if it’s dry then by watering can or hosepipe. Because overwintering onions bulk up earlier than spring-sown onions, there’s usually less or even no need to water them, depending on how lucky you are with the rain.

The main disadvantage is that the soil is left almost bare over winter – in the UK they are normally planted around September/October whereas spring-sown sets are normally planted around six months later in March/April. Since onions don’t grow much over winter, they only sparsely occupy the soil. That’s six months of minimal soil occupancy where an overwintering green manure could have been planted instead, which would have covered the soil much better, producing much more green growth and also taking up some of the soil nutrients that would otherwise have been washed down into the subsoil. The abundant growth and the absorbed nutrients are beneficial to the compost that the green manure produces – by turning it into the soil, by cutting it down and using it as a mulch, or by composting it in a pile or bin.

Another annoyance is that they are more likely than spring-sown onions to bolt, which means that they produce a flower and run to seed, and although onions which bolt are still usable, the eating quality is impaired.

It’s advisable to only grow them if your soil doesn’t waterlog over winter, otherwise they may rot and die in a very wet season.

Overwintering onion sets aren’t so widely available as spring-sown ones, but bigger garden centres often sell them, and buying by mail-order gets around this problem.

If you do decide to grow them, it’s a good idea to only grow as many as you need to fill the time-gap before spring-sown onions are mature, and harvest them to eat straight away. Don’t store them – only onions grown from spring-sown sets should be stored for use through the autumn and winter.

Sets or Seeds?

Onions are available as sets or as seeds. Most people grow them from sets – which are very small immature onion bulbs – rather than seeds, since they are easier to grow, requiring less soil fertility than seeds, and unlike seeds they rarely suffer much if at all from onion fly. They also occupy the soil for a shorter time period. They do cost more than seeds, but aren’t particularly expensive, especially if you buy them in a garden centre or hardware shop that sells them loose, rather than in pre-packs of for example 50 sets.

Buying loose also lets you select your sets individually – not too big (more likely to bolt and since they’re sold by weight they cost more) and not too small (less likely to do well). This article only discusses growing them from sets, but an internet search should throw up information on growing them from seed if that’s your preference.

Note that autumn and spring sown sets are different varieties optimised for their individual growing seasons, so don’t use one for the other.

Varieties, planting time, soil preparation, and planting

Both red and white varieties are available. I don’t have enough experience to give advice on varieties, but for whites, Senshyu and Radar are commonly available. For reds, the popular choice is Electric. In the UK the usual advice is to plant them in September or October. Planting in September can be a challenge since they aren’t always available in garden centres in early September.

Onions prefer loose soil, so if your soil is badly compacted you might want to shallow-dig it to a depth of 10cm/4″. With compacted soil, I expect you’ll still get a reasonable crop, so don’t dig if you don’t dig digging. The soil should be reasonably but not especially fertile, so I don’t add compost to the soil before planting them.

I normally plant the sets where maincrop potatoes were – an ideal rotation I think, since our main crop spuds come out in early September just before the onion sets are planted, which avoids unproductive bare soil. Harvesting spuds invariably involves quite a bit of digging which loosens the soil – ideal for growing onions. I always add compost just before planting seed potatoes earlier in the year, so there should be a reasonable amount of nutrition and moisture-retentiveness still left in the soil after harvest to be of benefit to the follow-on onions.

Onions dislike very acid soil, and they prefer a reasonably sunny spot.

Planting distances are a matter of opinion. Most books say something like put them in rows 30cm/12″ apart, and separate them within rows by 10cm/4″, then they go on to say that wider spacing produces larger onions. I plant them on the square, normally separating them around 15cm/6″ between rows, and within a row. I would suggest that if your soil is poor, increase the separation so that there’s less competition from neighbours for your limited soil nutrients, or you can reduce the spacing a little if your soil is very rich.

If your soil is very loose, it’s OK to push them in, but if it isn’t, pushing them in can damage them, so instead use a dibber or stick to make a hole, pop them in that, pointy bit upwards, rooty bit downwards, and gently draw in and firm some soil around the sets. The top of the set should be just protruding or only just covered with soil.

Winter hardship

I’ve found that they cope well with very wet winters – but the soil in our garden doesn’t get waterlogged, and from what I’ve read, I would have been disappointed if it did – they rot in waterlogged soil. A very cold winter has caused us more problems – most sets survived, but many only developed into very small onion bulbs, resulting in a poor yield overall. If only we could predict what the winter is going to be like…

Problems and solutions

Small bulbs Assuming you didn’t harvest them too early, next time don’t let them dry out, especially when they are bulking up (ie when the bulbs start to get noticeably bigger at a much faster rate). I’ve found by accident that onions always seem to be bigger after a wet spring, which I think is trying to tell me that I should water them more in an average or dry spring. That’s easier said than done though. You think the soil is moist, but it may only be the surface that’s moist. An infrequent thorough watering is needed rather than little but often watering. The soil needs to be reasonably fertile to get big ones and, especially if your soil isn’t very nutrient-rich, don’t put them too close together otherwise they’ll compete too much for nutrients and water.

Bolting This was discussed above. Choosing varieties that are resistant to bolting helps or, if available, heat treated sets. If they do bolt, the bulbs stop getting bigger and they are much more likely to rot, so harvest any that bolt and use them first.

White rot This is a serious fungal disease. Foliage goes yellow and the bulbs rot and are unusable. You’ll see a grey-white mould around the base of the bulb. Once you have it you’ll have it worse in the years that follow. The fungal spores survive in the soil for a long time, so the only way to eliminate it is to not grow onions for at least 15 and preferably 20 years.

Slugs and snails These rarely get mentioned in growers’ guides. I’ve found that the leaves can host lots of small slugs, but they don’t seem to do much damage.

Photo taken by John Curtis in mid April 2014. It shows spring-sown onions (sets planted in mid-March) on the left, and overwintering onions (sets planted in early Oct 2013) on the right. The purpose of the photo is to show that in spite of being set around five or six months earlier, overwintering onions don’t actually have that much advantage in terms of growth and, even at this stage, their grown coverage is low.

This article appeared in Growing Green International magazine Num 33 (Summer 2014), p36.