Growing onions

Growing onions is a skill. On the one hand you put them in the ground and they grow – that is if you are using onion sets. They will produce a decent crop and you will think onions are easy. However, there are a number of pitfalls, and you can lose a large amount of your crop needlessly.



seedlingsOnions need good soil, well manured, well dug, with good drainage – rich in nutrients. I think we have said this before, but if you have any plant that produces strong flavours, or rich bright colours, it will need a lot of nutrients to make these chemicals. The onion makes brilliant chemicals and flavours, and consequently needs a rich, well-manured and nutrient-rich soil.


The problems with onions are many. First of all they accumulate glucose in the leaves of the bulbs as well as a lot of water. Sugar and water is the ideal growing medium for fungi, and consequently onions are very susceptible to disease.


The onion has evolved a defence mechanism in the form of sulphur-based compounds, which have to be created in the plant. To do this they need lots of water, sunlight and nutrients.


The next problem is the roots – they rot easily. In particular they have to get a lot of oxygen, otherwise they die quickly. This is supplied by having soil with lots of air spaces in it, and you achieve this in two, seemingly opposite, ways.


First, the soil needs plenty of organic material in it. This acts as a sponge, soaking up water. Oxygen has to dissolve in the film of water before it can get into the root. Second, the oxygen comes from the air spaces created by adding grit to the soil – about two spades per square metre. This allows excess water to drain away and at the same time creates air spaces.


So, getting enough oxygen into the root is a balancing act.


A lot of serious onion growers have a bonfire on the soil they intend to grow on and then dig in the ashes with some grit, forking it in and then going over with the hoe. This adds nutrients and air, and has the advantage of sterilising the soil too, at least a few inches from the surface.



The allium family all form various bulb-like structures. The leek is simply a swollen set of leaves, which have, in the onion, pulled down to form what we know as a bulb. A bulb is simply a collection of swollen leaves, and when you cut through it you get to see all the leaves. A corm, on the other hand, as produced by garlic, is a swollen underground stem. When you cut through a corm, what you see is a single mass of tissue – no leaves.



The physiology of the onion, like most plants, responds to the length of the day. They come as long-day, intermediate-day and short-day forms.


Long-day onions will only form bulbs when the day is fourteen hours or more long. This means they are summer onions, and the combination of hormones that promote leaf manufacture and consequent clumping into a bulb, only kick in when there is a lot of light around.


Short-day onions will create bulbs when the day is much shorter – around eleven hours. These are winter-growing onions, which ‘bulb up’ in the spring. Short-day onions are also able to withstand winter conditions and they start growing much earlier in the year.


We also have maincrop, early and Japanese onions. Maincrop are long-day onions, planted anytime from April – grown from seed under heat in January. Japanese onions are planted in autumn and overwintered. Japanese are short-day onions; early onions are intermediate-day onions.



Onions need heat to germinate. Sow in modules at a temperature of 21oC. Actually, a kitchen windowsill will do just as well. The traditional day for sowing onions is Boxing Day, but you can sow well into January or even February, too.


Keep seedlings warm, well ventilated and transplant them to their growing positions in soil that has had a chance to warm up.


Young onion seedlings need not to be shocked – so water with mildly tepid water, but keep them on the dry side to avoid damping off.



This can be done in April – the seedlings are carefully popped into their growing positions – in holes made with a dibber. You are best covering them with horticultural fleece to keep the wind off them. As well as helping to create a microclimate for good growth, fleece also has the benefit of keeping onion flies at bay, which dig into the neck of the onion and lay eggs.



The onion is a biennial. In the first year it grows a bulb and in the second year it produces flowers and seeds. An onion set is a specially grown onion, which has been frozen a little at the tip. The growing point of the onion dies and instead of going on to grow a flower, the bulb grows out to a proper onion as though it were only one year old – but in fact it is two.


When planting onion sets, don’t push them in the ground. Instead, make a small hole with a dibber and plant in that. Firm the onion in afterwards, and then watch. After about a week or so you might find onions popping out of the ground – this is fine – just pop them back.


What you are looking to create is a well-seated onion whose neck protrudes from the soil. Don’t plant them too deeply, as they will rot. This begs the idea of growing onions in a growbag. It works very well, and we grow a lot of shallots and onions this way. In the growbag we plant them quite close together and feed them during the season once a fortnight with general-purpose liquid manure. This way of growing is more like hydroponics than anything else – the compost is just there to hold them upright!


Onions need to be spaced about one and half hand widths apart. Use them as a border crop – plant them around carrots and salads to create a confusing chemical atmosphere. Insects ‘see’ the world partly by their sense of smell. Plant onions near lettuce, and instead of ‘seeing lettuce’, cabbage white butterflies ‘see’ onions – which they hate.



You can tell when they are ready – the leaves begin to fall over and the bulbs are full. In truth you can harvest them at anytime – they are all onions after all, but if you keep them in the ground past mid September you will get rotting bulbs.



Onions harden and protect themselves if allowed to dry out. This is an important part of the process, because the skins harden and this is what protects the onion in storage. Short-day onions don’t do this so well and consequently do not store as long as maincrop.

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