This must be one of the easiest vegetables to grow, especially if you start from sets. Each small set will grow into a full-sized onion during the summer. On a vegetable plot, onion plants need very little attention and can be left to mature and dry off for winter storage. It should be possible to store homegrown onions until the following spring. Onions will grow and produce a worthwhile crop in large containers, too. There is now a good choice of varieties from sets. For even greater choice, try growing onions from seed, though this does take a little more effort.
How to grow onion from seed
If it is so easy to grow onions from sets, why bother with seed? Apart from the greater choice of varieties, seed also works out much cheaper if you want a lot of onions.
Sow the seed thinly into trays of multipurpose compost in late February or early March. The seed will germinate quickly at a temperature of 10-15’C. When the plants, which resemble grass, are large enough to handle, transplant them into pots or modular trays. Harden them off gradually until they are ready to plant out in late March to April. You can also sow the seed directly into the soil from February (under cloches) to April. Sow into seed drills 1cm deep and 15cm apart. Thin the seedlings to 2.5-5cm apart.
Multi-seeding works well with onions. Sow about 6-8 seeds together into 7-cm pots or modular trays, and do not thin the seedlings out. Plant out the whole potful together. Plant the clumps about 15cm apart each way. As the bulbs grow they will push each other apart to produce clumps of small, round onions.
Calendar
When you buy a bag of onion sets, you are buying immature onions that were raised from seed the previous summer. Because they are sown at a very high density, they do not reach sufficient size to bolt (produce a tall, erect flowering shoot) or produce flowers in the second season – they just carry on growing instead. Under normal conditions, onions are biennial, producing a bulb in the first year and flowering the second season. Sets are a convenient way of cutting out the seedling stage. Some varieties are more prone to bolting in their second year.
February
As soon as the soil is workable and not too wet, you can plant onion sets. In cold areas or on heavy soil, delay until mid-March or even April. Sow onion seed indoors.
March-April
Prepare the ground by digging the soil to loosen it. Soil that is too compact or firm will cause the onion roots to push the sets out as they grow. The ideal site is a piece of well-cultivated, weed-free ground that has been manured in previous years but not recently. Onions benefit from a little balanced fertilizer – i.e. up to 35g a sq m of growmore or similar – but do not add fresh manure before planting.
Push the sets gently into the soil so the tips are level with the surface. Birds will pull them out given the chance, so cover them with garden fleece or a cloche. The spacing will, to some extent, determine the final size of the bulbs. For lots of small bulbs, about 5cm in diameter, plant the sets 2.5cm apart in rows 15cm apart. This spacing allows room to get a hoe between the rows. This is important as onions cannot compete with weeds, so you will need to hoe or weed by hand. As the onions grow, the bulbs will push each other apart. For larger bulbs (up to 10cm across), increase the spacing to 10cm within the rows.
To plant sets in containers, push them into the compost 7.5-10cm apart each way. If you have left-over sets, plant them almost touching in a 15-cm pot and cut the shoots as spring onions.
May-July
Hoe between the rows, and if necessary hand-weed between the plants. You can buy special short-handled onion hoes that allow you to bend or kneel down to hoe. Even when fully grown, onions cannot shade out other plants so they are very sensitive to weed competition.
In most years, onions do not need additional watering.
August-September
The foliage will start to yellow and fall over naturally. Let nature take its course – if you bend them over too early, the leaves can get damaged and let in diseases. Lift the bulbs with a fork to break the roots and leave them on the surface to ripen fully in the sun. In a wet summer, cover them with cloches or move them to a greenhouse bench to complete their ripening.
You can sow Japanese onions early in September in the north, later in the south.
October
Lift the spring crop for storing. You can plant autumn sets now for an early crop next summer.
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