In this article we will cover the various methods that can be used to grow Onions from seed. While it is certainly true that for the new or inexperienced gardener growing onions from sets is the easiest options, starting them from seed is really not that hard to do.
Using seed rather than sets has several advantages the main ones being.
1. In general seed is cheaper than sets.
2. Onions grown from seed bolt less and store better than set grown onions.
3. There is a far greater choice of variety available to you if you grow onions from seed rather than sets.
The size of the onion grown from seed is governed by the final spacing and by the length of day. As the amount of daylight lessens the bulbs start to swell. Therefore if you want a decent crop the earlier you can start them the better. In the UK this is in early March although you can get away with Mid April, any later than this and you really will need to go with sowing sets rather than seed.
If you do intend growing onions from seed by direct sowing then warming the soil first is a must. There are several ways of doing this but the easiest and probably the cheapest is to cover the soil with plastic cloches about two weeks before sowing. The cloches should be left in place until the seeds have germinated.
Sow the seeds as thinly as possible in rows about 20 cm (9”) apart just under the surface. Once the seeds have germinated and are about 10cm (4”) tall thin them out to the final spacing. This thinning process is when the onions are at their most vulnerable to attack by the onion fly as the scent of the onion is released during the thinning process. To overcome this firm down the soil around the remaining seedlings and cover with fleece or the plastic cloches for a few days.
Depending on the size of onion you require the final spacing will be anywhere from 5cm (2”) for small onions suitable for pickling up to 15cm (6”) for the more standard size onion.
Another method of growing onions from seed is to start them off indoors in modules. This gives you a number of benefits, you can start them off earlier, gives protection from bad weather and removes the need to thin out.
Using a general purpose compost sow two seeds per module. If both seeds germinate remove the weaker one as soon as possible. In the UK you can start off as early as late February.
Once the seedlings are about 10cm (4”) tall move them out of the greenhouse to harden off before planting out. Once hardened off plant them out in rows 20cm (9”) apart and spaced in the rows depending on your size requirements (see above). To plant out make sure the hole is the same size as the module and then firm the soil around the plant.