Both methods are very common and have their advantages and disadvantages, and in the end it is up to you to make the choice. The onion sets are immature onion bulbs specially grown for planting and consequently more expensive than the onion seeds. On the credit side they can be planted in early spring, when the soil is too cold for sowing. Sets are also much easier to handle than the tiny seeds, and they will not have to be thinned or transplanted. The food reserves within the sets mean that they take a shorter length of time to mature than seeds, which need to be germinated before they start taking in nutrients.
The biggest drawback of sets, apart from the cost, is that they are more likely to flower prematurely, or bolt, than onions grown from seeds. There is no reliable way to eliminate the risk.
If you decide to grow onions from seed, you will have more varieties to choose from and you will also have the option of using the thinned seedlings as spring or salad onion.
Sow the onion seeds thinly into small drills inside in March or outside later in April. Just cover the small onion seeds with some fine compost, this will let the fine shoots come through the soil easily. If you have a bigger container in your greenhouse you can grow spring onions from seeds almost all year around. Some varieties like White Lisbon are designed for autumn sowing. If you sow these spring onion seeds in October-November, you will have a great early spring onion harvest.
When planting the seedlings into their final position, make the planting holes 6in apart with 9in between the rows. The wholes should be deep enough to let the roots fall in vertically, with the white portion of the bulb buried and only the green parts above the surface. Plant the onion seedlings firmly. After the bulbs have made some growth, scratch out some soil around them to allow them some room to expand.
You can avoid transplanting if you sown the onion seeds where they to grow all year, in this case you just have to thin the rows to give more space for those big bulbs to grow.
Feed the onions with a high nitrogen fertilizer once every two weeks from planting out until the end of July to promote growth. After this, switch to a high potash feed to harden the bulb tissues and aid ripening.
When onions start running to seed, called bolting, it can be down to a number of different factors. It may be because the seed was sown too early, or perhaps the onions were planted during an excessively cold spring. Or perhaps the seedlings were planted into a loose soil, as onions like their soil pretty firm.
Onion sets are specially treated to discourage bolting, although the process is only partially effective. The little bulbs loose moisture in the process and it turns their outer skin pale brown.
If your onions do start to bolt, you can tell when the leading shoot begins to swell into a flower bud, cut off the flower stalks and lift the onions. Do not let them stay in the ground until all the normal plants are beginning to ripen, bulbs that have bolted will not get any bigger. Use them in the kitchen as soon as possible because they will not keep for a long time at all.
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