Onions for cooking or fresh use are best started early indoors to get a good jump on the season. They can be sown directly into the garden but the crop will simply be a little later.
When to Plant Onions
The trick to sowing direct into the ground is to sow early. Seed that is not in the ground by the first week of May will not mature before frost.
To beat the season, sow indoors. Sow seeds .6 cm from each other, cover very lightly with soil, water with warm water and give full sunlight. Grow them with full light until the danger of heavy frost is over; then transplant to the garden.
If you want huge specialty onions such as Spanish onions, then I’d buy sets and plant these in late April.
Where to Plant
Full sun in a well-drained, fertile soil.
How to Plant
If you want to sow outdoors, put the seeds 1-2 cm apart. Plant .6 cm deep. Note that deep sowing will simply kill the onion seed. Rows should be 30-45 cm apart. You want to encourage air circulation in the garden to reduce or eliminate fungal problems.
Sets are planted so their tops are just below the surface.
Care & Maintenance
When the growing onions start to touch at this close spacing, thin and use the smaller tender onions in cooking or pickling. This leaves the remaining onions adequate space to reach 5-7 cm in size.
Onions are a shallow rooted vegetable so a liquid compost tea feeding early in the spring combined with a mid summer feeding of compost tea or fish emulsion will give you excellent results. This is over and above the normal application of compost in the spring. This shallow root system also means that you should not allow the onions to get too dry during the heat of the summer.
Avoiding Internet Advice
Do start your harvesting by by thinning the plants as they mature.
There’s a lot of advice about knocking over the tops of bulbs to make the bulb grow larger. The reason you knock the tops over is to prevent heart rot if there is heavy rain just before you cure the bulbs.
If there are no heavy rains forecast (and the bulb will not be absorbing excessive amounts of water) there is little reason to knock over the tops.
Curing For Storage
To cure the bulbs for storage, pull the bulbs out of the ground and allow them to sit on top of the soil for 7 to 10 days to “cure”. Avoid leaving them to be exposed to frost. Leave the soil on the roots to dry out naturally.
The neck will dry out during this time and you can remove it and clean the bulbs just before storing the bulbs. (necks are the long leaves that will wither down when pulled from the soil – you’ll understand when you see them) Do not cut into the green growth of the bulb before storage.
(all numbers rounded out)
1/4 inch = .6 cm
1/2 inch = 1.3 cm
1 inch = 2.5 cm
6 inch = 15 cm
12 inch = 30 cm
18 inch = 45 cm
36 inch = 91 cm