Growing onions and leeks from seed
When I first started gardening, I grew my onions from small bulbs called onion sets. Onion sets are large and easy to plant directly into the garden. But what is an onion set and why can you grow from sets or seeds seemingly interchangeably?
An onion set is an onion that has been started from seed and allowed to form a small bulb the previous fall. Sets are dug and stored, and sold to be replanted in spring. So an onion set is really a second year onion. Being a second year onion, most of your onion sets will flower and set seed that year if you don't harvest them first. They will also typically grow to be smaller than those you would plant from seed. Onions grown from sets wont have the ability to store very well either.
Many people don't realize how simple growing onions and leeks from seeds really is. Here's the simplest way we have found to succeed with onions and leeks from seed:
Start onions and leeks early. Onions and leeks are the first things we plant in the greenhouse in spring. Both germinate and start growing slowly, and they are frost-tolerant so can be transplanted out prior to the last frost in spring. We start our onions and leeks 12 weeks before the last average frost.
Use fresh seeds. Leeks and onions seeds are notoriously short-lived, so aim to plant seeds which are no more than 1-2 years old.
Plant in one-gallon plant pots. We plant our onions and leeks in fresh, sterile potting soil in one-gallon pots. Fill the pot with soil to within about 2 inches from the rim. Aim for about 75 seeds evenly spaced on the surface of the soil. Then sprinkle a light layer of soil over the seeds. Keep soil moist but not overly wet throughout the germinating process. Onions germinate best at 65-85 degrees F.
As the onions come up they do so in a grass-like chia pet manner in the one gallon pot. As long as you have no more than around 60-70 plants in one pot, they will have enough room to grow. If they are coming up too crowded, thin them. The idea now is that they continue to grow in a cluster until transplant.
Fertilize weekly with fish emulsion. About two weeks after germinating, begin fertilizing your onions with fish emulsion mixed with water. You have a lot of onions growing in that little pot, and they need nutrients to get up and growing.
Transplant when about the thickness of a pencil. This is the fun part. Gently dump your little onion pot out, exposing the whole plant. Gently pry the onions apart, breaking as few of the roots as possible. Wet the soil well before you transplant the onions into it- this seems negligible, but it really helps with the transplant shock. Plant the onions at least a foot apart in all directions. More space will yield larger bulbs.
When transplanting leeks, transplant with half of the leek under the soil and half above. This gives you that blanched white shank that makes leeks so valuable.
Both onions and leeks need a lot of water at first to transplant well. Try transplanting on a cloudy day, or in the evening to reduce transpiration and transplant shock. Keep soil wet for a least a week after transplanting.
Give it a try with our onions here: