In my opinion, the best way to grow leeks is to buy them as starts and transplant them. I have tried to grow them from seed, but I much prefer the starter method. Growing from seed is a bit tricky. It requires that the seeds be refrigerated over night before sowing, and then soil temps of between 68 and 78 degrees (20 to 25 Celsius). After germination, they need thinning, and they do not give themselves up easily, often breaking during the process, necessitating another thinning of the regrown stumps. So, since I already have my hands full of other things that absolutely require direct sowing and careful (read: time-consuming) thinning, I choose the starter method of planting leeks.
I buy them in "sets," meaning a bundle of plants (you can do the same with onions as well). Seventy bucks gets me 30 bundles of 60 plants, about 1800 leeks. I get them from Dixondale Farms in southern Texas. The photo above shows the last few bundles in their shipping container.
Before we plant, we use our standard bed prep, see this post for how I do that. Once prepared, and the bed is quite fluffy from incorporating the amendments, we dig a trench about six inches deep with a trowel:
When we finish a row, we fill in the trench. This is the important part: only a bit of the leek is then above the soil surface. Most of the leek is underground. This will enable the leek to grow tall and strong, as well as have a nice long white neck, before the above ground plant tissue is turned green from photosynthesis. We then dig another trench about six inches next to the first row and continue planting. Once we are finished, we water them in well.
Leeks can be grown year round in some climates, like my California garden. If you live in a snowy area, they should be planted well before winter, so that when the snow comes, the leeks are big enough to continue living in the cold ground with a thick mulching of straw. You can go out and harvest them as you like, big or small. We typically pull them small for Manresa. Here is a picture of some of our leeks prepared by Alain Passard when he came to the restaurant last spring to cook a few special dinners. Photo courtesy of Chezpim.com