Leeks are not the fastest growing edible garden plant, most take over 100 days from sowing to harvest. As a result it is important to get started early in the spring for a fall harvest.
In mild climates leeks can easily be overwintered (planted before winter and harvested the next spring/summer) by sowing in late summer. To do this with success you may also need to mulch around them for the coldest part of the winter.
Every type of seed requires a specific temperature range for optimum germination. In general, leeks require higher temperatures than most, with an ideal temperature of 65-75°
F. Leek seeds are very small black and sort of a flat teardrop shape. Sow leek seeds indoors in an appropriate seed starting medium 4-6 weeks before your estimated last spring frost. Sow seeds outdoors after the threat of frost has passed and the soil has warmed to 65-75° F. In each instance cover the seeds with 1/4” of soil and water lightly. Keep moist for 5-14 days or until you see the first shoots emerge.
Leeks should be grown in full sun and need an average amount of water. Watch your leek plants and learn to see the difference when they have enough water and when they are water stressed. This is easy to do as they will look pale and wilted when they need water, especially so when they are young and have small root systems.
Have you ever wondered how the leeks in the store have such a beautiful, pure white stalk? If you have tried to grow leeks and didn’t end up with similarly white stalks here are a few suggestions. To have leeks with white stalks it is necessary to exclude light from that part of the plant. There are several well-established methods of doing this.
The first, and most fun in my opinion is the trench technique. Start by digging a trench that is 6” deep and 4-5” wide and as long as you like. It doesn’t have to be exact; you may find a deeper or shallower trench will work fine in your garden. Sow your leek seeds in the trench with about 2” between each seed since you will later thin the plants to about 6”. Cover the seeds with ¼’”of fine soil and water until the soil is moist. As the leeks grow, slowly fill in the trench so that their stalks end up with 6” covered by soil.
The second method can be called mounding and uses the same principle of covering the stalks with soil but instead of digging a trench the seeds are planted at ground level and covered with ¼” of fine soil. Spacing and water requirements are the same as above. As the leeks grow, begin to move soil around them so that over time you end up with mature leeks that have a 6” mound of soil around their stalks.
Using either of these methods will produce delicious, white stalked leeks.
When leeks reach about 1” in stalk diameter it is time to thin to one plant every 6”. You can use the thinned leeks in the kitchen for any number of dishes. At maturity, leeks are typically 2-3” in diameter and can be harvested at that point. However you don’t have to harvest immediately. The great thing about leeks is that when they reach maturity they can be left in the ground as a live plant until you need them to make a soup or other dish. To harvest simply clear the soil around the stalk, grab the stalk as low as possible and gently pull. Trim the roots and rinse with water before taking them inside.
Look here for more specific information about leeks.