Ensuring leeks grow to the fattest possible diameter takes special care. A fat leek is a well-cared-for leek, left to grow throughout its entire season and protected from drought. Leeks usually grow to a maximum of 2 inches in diameter, so if you can get the leek to reach that size, harvest at that point. Don't wait too long or the plant will go to seed.
Start seeds indoors early to give the leeks the longest possible growing time. Keep them indoors for about two months before planting them outdoors. The exact timing will depend on weather in your area but usually ranges from late April to June. Contact your county or university extension office to find out when to start and transfer leeks outside in your region.
Apply a balanced fertiliser to the soil when transplanting and once more generally about three months after transplanting. This second application is called side dressing. The type of fertiliser to use will again depend on the region; the University of Minnesota Extension suggests balanced fertiliser like 10-10-10, but Utah State University recommends 21-0-0 and says this one application will contribute to a healthy yield of leeks. Don't apply fertiliser after this.
Keep the soil well-watered so he leeks do not have to deal with drought. Utah State University advises that any water stress will adversely affect the leeks. Adding mulch can help the soil retain moisture. Because leeks can extend several inches below the soil surface, Utah State University recommends watering until the top 1 1/2 feet of soil are wet each week.
Measure the approximate diameter of the leeks as they grow in. Once the leeks reach a certain width -- the common description for this is pencil-width -- begin piling soil up against the stalk. Continue this throughout the growing season. An alternative is to plant the seedlings in a trench and fill in with soil instead of hilling. Don't cover the joint areas where the leaves bend away from the main stalk. The growing mound of soil will help expand the edible white and light green portion of the leek.
Some leek varieties can take as long as five months to grow, while others need as little as three months. Label the leeks in the garden if you plant more than one variety to avoid confusion about when to harvest them.