Leeks are annual vegetables related to garlic and onions. The leek has a subtle flavor that is not nearly as strong as that of its relatives. It was first cultivated about 4,000 years ago in ancient Egypt, and is commonly used today in soups and casseroles. Leeks grow between two and three feet in height, and have an edible white shaft, which is usually six to ten inches in length. They require between 100 and 150 days to grow to maturity, resulting in a late harvesting season.
Start leeks indoors in a planter in late February. Keep the soil consistently moist, and continue growing inside for 10 to 15 weeks, or until the average daytime temperature is 45 degrees Fahrenheit or greater.
Harden leeks before planting by placing them outside for several hours each day, and bringing them back inside at night. Continue this process for one week, increasing the time outdoors each day. Alternatively, leeks can be transplanted directly into a cold frame for five to seven days.
Choose a location that receives between six and eight hours of direct sunlight each day and has well-drained soil. Do not plant in a site where other members of the onion family have been grown in the previous three years, as common diseases to the family can still be present in the soil.
Transplant each leek into the garden by planting in holes just large enough to completely encompass the root system. Maintain a distance of four to six inches between separate leeks. Gently refill the hole with soil, and water thoroughly to compact the soil around the roots.
Fertilize using a 20-20-20 NPK fertilizer immediately after transplanting to provide the leeks with plenty of nutrients to begin growth. Feed again in midsummer using a 10-10-10 NPK fertilizer. Ensure the fertilizer doesn't directly touch the leeks. Always follow the manufacturer's directions for proper application and dosage of fertilizer.
Water leeks once per week, and only on weeks that don't receive at least two inches of rainfall. Both excessive watering and splashing moisture on the foliage can result in fungal disease, and should be avoided.
Harvest leeks when the foliage is dark green and the stem is at least one inch in diameter and at least three inches in length. Pull the leeks from the ground while gently twisting until the entire plant has been uplifted from the soil. The ideal time for harvest is late fall, just before the first frosts of the season.