|'London Flag' is an English heirloom variety with a fat base and overall large size. A Leek is a biennial grown as an annual. This onion relative does not form a bulb, but rather a fat stalk and strap-like leaves. A cool weather crop, they are at their best when day temperatures are below 75 degrees F.
Leeks may be planted from seed 4 to 6 weeks before the average date of the last frost or from transplants in the fall. Transplants may be set out in early spring to mid summer.Plant in full sun in rich, well-drained soil that has been amended with 1lb of complete fertilizer per 100 sqare feet. If planting seeds, do so shallowly, at only 1/8 of an inch, then thin to 9 inches apart. If setting out transplants, dig holes 6 inches deep and 9 inches apart. A double row will help save space. Drop the leeks into the holes, but do not fill with soil. Water will collapse soil into holes over time and leeks will position themselves.
Keep leeks watered and never let them completely dry out. Once leaves have developed ample growth, mound soil around base to keep bottoms white and some say milder tasting. Harvest leeks as you need them, but try to harvest all by first frost.