Leeks are cool-season crop and will grow best at temperatures below 24 Celsius degrees (75 F), but will tolerate some higher temperatures if it is kept moist. Leeks cultivars vary from early and moderately hardy to late and very hardy. To develop well, leeks need an open site with fertile, moisture-retentive soil. Before planting, add plenty of manure or compost to the soil and add a nitrogenous fertilizer if the soil is low in nitrogen. Do not plant leeks on compacted soils and remember that this culture must be rotated.
Leeks need a long growing season so you should start them as early in the year as possible, sowing in succession. Sow 1 cm deep in an outdoor seedbed. Transplant them when they reach 20 cm tall. Alternatively you can sow in situ in rows 30 cm apart then thin or transplant the seedlings at the three-leaf stage to 15 cm apart. If you are growing smaller varieties you can use closer spacing.
For well-blanched stems, make holes of 15-20 cm deep with a dibber and drop a seedling in each, making sure that the roots of each plant are reaching the soil at the base of the hole. Water gently and then allow soil to fall in around the plants as they grow. Leeks may also be planted on the flat and blanched by hoeing up soil around the stems, 5 cm at a time, as they grow.
To extend the growing season sow under cover as for bulb onions. Leeks respond well to being multi-sown in modules with up to 4 seeds per cell. If grown this way, plant out the groups of seedlings at an even spacing of 23 cm.
After planting, water thoroughly until the leeks are well established. After that water only in exceptionally dry conditions. Keep the beds weed-free and mulch to retain soil moisture if necessary.
You can harvest the leeks 16-20 weeks after sowing but they can stand for many months. Lift and use them as required from summer onwards. The hardy cultivars may be left in the ground and lifted throughout winter into spring, except in very severe climates. Leeks do not store well if they are out of the ground. If you need the site that is occupied by leeks for something else, then you can prolong the season by digging them up and heeling them elsewhere. Lean them against one side of a V-shaped trench and cover the roots and white stems with soil then firm lightly. From here you can lift and use them as required.