Leeks prefer a sunny, well cultivated, sheltered site with rich well-drained soil. As they will sit in the soil for a long time, they are ideal crop for the allotment, although many have fantastic foliage that makes them an ideal vegetable to grow in flower borders or an ornamental potager.
Preparing the soil: If possible, prepare the soil for planting in the winter. Dig the site well, removing weeds and working in plenty of well-rotted manure to improve its ability to retain water. Leeks can be planted in heavy soil, but improve the drainage by mixing in some horticultural sand. This is a hungry crop – spread a general balanced fertiliser over the soil a week or so before sowing and rake in. A rate of 60g per square metre is ideal.
Timing: If you live where the autumns are long and cool and frost is rare, you can plant two crops. Sow the first crop 12 to 14 weeks before the last frost in spring. In mid-January, sow the second crop indoors. If your area could experience frost during the winter, plant a frost-tolerant variety for your second sowing.
Sowing: Sow seeds indoors 12 to 14 weeks before the last frost date. Seeds can also be sown direct later but will give smaller plants.
Sowing Indoors: Sow the seeds thinly and evenly 6mm (1/4in) deep in moistened potting mix and cover them lightly with vermiculite or sand. Keep the soil temperature at about 70°F until the seeds germinate. Move the seedlings under grow lights or into a very bright window. Thinning the seedlings will encourage more rapid growth, but it isn't necessary if you keep them well fertilized. When the grass-like seedlings get to be 15cm (6in) long, cut them back by 4cm (1½ to 2in) You can use the part you cut off as you would chives. Harden off the plants before transplanting into the garden starting in late October or early November (the plants will tolerate light frost). You can also transplant later or sow seed directly outdoors for smaller plants.
Transplanting: When the seedlings are about the diameter of a pencil, they are ready to transplant outside. Planting deeply helps to blanch the stems. Use a dibber (or a rake handle - great for making perfect holes), and make holes 15cm (6in) deep and 22cm (9in) apart. Make the rows 38cm (15in) apart. Mark the row clearly so that, when weeding later you don’t remove plants by mistake. Drop the leek seedlings into the holes leaving just the tips of the leaves showing. Do not fill in the holes or try to cover the roots with soil or even firm them in. Just fill each hole with water from the watering can and this will wash some soil over the roots and be just enough to tighten the little plants in. Over time the holes will fill up gradually.
Sowing Direct: On the allotment seeds are best sown in rows, 35 to 40cm apart. Mark a straight line and use the corner of a rake to make a shallow groove in the soil, about 1cm deep. Sow seed thinly along the trench, cover with soil, water and label. When seedlings have three leaves each, about four to five weeks later, thin to leave plants every 15cm – the seedlings you remove could be used to plug gaps elsewhere.
Cultivation: Keep the leek bed moist in dry weather and hoe regularly to keep the weeds down. Except for exhibition plants there is no need to feed the leek plants. But if you want to be sure of a good crop you can feed with weak liquid manure and hoe in a small dressing of nitrate of soda. After the holes the leeks were planted in have filled up, push some soil up to the stems with the hoe. This will make sure you will have a good length of white (blanched) stem. Do this earthing up gradually over a period of three weeks because if done too much to soon, the leek plants may rot. Mulch will help to retain moisture over summer.
Rotation considerations: Avoid following onions, shallots, garlic and chives.
Good Companions: Beet, carrot, celery, garlic, onion, parsley and tomato.
Bad Companions: Beans, peas.
Harvesting: Pull up as and when required. Harvest them by lifting carefully with a fork, aiming to avoid damaging neighbouring crops.
Storage: They will keep in your refrigerator for at least a month, but the quality will not be as good as when freshly harvested. The variety and packaging will affect the storage life. And digging the plant up doesn't stop it's growth - it will continue to grow (very slowly) for months even under refrigeration, but this growth reduces the quality of the stems.
Types of Leeks:
Leeks are relatively easy to grow, and have a pleasantly sweet mild onion flavour, and depending upon variety, can be harvested from mid-summer, through the autumn and winter, until late spring. This ability to provide a crop through the winter months is very useful at this lean time of year.
A range of varieties (cultivars) with differing times of maturity ensures the widest cropping period, though leeks have the ability to stand in the ground over a lengthy period without deterioration.
Although many growers will choose to grow just one or two varieties, leeks are available six groupings to ensure the very longest cropping season, from late July stretching through to April of the following year. These six groups are derived from commercial usage, rather than botanical differences.
Early Leeks (type 1)
These are varieties possibly derived from ‘Copenhagen Market’ which give tall slender stems (greater than 250mm) with pale green leaves that form an acute angle to the stem. On the continent they are known as ‘summer’ leek, and in the UK are suitable for cropping from August, September and October, but cannot stand in the ground later than this as they are not winter hardy.
Early Leeks (type 2)
Also called Gennevillier types, they are similar to early leeks (type 1), but with stouter stems. Some varieties are a little shorter in length but their season of cropping may be extended into late November/early December.
Swiss Giant Leeks
These are early ‘mid-season’ leeks suitable for harvesting into the early new year. They have medium long stems (c.150mm) with mid-green leaves.
Autumn Giant Leeks
These are the true mid-season leeks, recognised as traditional crops at this time of year. These varieties are usually very vigorous, with a stem of medium length (c.120mm) and very stout. The leaves are broad and extensive and mid green in colour. ‘Elephant’ and ‘Mammoth’ frequently occur in the variety names. They are suitable for harvesting from April through to August in all but the most severe winters.
Winter Giant Leeks
These are the continental ‘winter’ leeks - medium short stemmed (c.100mm) with dark green foliage. They are useful for cropping during the June to October period.
These are the most winter hardy of all the leeks, immediately recognisable by their dark green leaves which assume a purplish tinge in cold weather. The leaves extend from the stem at an obtuse or broad angle. They have short stems and are really only suitable for growing as transplants in dibbled holes (to increase length of stem blanching) These types, which include such varieties as ‘Blue Solaise’ should only be harvested late in the season, as much of their yield increase is derived from early re-growth in August and September.
The leafy top is often referred to as the ‘flag’ and its colour can vary from variety to variety, from pale bright green through to a darker deep green to a blue-grey green. Leaves may grow at varying angles to the stem, again depending upon variety; the earlier maturing varieties usually being at a more acute angle.