Date: Aug 2007 Author: Eric Darley
Leeks (Allium porrum) are of the onion family, Amaryllidaceae. They are grown for their blanched thickened ‘stems’ which are actually elongated leaf bases. Delicious in soups, stews and other dishes, they can also be cooked on their own or eaten raw in salads. Although their odour is somewhat pungent they have a sweet flavour.
Leeks are mostly grown in small, outer-urban market gardens, but are becoming popular with home gardeners. A good substitute for onions, they are very adaptable and can be grown under a wide range of conditions.
While onion varieties are fairly specific in their temperature and daylength requirements, leek varieties are less so. The main reason for this greater adaptability is that, unlike onions, leeks do not form bulbs, nor do they have a rest period. Thus they can be harvested over a long time. Leeks are also more cold tolerant but, like onions, are induced to bolt (form flower stalks) by low winter temperatures. The best temperature range for growth and production is 13°–24°C.
Leeks prefer a deep, fertile, friable soil with plenty of organic matter. A well-drained, light-textured soil makes planting and harvesting easy and contributes to production of clean and presentable produce. Leeks do not like acid soils and prefer a soil reaction of pH 6 or higher. If the soil is too acid, agricultural lime or dolomite can be applied at about 3–5 t/ha and mixed in a month or two before planting.
Thorough land preparation is important to kill weeds and produce a suitable tilth for planting. The first working should be deep, with subsequent cultivations aimed at producing a moist, friable tilth. If the soil is not very fertile, apply a liberal dressing of well-rotted animal manure to the preceding crop. Do not apply organic animal products, such as manure and blood and bone, immediately before planting because they attract the corn seed fly whose larvae will attack the germinating seeds.
If a liberal dressing of animal manure was applied before the previous crop, use a mixed fertiliser such as Grower 11® ( 11% nitrogen, 14.6% phosphorus and 9.1% potassium) at the rate of 150–200 kg/ha. (Other mixtures with a ratio of about 1:1:1 are also satisfactory but use them at a higher rate because of their lower analyses.) Band the fertiliser into the soil below the crop row either at or before planting. Side-dress with a nitrogen fertiliser at 30 kg/ha of nitrogen if growth lacks vigour and is not bright green.
In New South Wales, leeks can be either sown direct, or in a seedbed and transplanted later. Planting periods are February to May in coastal and inland districts, and September to December on the tablelands. Musselburgh and London Flag are the most popular varieties.
There are two systems of production with direct drilling. One is to have beds with five or six rows each about 30 cm apart. Earthing up is not possible with this method and thus the ‘stems’ are shorter. With the other system the rows are 45–50 cm apart so that the ‘stems’ can be blanched by earthing up. Plants need to be at least 10 cm apart to develop a ‘stem’ of reasonable size. From 1.5–3 kg of seed is needed per hectare.
For seedling production, sow the seed 1 cm deep in rows 10–15 cm apart in nursery beds. When the plants are almost pencil thick and about 20 cm high, transplant them into trenches about 23 cm deep and 60 cm apart. Space the seedlings 15–18 cm apart along the row and then water.
It is essential to control weeds. Where plants are in trenches, gradually filling in the trench and banking up the soil may be sufficient to suppress weeds. A herbicide must be used with direct-sown crops.
As a monocotyledonous plant, leeks share some characteristics with grasses. They have a fibrous root system and these roots develop from the base of the plant. Periodic wetting of the base is necessary to encourage formation of a large number of roots. When plants are four months old the roots will have penetrated to a depth of 45–60 cm.
In general, water often enough to keep the soil moist and so avoid any check to growth.
Leeks are harvested when large enough, and this is mostly 5–6 months after planting. They are more difficult to harvest than onions because the plants are deeper in the soil and the roots more active and matted at the time of harvest. One method is to undercut the plants with a tractor-drawn knife, pull the plants from the soil, wash the roots and stems, trim the leaves and roots, and tie in bunches of three to six.
Although leeks should be eaten as soon as possible following harvest, they will store quite well for up to three months if held at 0°C and 90–95% relative humidity.