Leek production

Introduction

The leek (Allium porrum) originated in Middle Asia, with secondary centres of development and distribution in Western Asia and the Mediterranean countries. The leek has been cultivated in Western Europe since the middle ages and found its way to North America with early settlers from Europe. It is a more popular vegetable in Europe than in North America, but potential exists in Ontario for replacement of imports from the United States and market expansion by increased domestic consumption as consumers' eating habits become more varied.

Nutrition

Leeks may be grown successfully on a wide range of soil types but deep topsoil is preferred for vigorous plant growth and above average yields. Soil pH 6.5-7.0 is most desirable. Coarse sands should be avoided because sand particles under the leaf sheaths are not palatable to the consumer. The soil should be prepared with green manure plough down or farmyard manure to enhance organic content and provide nutrients and extra moisture holding ability for the crop. Leeks require about 200-250 kg N (nitrogen) per hectare, preferably in three installments - one-third pre-plant incorporated, one-third as a side dressing, and one-third as a top dressing when the leaves are dry. Phosphate requirements of leeks are not very substantial and applications of 50 to 100 kg P₂0₅ per hectare are adequate. Potash requirements are also low and 150 to 200 kg K₂0 per hectare as sulfate of potash are adequate.

Varieties

There are four basic groups of leek based on season of maturity:

1. Summer leek
2. Autumn leek
3. Autumn and Winter leek
4. Winter leek

In Ontario only the first two groups are feasible as the climate does not allow overwintering leek to be of suitable quality to market in the early spring.

Leek cultivars differ significantly in growth habit which affects the final product. They vary from long, green narrow-leaf types with long slender white stems to long wide-leaf types with thicker shorter white stems and blue green leaves.

Growers should check with seed company representatives for varieties most suitable for the climate and market requirements.

Transplanting

Traditionally, transplants for summer leeks are started in flats with soilless mix in early March in the greenhouse and transplanted into the field in late April or early May.

If "288 cell" trays are used for raising transplants a four cone type or carousel type transplanter can be utilized to plant the young plug cell plants.

Bare-root transplants for late fall maturing should be seeded in outside seedbeds late April or early May to be of sufficient size to transplant late June early July. Seedbed row spacing is determined by the equipment available to keep the seedbed weed free. In-row spacing of seeds should be such that 70 vigorous plants per metre of row can be produced for transplanting. 

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Figure 1. Plug transplants offer greater uniformity and labour saving with mechanical transplanting over bare-root plants.

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Figure 2. In-row spacing is important to maintain uniform sizes till maturity.

Spacing of the leek crop in the field is critical to maximize returns per unit area. Usually the in-row spacing is 10-15 cm and the between-row spacing is more dependent on available equipment to maintain a weed free crop. Depending on weather conditions, a post planting irrigation is desirable to ensure rapid establishment. Further irrigation will be necessary if rainfall is deficient during the hot summer days when rapid growth should take place.

Weed Control

There are no registered chemicals for weed control. Alternatives that can be useful are: stale-seedbed technique preplanting, selecting fields with a low weed population (crop rotation), and using row spacing that can be easily cultivated. If the size of the crop warrants, special row crop tillage equipment is a good acquisition.

Insects and Disease

Thrips (Thrip tabaci)

Onion thrips are quite common and migration from surrounding grassy weed hosts is likely to occur. When thrips infest leeks, feeding produces silvery-white mottled lesions on the leaf surface To examine for thrips (if they are suspected) remove leeks from the soil and peel back leaves one at a time to reveal younger emerging leaves in the center of the plant (see OMAF Factsheet, Thrips on onions and Cabbage, Order No. 99-027).

Onion Maggot (Hylemya antiqua)

Onion maggot is widespread and three generations of maggots occur during the growing season. Visual checking of the plants is required to determine if there is maggot activity (see OMAF Factsheet, Onion Maggot, Order No. 91-005.

White Rot (Sclerotium cepivorum)

This soil-borne fungal disease can be devastating if present in farm soils. The fungus survives as sclerotia in the soil for long periods. Leeks should be grown on lands that have not grown an onion family crop recently. Sanitation through cleaning of field equipment and disposing of cull leeks away from production areas is important in preventing the spread of this disease. The first signs are yellowing and dying back of the leaves beginning at the tips and progressing downwards. Young plants wilt and collapse and are easily dislodged from the soil, revealing a dense white mass of mycelium in which minute black sclerotia are embedded. Cool, wet growing seasons favour the development of white rot.

White Tip Disease (Phytophthora porri)

White Tip disease is a fungal disease that can become prevalent after heavy rainfalls later in the summer. Affected areas have a water-soaked appearance at the leaf margins near the tip of the leaf. Older plants when slightly affected, wilt rapidly after harvesting. Fields with low lying areas where drainage is poor are the most likely places for white tip disease to develop. This disease can persist in crop residues.

Rust (Puccinia porri)

Rust is a fungal disease that shows up frequently in a mature crop in dry summers. It can reduce market value and yield of the crop severely. The disease is recognizable by the rust-coloured spores on the upper and lower leaf surface.

Leeks are also subject to diseases that are usually found on onions i.e. pink root, purple blotch, downy mildew, botrytis leaf spot, botrytis neck rot, and smudge. See OMAF Factsheet, Identification of Diseases and Disorders of Onions, Order No. 95-063.

In case of any problems in the leek crop, contact an OMAF Vegetable Specialist for appropriate remedial action where possible.

Harvesting

When leek plants are mature, the outside lower leaves display some senescence that is readily detectable. Physical size should meet market requirements for thickness and length.

Many different methods of harvest are possible but the main method is with an undercutting knife similar to one used to loosen cole crop seedbeds. A modified version of this under cutting knife incorporates a vibrating share of various widths.

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Figure 3. A vibrating lifter loosens plants and removes a large part of the soil from the roots.

Equipment is available for mechanical harvesting, leaf trimming and root trimming in the field. The machine undercuts the roots of the leek plants with a vibrating knife that aids in removing excess soil from the roots. As the machine travels forward, the stems of the leek plants are held upright between two belts and in an almost simultaneous operation, the excess roots and the leaf tops are trimmed by sets of rotating disk knives. Small scale growers cannot justify the cost of this extensive mechanization, hence, all the above operations after lifting and loosening are done by hand. The dead outer leaves are stripped in a packing shed after which roots and leaves are cut off on a trimming and rinsing table. Once rinsed off by spray nozzles, the stalks are packed into cartons, 12 bunches per carton.

The harvesting season starts about mid-August and continues until the ground freezes in November. Most growers then start marketing from storage. During the warmer days of summer and autumn it is most desirable to ice the product before shipment to retain maximum freshness. Some growers pack with ice all season long.

Storage

Leeks store well for 2-4 months at 1-3°C and high humidity provided they are harvested and placed immediately into storage. For ease of handling, leeks are stored in 40 cm high pallet bins made of planks for better aeration and conditioning while in storage. The bins may be stacked several levels high depending on storage facilities. The leek crop can then be removed from storage as time and market conditions permit over a four month period.

Marketing

Markets usually accept a wide range of stalk sizes. The standard method of packaging leeks is three uniform sized stalks per bunch and twelve bunches per box. The grower usually selects bunches to give a uniform grade standard in a box. Physical size of leek is not important but bigger stalks command better prices than petite stalks. Wholesalers prefer bunches that are uniform within the bunch and uniform throughout the box.

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Figure 4. Uniformity of bunches is more critical than physical size in a bunch.

Yield Potential

Leek yield potential is dependent on plant population. Row spacings of 60 cm (24 in.) and plant spacings of 10 cm (4 in.) will give a stand in excess of 160,000 plants per hectare. If we consider a harvestable crop at 80% of original stand, approximately 3600 cartons containing 12 bunches of leeks will be marketed. Similarly, a row spacing of 91 cm (36 in.) and plant spacing of 15 cm (6 in.) will give a stand in excess of 70,000 plants with harvestable crop yield of approximately 1600 cartons containing 12 bunches.

Quality

Leeks of good quality have fresh green tops and well-blanched stems or shanks. In order to attain 15-20 cm or more of white shank, a common practice is to plant the young transplants in a shallow trench 10-15 cm deep and as the plant grows the rows are cultivated and gradually hilled to promote more white stalk development. The greater the length of white shank, usually the more premium is the product. Wilting and yellowing of the top will downgrade the quality. Bruised tops are unimportant if they can be trimmed without spoiling the appearance.

Crooked stems and bulbous bases are not desirable characteristics and should be avoided in order to maintain a premium pack.

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Figure 5. Leek plants that are hilled in the row yield greater quantities of high-quality blanched shanks.

Related Links

• Thrips on onions and cabbage
• Identification of diseases and disorders of onions