Leek

Allium porrum

1. Identification and origin

Allium porrum L. is a biennial herbaceous species of the Amaryllidaceae family, known as leek. It is native to the Mediterranean region and the Middle East, having been cultivated since Antiquity. It is currently produced worldwide, especially in temperate regions, being one of the most important leafy vegetables for fresh consumption and cooking.

2. Economic importance

Leek is widely used in gastronomy, both fresh and in culinary preparations and the food industry. The crop has great economic importance in intensive horticulture, outdoors and in greenhouses, due to the high demand and the relatively long cycle, which allows staggered production. It is valued for the quality of the white and tender pseudostem.

3. Botanical characterisation

Biennial herbaceous plant grown as an annual. It has long, linear and overlapping leaves, forming a white pseudostem that constitutes the marketed part. The leaves are bluish-green and waxy. In the second year, the plant produces an erect floral scape, ending in an umbel with white or pinkish flowers. Reproduction is carried out by seed or by seedlings produced in a nursery.

4. Soil and climate requirements

It prefers cool and temperate climates, with optimum temperatures between 12–20 °C. It tolerates moderate cold, but heavy frosts can damage young leaves. It requires deep, fertile, well-drained soils rich in organic matter, with a pH between 6.0–7.5. Irrigation must be regular and uniform, avoiding waterlogging. “Earthing up” the soil around the pseudostem favours blanching and commercial quality.

5. Main pests

• Onion fly: Delia antiqua causes damage to the roots and pseudostem.
• Thrips: Thrips tabaci causes silvering and reduction of the photosynthetic area.
• Nematodes: Ditylenchus dipsaci affects internal tissues and reduces vigour.
• Aphids (Aphididae): sap sucking and transmission of viruses.
• Slugs and snails: perforations in young leaves.

6. Main diseases

• Downy mildew: Peronospora destructor causes chlorotic spots and grey sporulation.
• Rust: Puccinia allii causes orange pustules on the leaves.
• Basal rots: Fusarium oxysporum f. sp. cepae affects the collar and pseudostem.
• Botrytis: Botrytis porri causes necrosis and foliar rots.
• Root rots: Pythium and Rhizoctonia affect seedlings and young plants.

7. General crop management

It includes the use of vigorous seedlings, planting in well-drained soils, balanced fertilisation and regular irrigation. The progressive earthing up of the soil improves the blanching of the pseudostem. Crop rotation with non-host species reduces soil problems. Monitoring of pests and diseases, together with good ventilation in greenhouses, is essential to maintain commercial quality. Harvesting must be carried out when the pseudostem reaches the desired diameter and length.

Bibliographic references

• EPPO Global Database – Allium porrum.
• CABI – Crop Factsheets – Allium porrum.
• Rubatzky, V. E., & Yamaguchi, M. (2012). World Vegetables. Springer
• Kays, S. J. (2011). Cultivated Vegetables of the World. Wageningen Academic Publishers
• FAO – Technical documents on vegetable production
• Brewster, J. L. (2008). Onions and Other Vegetable Alliums. CABI