Leek is a herbaceous, biennial plant that, in most cases, is cultivated as an annual crop in the hardiness zones 5 to 9. Its scientific name is Allium ampeloprasum var. porrum, and it belongs to the Alliaceae family, together with onion and garlic. Leek is also closely related to Chinese leek (Allium tuberosum Rottler), green onions, and elephant garlic (Allium ampeloprasum var. ampeloprasum). The plants are cultivated for their edible shank and leaves.
The plant consists of a white cylindric pseudostem (shank), formed by multiple layers of bundled leaf sheaths that can be up to 25 cm long (10 inches) and with 5 cm (2 inches) diameter. The base has relatively little bulb development. Leeks’ leaves are broad, elongated, and green; Leek plants can become up to 60-90 cm (2-3 feet) tall.
The plants thrive and give good yields when the temperature ranges between 13 and 24 °C (55-75 °F). The flower formation can be initiated if the plants are exposed to low temperatures (5 °C is optimum). This process is called vernalization. Under such conditions, the leek plant produces one inflorescence consisting of 300-400 small white, white, and purple flowers. These flowers are not self-fertile and need to be cross-pollinated by insects to produce seeds. These numerous, small, black seeds are used for plant (commercial) propagation. When cultivated as an annual, leek needs around 70-120 days (or even 150 days) to reach maturity (from sowing to harvesting) since it has a relatively slow growth rate (depending on the cultivar). The crop is propagated by seeds usually sown in a greenhouse. For successful germination, seeds require a soil temperature of around 21 °C (70 °F). The young seedlings are transplanted to the open field 4-8 weeks after seeding and always after the latest frost.
There is a large variation among the cultivars of leek available in the market. There are both hybrids and open-pollinated varieties of leeks available.
Leeks are categorized into four main groups depending on their maturity season.
- Summer leeks.
- Autumn leeks
- Autumn-Winter leeks
- Winter leeks
Each category includes numerous leek varieties of different characteristics, and not all are suitable for cultivation in different regions. Moreover, different or further classifications of the commercial leek cultivars can be done based on their resistance to cold and bolting, the length of the shaft, etc. Leek varieties harvested during summer have the shortest life cycle (early-season cultivars). Some common examples are: Varna, King Richard, Pancho, Columbus, Rival. On the other hand, the ones harvested in winter have the longest (late-season cultivars). Some common examples are: Otine, Titan, American Flag, Durable, Bandit, Giant Musselburgh, and Laura. In regions where more than one leek type can be successfully cultivated, many farmers choose to plant varieties from at least two types to supply the market for a longer period.
References
https://extension.umn.edu/vegetables/growing-leeks
https://www.sciencedirect.com/science/article/pii/B012227055X010403
http://www.omafra.gov.on.ca/english/crops/facts/91-004.htm
https://www.dpi.nsw.gov.au/agriculture/horticulture/vegetables/commodity-growing-guides/leeks
https://hort.extension.wisc.edu/articles/leeks/
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