Regeneration of onion and leek

Seed dormancy



Onion is vernalized in the bulb stage. The degree of cold requirement is very dependent on the type of material.
Leek is vernalized in the mature vegetative plant stage. Potted plants over-winter in a glasshouse at 5-10°C.

Crop phenology

Long and short day onion cultivars differ in sowing and planting dates. This will be indicated at the appropriate sections.
phen allium.jpg


Onion is sown in the first half of March in seed trays, or directly in peat pots. After germination seedlings, when appropriate, are transplanted to peat pots in greenhouses at a temperature of 14/10-12°C day/night. Short day cultivars are sown at the end of January. Short day cultivars that give serious problems with regeneration will be raised in the phytotron for flower initiation without bulb formation. Seeds are sown in August at 18° C, transplanted into pots and after the plants reach a diameter of about 7 mm transferred to 10° C and a day length of 14 hours.

Leek is sown in June in deep seed trays in a glasshouse at a temperature of 18-20°C. After one or two weeks the trays are placed in a glasshouse of 15°C.


Onion is planted at the end of April. Two weeks before planting, the peat pots are placed on flat trays in a sheltered area in the open air for acclimatization. Planting distance: 8 x 40 cm (appr. 250 plants per accession). The short day cultivars are planted in early April.

Leek is transplanted directly into pots. This measurement is taken to prevent Fusarium attack, which can be serious, because of injuries during uprooting the plants from the field. Pots are placed outside in July. Per accession 80 plants are regenerated.


Vegetative stage

For onion the bulbs are pulled out of the soil when at least 75% of the plants have dropped their foliage. The bulbs remain in the field until the leaves are completely dry (appr. 4 weeks). Leaf material and soil are then removed from the bulbs, which are put in boxes. After removing the bulbs from the field, they are dried under natural conditions in a well ventilated open greenhouse for 4 weeks. They are then potted and kept in a glasshouse for over-wintering at 5-10°C. The accessions of which the bulbs cannot be stored, are immediately desinfected and potted after harvesting.
Pots with leek plants are also transferred for over-wintering to a greenhouse at 5-10°C.

As soon as flowers appear, the plants are transferred to isolation rooms. As means of isolation, gauze screens are used to create compartments inside an unheated greenhouse. Insect pollination takes place by using blowflies.

Generative stage

Harvesting is carried out manually when the majority of plants of an accession have ripe seeds. If there are large differences in seed ripening between plants of one sample, the harvesting is done in phases. To prevent lodging, plants are staked. The entire inflorescence is harvested and put into labelled paper bags and directly transferred to the oasthouse for drying and cleaning.

Pest and disease control


  • Seed and seedling pathogens
  • Bulb pathogens before potting (only short day onions)
  • Neck rot (Botrytis aclada) at the stage the foliage starts to fall in the field (onion)


  • Downy mildew (Peronospera destructor) (onion) 
  • Botrytis (Botrytis squamosa) (onion)
  • Rust (Puccinia allii) (leek)
  • White tip (Phytophthora porri) (leek)
  • Purple blotch (Alternaria porri) (leek)
  • Onion fly (Delia antigua) (onion and leek)
  • Thrips (Thrips tabaci) (onion and leek)
  • Aphids (onion)
  • Spider mites (onion)
  • Leek moth (Acrolepia asectella)(leek)
  • Weeds (onion and leek)

The regeneration procedures for the wild related species of onion and leek can differ. Not enough experience has been obtained to give exact protocols. Part of the wild species with dormancy is sown in November and these without dormancy in January. Plants die off in June and their bulbs are stored and replanted in the autumn.