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Young onion plants in the midst of winter in a greenhouse.
Start a vegetable garden - Lesson 9: Onions

You can either sow onions or grow them from small bulb onions. The cultivation starting with plant onions is easier but the resulting onions are somewhat less suitable to store through winter. You sow or plant in March and April. It takes long before you can harvest so sowing in March is preferred.

Grow them on a place that is rich of humus but don't put fresh compost to the field. Grow them on a field that was well fertilized the year before.

It is also possible to sow in August. Cover the plants with straw when it freezes and plant them out in the spring. Onion can rather well tolerate frost.

Onions need long days for the growth of the bulb, ca 16 hours of daylight, depending on which cultivar you grow. The bulbs grow as soon as the temperature is above 43 F (6 C). In spring onions do well in a cold frame or a greenhouse, but not anymore when the weather gets hotter.

You can harvest the onions as salad onions when they are still green. Otherwise you let them grow until August and you take them out when the leaves get yellow and dry. Pull them out together with the leaves, hang them to dry and keep them in a cool but dry place where they stay well until next spring.

Chunks of information

  • Bulb onions can be planted in autumn (depending also on the species), but when done outside, many disappear because of the weather. When done in a greenhouse, they come through winter but don't grow well in spring. Better is to plant them outside in early spring, under a cover of plastic, and then remove the plastic as soon as the bulbs sprouted well.
  • Carrots can be affected by carrot flies. Crop damage is caused by the up to 10 mm long larvae (maggots) feeding on the outer layers of the carrot root. Onions can be affected by onion flies which resemble houseflies. They lay their eggs in groups on the shoots, leaves and bulbs of host plants and on the ground nearby. The larvae create large cavities in bulbs. But onion flies don't seem to like the smell of carrots and carrot flies don't like onions. So onions and carrots make perfect companion plants. I always plant bulb onions in the lines of carrot seeds, each one or two hand widths an onion. I've never had problems with either carrot flies or onion flies but heard of many gardeners who had this problem. This method also has the advantage that you can see where you sowed the carrots. Carrots take long to germinate but bulb onions sprout fast.
  • Onion has many medical uses.[1]


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