Onion

Young onion plants in the midst of winter in a greenhouse.

Chunks of information

Bulb onions can be planted very early. Onions anyway must be planted or sown rather earlier in the year as they don't appreciate warm weather but they do need long days for the growth of the bulb (ca 16 hours daylight, a little dependent on which cultivar you grow). The bulbs grow as soon as the temperature is above 6 C. So you want to sow and plant them early in the season so they can grow when the days get longer, but later the days will shorten again. In spring they grow well in a cold frame or a greenhouse, but not anymore when it gets hotter.
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.

Onion plant that grew from a small bulb, planted before the winter in a greenhouse, shown here after several weeks of freezing temperatures.
Planting the first onion bulbs in the season, the end of February 2012. A cover of plastic will advance the growth of the bulbs until it gets warmer. In a few months they can be harvested as spring onions, or later the bulbs when they have fully grown out.