It is often said that the traditional time for planting garlic is on the shortest day of the year – 21 December – with harvest at midsummer – 21 June; I am pretty sure this is one tradition my dad related to me. I have heard some folk say that Boxing Day is the traditional time for setting the onion seed. I think anytime from late December through January is good to give the onions a start if growing from seed.
Sets of Japanese onions can be planted in late autumn and other onion sets in spring, say, mid March to mid April. Planting sets too early just means that they sit unhappily in cold wet soil, perhaps losing a few in the process. Seed though can be started early in pots, modules, or seed trays, with a little protection.
Onion seed does not need much heat to germinate. Optimum temperatures might be somewhere around 20°C but they will do fine above 15°C and even at 10°C germination should still be reasonable and not too drawn out. Below this, germination rates will fall off and germination times increase markedly. Onions also take a while to get going and need a fairly long season to develop good bulbs; they bulb up in response to increasing daylight hours so in order to form large bulbs they need to put on a healthy amount of vegetative growth before day length increases significantly. This makes sowing direct problematic. Waiting until the spring, when soil temperatures are more conducive to germination, means a shorter growing period. Instead, onions can be started earlier and coddled in a propagator to give them the head start they need, before planting out when soil conditions are much improved.
If you wish to follow tradition, perhaps sow on Boxing Day, though the only reason I am heading to the kitchen garden at that time of year is to grab some veg for dinner. Early January is plenty soon enough for me, and this week we set four varieties of onion in module trays of multipurpose compost. This year we have Tropea Rossa, a wonderful red onion from Calabria, that is famed throughout Italy; Rose de Roscoff, the pink tinged onion of fine quality associated with the “onion Johnnies”, those Breton farmers of yesteryear selling beautiful strings of onions from the handlebars of their bicycles; Rossa Lunga di Firenze, an elongated red onion from Florence, which is good taken small and thus may be sown in several batches, or then sown early for larger bulbs; and finally Borettana, the classic ‘cipollini’ or small onion, again, from Italy, this one forming small flat bulbs with sweet white flesh that is ideal for grilling or pickling. I will probably use the Florence and Borettana onions partly for mature bulbs and partly for young onions.
We placed our modules in a propagator set to 18°C in our smallest greenhouse. In the current cold spell, with night time temperatures dropping below zero and day time temperatures not climbing far above zero, the propagator will struggle to reach 18°C. However, it should still be warm enough for onion seed to germinate well.