First off onions are day light sensitive and consist of Long Day, Intermediate Day & Short Day varieties. I am in the northern section of North America so I have to grow Long Day varieties which need 14-16 hours of day light to start developing bulbs. With them needing such long daylight hours to start developing bulbs it gives them a longer season to grow leaves and get big. The more leaves and the bigger the plant, the bigger the bulb will be when it starts developing.
This year I am growing Prince F1 for a large yellow storage onion and Armador F1 Shallots for a small storage onion.
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Planting Onions Seeds
There is a couple ways of growing onions, the first being in a community pot or flat. They don't mind growing close together and a community pot makes them very easy to look after. You can also plant them in cell packs and thin to 3 plants per cell. I choose to use terracotta half pots so I can use them year over year, those cell packs don't seem to hold up well for reuse.
Now fill your container of choice with potting soil, pack it down and water until the soil is completely saturated. Potting soil has a tendency to repel water so if it is not saturated to begin with it can be problematic to get wet later. Once the soil is good and wet, let the excess water drain out for about 10 minutes.
With the soil wet, start to broadcast your onion seeds over the area. I usually use extra seed and then thin when they germinate. You can use the extra sprouts to transplant into bear spots and eat the left over as onion sprouts.
Once your seed is in place top dress your containing with about 1/2" of soil and gently tamp the soil down. From this point no more watering is needed. The top dressing will become saturated from capillary action from the moist soil below.
Then label your pots and place them in a seed propagator. Place the propagator under a grow light or on a window sill and make sure that the light is on for less hours then need to produce bulbs. For the Long Day onions I am growing I will leave the light on for 8-10 hours a day.
The seeds should germinate within 4-13 days depending on soil temperature and need little care well in the seed propagator. Once most seed has germinated and grown a little bit, remove the pots from the seed propagator to avoid any mold growth. Once the onions start to look crowded thin to approx. one plant every 3/4" and the tops can be trimmed to 4"-5" to bulk the seedlings up as well as control leggy growth. When the weather is better I will move these into my cold frame and then transplant them into the soil once my area is frost free.
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