Early March is about the latest time to sow onion seeds if you want large bulbs come August. I have successfully grown onions by direct seeding in the ground in mid April, but that is really mostly going to create smaller bulbs for pickling or cooking soon after harvest.
Onion growth can be split into three real phases:
2. Growth of foliage up to the summer Solstice
3. Bulb development until foliage collapses in mid to late summer, when harvesting takes place.
Germination of seed is a bit of an art form, particularly early in the year (prior to mid February) when conditions are not promoting natural germination. Some suggest that the best natural conditions are soil temperature of 13-14C, with nights down to 10C (with germination occurring in 10-14 days), whereas I have often found better germination with a pulse of higher heat for two or three days, which can be delivered a week after sowing seeds in trays or modules. Ultimately, you have to try things out yourselves.
For the largest bulbs, transplanting seedlings at the ‘hook’ stage (2-3 weeks after sowing) is recommended. I do this into 8cm pots, transplanting 40 seedlings intending to plant out 35. For about a month, this means that onion seedlings may dominate your sunny windowsills, which is why I sow my Kelsae onions in early February and my main batch of tomatoes in mid March. This means that the windowsills become available for tomato seedlings just as onion plants can be moved elsewhere. It all depends how much indoor space you have.
Once seedlings have been transplanted, daily foliar water sprays benefit indoor plants, as will a weekly foliar feed with a dilute liquid seaweed. Watering from underneath (I use 600ml once a week for a dozen 8cm pots in a 40cm tray) is the way to ensure uniform moisture without flooding.
4 weeks after sowing, >90% of seedlings should have a well developed first true leaf (one cotyledon and one true leaf means two leaves). The ones that haven’t will likely be discarded. Of my 39 2017 Kelsae transplants, 38 have first true leaves on day 29.
Transplanting into the garden can happen from the equinox through to 21st April, depending on the spring weather and how far developed the young plants are. Early transplantation is accompanied by protecting the plants with fleece for a month, or until temperatures at night are around 8-10C.
My timetable for Kelsae onions this year was/is:
Sowing date: 5th February 2017
Transplantation date: 21st February 2017
Setting out date: April 1st or April 9th 2017.
IF you sow onion seed this week or next, you can still achieve good bulb sizes.
For an interesting read about some historically interesting contributors to the art of growing onions, try reading:
‘Growing Onions and Shallots’, Daniel A. Calderbank (Ross Anderson Publications) ISBN 0-86360-027-1, published 1986