Thanks to Nick Brake for his helpful grow guide
Below is the way I grow my onions and it seems to work for me. In 2018, I grew an onion weighing just over 17lbs (18.4lbs with foliage) and won the World-Famous Harrogate Onion Championships. This is probably not the way some growers do it, so this is only a guide, and some may not agree with the way I do things.
Starting Off the Seeds
Seed (good seed is essential to grow big onions) are set to germinate in the first week of October at around 16c in a seed compost and cover seeds with just a couple mm of soil – nice and fine is best, I use Levingston f2+S. It takes around 7-10 days for the seedlings to emerge and I then leave them until you get to the crooked stage.
I then carefully repot them again into 7cm pots into Levingston f2+ s / 25% vermiculite to give the soil some structure as onions do like open structure and not much resistance in the soil. These will stay in these pots for probably about 4 weeks or until the pot is nice and full of roots but do not let them become root bound.
Next step is potting on again into about 15cm pot and this time I use about 75% general purpose compost (good quality) and JI 1 + 25% vermiculite. I also when watering them give a bottom feed of diluted liquid seaweed feed, just to boost and give the roots some nutrients. Again, these now stay in their pots until outgrown. Next pot up is 50% general purpose compost and JI2 50% and perlite.
Final pot is JI3 / m3 and vermiculite. I do add other nutrients to the final mix but not much, just some Blood, Fish and Bone, a hand full of general-purpose slow-release pellets per 50 ltr pots
Make sure you support the plants all the way through its grow and try not to restrict the leaves as the more exposed to light the better they grow and stops the flutes from getting stuck
Avoid overwatering and under watering – best water little and regular (do not let them dry out) – no feed is needed in the soil at all until final pots then it is up to the grower. I use a high NPK for growth and when bulbing up picks up I increase P and drop back the K a bit. I would recommend using the Plant Magic Old Timer Organic Grow to start off with and then from July onwards, Plant Magic Old Timer Organic PK 4-8 until the end of the grow as it helps to firm up the bulbs for heavier bulbs and better storage. This technique worked for me. It is a good product and easy to use.
Lighting is of a personal choose and type. I use a mixture of full spectrum LED, T5 and CFL , duration of lights varies with growers. Some use 12hrs on and 12 hrs off, continual through the growing season until the day hours take over, some do 24hrs for a period of time then follow daylight hours. There is no real science with this, but it is essential that they get light and require lots of it as onions and most plants will become leggy and generally look unwell without the correct light levels.
Heat is very important, and I heat from the bottom and keep about 16c at night through the winter and spring and try to keep the heat below 25c during hot weather although this is not easy but with forced air circulation you can get near it
Rules of growing onions
1. Good seed required for really big onions and not easy to source good seed
2. Good quality compost throughout every pot on
3. Water regular and do not let them get wet or dry – water from the bottom if you can and make sure water is at 12c + – I always use rainwater
4. Feed if you think it is needed but generally, they are fine until the final pot on then you add some nutrients so they can go the season
5. Lighting needs to be good and of a high quality throughout the winter and spring months
6. Keep the soil temps about 12c, otherwise they may run to seed later in the grow
7. Tackle disease before it happens and treat as needed
8. Support your plants when needed
9. Ventilation and air circulation are critical as without it is a home for disease and insects to attack your plants
Dedication, time, and commitment is needed, growing big onions in my experience is about this and if you put the effort in and commit you can do well. It is a 11 month grow but extremely rewarding when it all comes together …… Good luck and enjoy your grow, Nick.