I planted onion seeds in flats. They are now up and the tops are growing tall. Can the tops be trimmed to about 4 inches, or do I have to keep all the top growth until they are set out?
I would not trim them. This is how they build their energy that they then store in the onion bulbs. Trimming them may result in a smaller harvest.
I have mold (I think) on top of my onion seeds. They're in a container with a greenhouse top and I am using organic soil for the first time. I have them in a window on the south side of the house.
What color is the mold? If it is green, it is likely algae and indicates that the soil has too much water, but it is not harmful to the seeds.
Can I plant regular onions to make an onion plant? What do you mean when you say of onion sets? I live in NH. When should I plant?
If you meant can you plant onions from the store to get more onions, the answer is not really. You can plant them to get seeds, which you could plant next year to grow onions, but that would take awhile.
Onion sets are basically baby onions. If you go to a plant nursery, they will have them for sale. You can plant them after your last frost date in your area.
I had an onion in my fridge that had started to sprout. I left it for a couple of weeks, during which time it grew quite a bit more. I planted it in an indoor pot and it has shot up! It has quite a few long 'rubbery' leaves around ten inches long. Am I likely to see anything from planting this bulb, and how will I know when to pull it up?
You won't get more onions from it, but you will get flowers. You can harvest the seeds from the flowers and grow more onions from those next year.
I would like info on how to plant onion sets.
This article has instructions about 1/2 way down:
https://www.gardeningknowhow.com/edible/vegetables/onion/find-how-to-grow-onions-in-your-garden.htm
I live in Zone 9. . . Arcadia, FL. What in the best time of the year to plant onion seeds?
For a general planting guide in your zone, this link should help:
https://www.gardeningknowhow.com/edible/vegetables/onion/growing-onion-seed.htm
I planted Candy Onions three weeks ago. They have taken root and started to grow new leaves, but the new leaf is brown on the tip (1 inch). We had 1 1/2 inches of rain the day after planting, then it was dry for two weeks, and then again two more rains about 3/8 of an inch each. I applied 23-0-0 fertilizer three days ago and then watered (overhead) just enough to desolve the fertilizer. I did not notice brown tips when watering.
It sounds like you burned the plant with the nitrogen fertilizer. Next time, water the plants well before applying nitrogen fertilizer.
For what is it worth, you do not want to use only nitrogen on your onions. This will produce big tops but small bulbs. Instead, make sure they are getting a good amount of phosphorous.