When I was younger, my family would grow scallions by taking an old glass bottle, filling it with water to the brim, and putting a regular onion head on the bottle so its bottom would touch water. It would grow roots and start growing little scallions.
I tried doing that again, and met with varied success. Some onion heads produced scallions after a week or two. Some took 2-3 months. Some never did.
Is that a good way to grow scallions, and if so what can I do to improve the success rate? Buy specific kinds of onions? Add something to the water? Anything else?