You may plant in clumps if you want more produce per square foot even though they will be a little smaller. Harvest scallions whenever you are ready to eat them. You can try just cutting the greens and see if they will grow back or you can pull them out. Harvest sweet onions when the bulb reaches a decent size and storage onions when the leaves are 2/3rds brown. Leeks will stay in the ground through October into November.
Set plants 6–12″ apart, onions 4–6″ in trenches in well-dug beds with generous quantities of organic matter. Avoid transplanting next to grass strips; slugs love to dine on tiny allium seedlings. Irrigate seedlings whenever the topsoil dries out (courtesy of Fedco Seed Catalog).