Any of the alliums can be sown in the garden March/April and can be planted provided the plants are hardened off. They like a loose, moist (not wet) well-drained soil.
Sets: Small bulbs are sold bagged and usually develop into stewing type onions. Plant with the pointed side up, just under the soil line.
Scallions/Bunching Onions/Green Onions: Easily seeded in the garden early spring or started indoors late winter. The whole plant is harvested and used. The tops can be used much like chives. The small root bulb is mild in flavor. Sow in a shallow trench, use a seed tape or plant young seedlings. Plant with the white bulb just slightly below the soil line.
Slicing Onions: Transplants are usually young onion plants sold bundled or in soil. Starting them indoors early guarantees a larger onion earlier in the growing season. Plant so the white (yellow or red) roots are concealed by dirt. The may be flimsy at the start but will firm up in days. As they mature the top foliage yellow and shrinks. They can be harvested and stored by hanging them (including the tops) or clean up the bulbs and store in a refrigerator.
Leeks: Started by seed indoors late winter. They don’t develop a bulb like onions, rather develop a thick stalk used for soups, egg dishes or general flavoring. Plant by seed or young seedlings. Harvest when the stalk is large enough for slicing.