Are green onions, scallions, and chives the same plant or different plants? And is a green onion just a young onion or is it something else?
green onion = scallion. Chives = chives.
There are quite a number of cultivated specise of Allium - ‘green onion’ can refer to a number of things; scallions (which I think we call spring onions here), which are essentially ordinary bulb onions picked at an immature stage, but it could also encompass Chinese bunching onion, Welsh onions, and a few other things, some of which are different species.
All are allium; scallions are “bunching onions,” not “globe onions” (i.e., a different variety from the big old yellow or white onion).
Scallions go by various names: Scallions, Bunching onions, Welsh Onions. The species is Allium fistulosum, and unlike most onions, they form clumps, like spreading grass. In fact these will give you onions all year when grown, because one just has to take a few stems, leave the rest and the plant continues growing. But, Spring onions, which are also green onions are immature bulb forming onions (when proper bunching onions cannot be found).
Chives are Allium schoenoprasum, and stay small and fine leaved.
Bulb onions are Allium cepa. These come in two types, short day (forming bulbs when days are 12 - 13 hours long), and long day (forming bulbs when the days are 14 - 16 hours long). Interestingly, long day onions will never form bulbs at the equator.
Everything you wanted to know about herbs and spices but were afraid to ask.
Gernot Katzer’s terrific encyclopedia covers everything about plants and spices, including etymology and genealogy, so to speak. Appropriate pages: