Taxonomically, a green onion is the same species (Allium cepa) as a common onion and a “sweet” onion. Or it is a different species, Allium fistulosum. Or it is a cross of the two. In case you don’t think this scientific description is helpful, just try finding a common one, as I have tried, by showing farmers, distributors, and “just plain folks” in different regions of the country the photo at the top and asking for names of the green-topped whatever-they-may-bes!
I asked Bruce Sanbonmatsu, co-owner of Sanbon, a grower in El Centro, California: “Is there a distinction between scallions and green onions?” Without hesitation he replied: “Of course. People from New York and Boston call scallions what most everyone else calls green onions.” (“Most everyone else” means people who call them spring or salad or bunching onions.) Sanbonmatsu’s reply may be as close to a truth about common names as I can get—a truth that has taught me to list the provincial New Yorker’s scallion and green onion in the ingredients lists after extensive cross-country questioning.
In the marketplace, the terms “green onion” and “scallion” generally designate slim, bulbless green onions—despite the fact that in all current American and British dictionaries I consulted, the first definition of “scallion” is “shallot”.
The terms “spring onion” and “salad onion” are more likely to describe a later stage of the same onions, green-topped with more developed bulbs (see photo). Both the bulbless and the bulbing forms come from the same seed as common storage onions, planted in tight quarters and harvested very young.
Chef Jan Birnbaum describes them this way: “Spring onions have a real freshness that regular onions don’t have. You know they’re going to be grown-ups—but they aren’t yet. The tear-producing and heat-producing aspects are not as strong, and they’re much more tender.”