zxcvbob said:

What will happen? I never got around to planting them in the early spring,
and I know the seeds will be no good next year. These are cheap hybrid
long-day onions (I don't recall the variety). The days are already about
16 hours long up here. The weather is also unseasonably cool and damp,
which would be a good thing for the young onions.

I know there's no chance of getting big onion bulbs because the days are
too long already. But will I get something useful for "spring onions" or
scallions, or will they just get confused and die?

They will attempt to bulb but won't mature (and won't store). You'll have
to harvest them as green onions (bulby ones, likely).

Anything after the very first of May is too late to seed onions here in Michigan,
if you want mature storage onions. Even that is cutting it close. (One of the
bulletins from the extension service this May was specifically about it being
too late to plant storage onions.)

--
Pat in Plymouth MI

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)