I've read about grapefruit seedlings getting 1 terminal flower (a flower at the end of a branch) and then not flowering again for years. It is apparently pretty rare, but does happen. It must have been a big shock!

that's very interesting. Could you provide us a picture? :)

It's quite common on grapefruit seedlings, but I'd not seen it on a pummelo. Still, it makes sense, since pummelo is one of the parents of grapefruit.

By the way -- I'll make my usual apology about being pedantic. But just to keep us all "correct," "pomelo" (with an o and one m) is the old-fashioned name for grapefruit, Citrus paradisi. "Pummelo" (with a u and 2 m's) is the alternate name for the shaddock (Citrus grandis / C. maxima), which I suspect is the plant you refer to.

@kemistry, Pictures like you asked. I don't know how to resize them so they ended up huge.

The two pots:
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Close up under the flowering one:
{{gwi:606490}}

It has been week and a half now since blooming so I guess it either got pollinated or it gave up:
{{gwi:606493}}

What a normal SAM should look like for the species:
{{gwi:606506}}

Both together to show the differences:
{{gwi:606508}}

@malcolm , Pardon me, Pomelo is what we use in Eastern Europe so I thought it was the same state-side. But of course we also still use Oceania, Britannia, and Germania daily so I should expect more words to be out-of-date.

gauche, your seedlings look great! And the flowering one is interestingly cute! Thanks for the pictures :)