Slam Bang

I happened to see an issue of The Globe and Mail today, which is never a good idea, because pretty much without fail I'm also going to see a typo or a grammatical error, and I'm not going to like it, because these things are not supposed to happen, because there are supposed to be editors of various sorts, but there just aren't, because they (as business people like to say these days) impact the bottom line.

In this Olympics story is the following sentence:

Slam and spoken word, two offshoots of what's known as performance poetry, has been slowly growing in popularity.

It's a slightly tricky construction, granted, at least for a complete novice, because immediately following the noun "performance poetry" is a verb, and so it looks very much as if that verb should also be singular. But of course that little clause, set off by commas, is what's called a non-restrictive clause: it tells you something about the actual subject without limiting it in any way, and so could be discarded without changing the meaning of the sentence. That means it's not the subject of the sentence: the compound subject is "slam and spoken word", which is plural, and therefore requires a plural verb. The sentence ought to have read, "Slam and spoken word, two offshoots of what's known as performance poetry, have been slowly growing in popularity."

A little thing, but the difference between correct and not.