As you have no doubt noticed, usage varies, and so do rules. In my
career as a copyeditor, I have enforced some pretty silly ones. The
usual practice for this example is to hyphenate it when it stands
before the noun, but not in predicate.
"Growing", here, is an adjective (in the broad sense of something that
can modify a noun). "Fastest" is an adverb (in the broad sense); here
it modifies "growing". Ordinarily, when an adverb precedes & modifies
an adjective, and the adverb is a word that can only be an adverb, no
hyphen is needed: "rapidly growing city". However, there is a
fair-size set of English words that can be either adjectives or
adverbs; "fastest" is one; some others are "well", "still", "kindly",
and "worse". It mildly improves readability if those are hyphenated
when the adjective precedes the noun. However, the
when-in-doubt-leave-it-out school would write "fastest growing city"
on the ground that no-one could possibly think that it meant "fastest
city that is growing".
Some stylists, of the kind who delight in behaving like badly
programmed computers, draw the line, not between adverbs that can and
cannot be adjectives, but between adverbs that do not and do end in
"-ly" -- I presume because that rule requires less thought to enforce
it. They would write "a kindly meant remark" and "an often-heard
remark", whereas for sensible people IMO it is the other way around.
--
--- Joe Fineman
jo...@verizon.net
||: Here's to the fizziness that gets us from busyness to :||
||: fuzziness! :||