gerund doubt, 高手入

We should use adjectives to modify gerunds. For example, casual cycling along the river is refreshing.

Casual - adjective
Cycling - gerund (subject)
Along - preposition
The - article
River - noun
Is - verb
Growing - present participle (adjective)

In your example, we cannot tell whether "rapid growing" and "rapidly growing" are correct or not. We dont know the word "growing" is a gerund or not.

In "a growing tree", the word "growing" is an adjective, not a gerund. In that case, it should be "a rapidly growing tree". We use an adverb to modify the adjective "growing".

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依照文法原則，形容noun的是adj，所以形容gerund(一種noun的form)應該是用adj。

rapid growing正確
A rapid growing tree...

但是如果-ing不是gerund，而是continuous tense的話，按文法原則形容動詞是用adverb，例如作一句擬人法的句子：

I am a rapidly growing tree.便用rapidly，因為am growing是動詞。