Learn Spanish (Latin America): Dudo que el girasol haya florecido; ¡cuánto llueve! - I doubt that the sunflower has bloomed; how much it rains!

Both are correct, but they focus on different things. ¡Cuánto llueve! focuses on the sheer amount of rain (how much it rains). If you used ¡cómo llueve!, it would focus more on the intensity or the way it is raining (how hard it is raining).

Why doesn't cuánto change its ending in this sentence?

Because it is describing a verb (llueve). When cuánto modifies a verb, it acts as an adverb, and adverbs never change gender or number. It only changes to cuánta, cuántos, or cuántas when it describes a noun (like ¡cuánta lluvia! meaning "how much rain!").

Why is it haya florecido instead of the regular present perfect ha florecido?

The phrase Dudo que (I doubt that) shows uncertainty, which triggers the subjunctive mood. Because the blooming is an action that would have already happened before now, we use the present perfect subjunctive (haya + past participle) rather than the present subjunctive.

What is the infinitive of florecido?