Word order in a descriptive statement

Either is correct. Which order you use is a matter of emphasis - the first is considered to have been given more emphasis by virtue of being first in the list, the others in the list will be then placed in decreasing order of importance.

Plants were exposed to freezing temperatures for 2 hours [for 2 hours to freezing temperatures].

An adverb is very versatile in a sentence. Its position in the sentence does not have to be right next to the adjective, verb, or another adverb that it modifies. This is why the adverb prepositional phrases, one answers "What?" and the other "To what extent (how long)?" can be moved around.

What were the plants exposed to? Freezing temperatures.

To what extent or how long were they exposed? For 2 hours.

Both phrases modify the verb phrase were exposed.

You can even place them at the beginning of the sentence.

To freezing temperatures, plants were exposed for 2 hours. [sounds better on the other side of the verb phrase]

For 2 hours, plants were exposed to freezing temperatures. [this one too would sound better on the other side of the verb phrase]