transitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means cultivate and the verb vocab version means to cultivate.
You have two cheese cars (つちか) that are growing mold. But that's a good thing, because it's the good kind of cheese mold. So, you cultivate the mold on these two cheese cars, comparing the difference between the two.
I learned a lot about the importance of cultivating friendships through the club activities in high school.