Se cultivaban porque es en plural, si fuera una sola hortaliza sería en singular. Se cultivaban hortalizas... Se cultivaba una hortaliza...

The previous answer is correct, I will just translate since it might be easier for you to understand the answer in English. The conjugation for Singular (una hortaliza) is "cultiva" (present) "cultivaba" (past), and for plural (muchas hortalizas) "cultivan" (present), "cultivaban" (past) So: Singular Se cultiva hortaliza (present) se cultivaba hortaliza (past) Plural Se cultivan hotalizas (present) se cultivaban hortalizas (past)