This is a really fun question. If you are doing a paper on Roman gardens, there are many questions that you can ask and seek to answer. Let me provide a few of them.
First, you can ask how many people had Roman gardens in the Roman world. In other words, were gardens only for the wealthy or did not-so-wealthy people possess them as well.
Second, you may want to ask what the purpose of these gardens were. Were they for beauty and relaxation, the showing off of wealth, or for sustenance? For example, many wealthy Romans had huge fishponds in their homes during the late Republic.
Finally, you might want to ask about the importance of public gardens and how many there were in any given city. All of these questions will give you great insights into the mind of the Roman world.