"从事庭园设计" can be translated into English as "engage in garden design" or "be involved in landscape garden design" (where "landscape garden design" emphasizes the broader aspect of designing outdoor spaces, including gardens, though "garden design" is more straightforward for the general context).
For simplicity and directness, "engage in garden design" is often sufficient.
Example sentence:
"She has been engaging in garden design for over a decade."
（她从事庭园设计已有十多年。）