Would be, code-wise, very very similar to janitor. The idea is that they would deal with all plants in pots (not flowers or trees) that enhance the terminal beauty (in my opinion their effect is still too visual as passengers only find terminals better with statues from what I see lol).
Just like janitors go around and clean sinks and bathrooms, gardeners would water the plants. Hopefully easy to implement for devs, many times seen it in real airports (unless plants are fake of course, but for the sake of the game let’s assume they are real) and should not really increase difficulty (I play on hard and plenty of profit to spare with a handful of small aprons and many commoditieis).
Curious what the community thinks about this suggestion? 
but there it’s the janitors job as well. (Imagine you will need 500 janitors then in ACEO)