(covers information from several alternate timelines)
A gardener was someone who looked after ornamental plants or engaged in minor agriculture, such as growing herbs or vegetables, typically in a garden.
The Earth movie Appointment with Danger featured a gardener character. In 2153, T'Pol dreamed she was watching the film on movie night on Enterprise NX-01, during which Trip Tucker speculated that an unidentified corpse might be that of this man. However, Doctor Phlox disagreed, saying he was too tall. (ENT: "Impulse")
In 2269, Flint described M-4 as his "butler, housekeeper, gardener, and guardian." (TOS: "Requiem for Methuselah")
Elim Garak claimed to be a gardener while working at the Cardassian embassy on Romulus, during a period in which several prominent Romulan officials mysteriously died. He also claimed that Edosian orchids were his specialty. (DS9: "Body Parts", "Broken Link", "Inter Arma Enim Silent Leges")
Towards the end of the Cardassian Occupation of Bajor, Bareil Antos took a job as a gardener in a monastery. He had no ambitions other than to grow the "loveliest" Feloran bromeliads on Bajor. (DS9: "In the Hands of the Prophets")
In an unknown, alternate future timeframe, Geordi La Forge claimed that Leah Brahms was "quite a gardener". (TNG: "All Good Things...")
Boothby was the groundskeeper at Starfleet Academy and thus did a good deal of gardening. (TNG: "The First Duty"; VOY: "In the Flesh")