Self employment as a gardener is not very profitable for the amount of work it takes. You either are constantly planting, tending, harvesting, selling or running around finding plants and selling and all for very little profit. As the quality of your plants increases so does the amount you earn from it but my Sim barely could keep herself at a nice level and afford occasional home emprovements and upgrades. When she married we remodeled the house so it makes her and her spouse very happy wherever they are but he's the one bringing in the money. How traditional!
He's a chef but he's got creative for a trait and he gets quite inspired painting, which helps his cooking as well. He brings in a nice profit from his paintings even at a fairly low level. If he pays $100 to paint a large picture he sells it for $160 so that's not bad. I noticed pop art does not sell well so don't waste your time and money there. Maybe he's just not good enough at it yet since he's only painted one pop art picture value 0.
The way I kept my gardener in funds was dig, dig, dig and sell off what you find.