It's a bug, plants aren't turning green when they ought to. They should be healthy and produce the way they should, they just don't look as nice as they're supposed to.
"Hire a gardener" will do the same things you can do yourself. It's just an option you can choose, for example maybe you have a big garden and your sim needs to go to work. The hired gardener will take care of whatever watering, weeding, and bug spraying that needs to be done-- all things you can do yourself, but maybe you don't have time. Also, buying Patchy the Scarecrow will basically be like having a live-in gardener. Once you do some interactions with him, he will start coming to life every day. He hangs out with your household and is a bit of a nuisance sometimes, but he will also go take care of your garden for you.