Another way to do it, if someone doesn't have the greenhouses and doesn't want to use moveobjects, is to plant the garden, then build the room around it. The interior will give the unfinished room moodlet, but plants don't go dormant. To replant, delete one section of the wall, which turns the room into the outdoors again. Don't quote me, but I think if you place the sprinkler and turn it on before building the walls, it will stay on and continue to water the plants.
Personally I tend to use the dirt-coloured flooring + moveobjects method for my indoor gardening needs. I have the sim plant everything in the yard, then arrange the plants inside with moveobjects.