Tip: Give your Sims "likes Gardening" and perhaps gardening related traits for a boost. (Or even an aspiration and/or gardening career work from home) I find they autonomously do TOO much gardening. Which is ok, though I'm often cancelling "talk to plant" when there is nothing else to do in the garden, heh heh!
The only thing I've noticed they don't do autonomously is fertilize plants. I always have to get them to do that manually. (Also "Evolve" a plant.)