Well we already have "Home Improvement" which has had a ton of "Home Care & Maintenance" type questions on topics like cleaning vinyl siding, pavers, etc. There's a ton of overlap between "improvement" and "maintenance" experts. For that reason, if it was up to me, I'd close home care & maintenance as a dup of our already existing site.
There's also the question of viability. I'm hopeful Home Improvement can make it out of beta, but its struggled in the 4-5 questions/day range for a while with the maintenance kind of questions. Creating a second site would I fear have the effect of taking users away from a struggling beta site.
I'd stick with creating a "Gardening" site which will have some overlap with home improvement, but will have a whole world of questions onto itself. It would cover the 1st and 3rd proposal you list above and attract true gardening experts -- landscape designers, serious home gardeners, plant experts, etc.