If I am unable to log into the game for an extended period of time (weeks or months), will my plants all be dead when I return?
I do not think so. I believe gardening data is loaded every time your house loads and is not recorded independently, meaning you can take extended breaks and still return to find your Plants alive. (Though, of course, you will still have to take care of their Needs and Pests.)
They will be dead if they had a need or pests when you were last in the house and then do not return for an extended period. If they were fully cared for when you left, they should be fine when you return.
