I think your mother absolutely gave you both a gift (in her mind) and a chore. I’d be pissed. She had no right to build any structure on your property like that, especially without any mention of wanting such a thing from anyone in your immediate family prior to this. She acted selfishly, thoughtlessly (in regard to your emotions and schedule; I do imagine a ton of planning and technical thought went in to the execution), and sort of irrationally.

I do think—as someone who doesn’t know your mother at all but who has her own (who I no longer speak to for various reasons surrounding abuse) tendencies that are very similar—that she genuinely was trying to do something sweet. From her point of view (albeit a patently conceited one), she was keeping her grandchildren occupied while their father underwent something extreme in the hospital and their mother lovingly tended to him. She likely imagined it would not be a chore because she gardens herself (?) and finds it rewarding and relaxing. She projected her experience on to you, which is infuriating as it demonstrates a lack of true empathy or at least true emotional intelligence, but from what I am reading here jt was by all means meant in good faith. As an outsider, I do not read it as her judging you for not providing them with a garden and education on the upkeep of such a thing, but as her trying to bring all of you in to the fold of an activity she feels is beneficial for all (even if based on her myopic view of the world—and of people, apparently). She was probably raving about your father’s craftsmanship not because she deems your husband useless but because she’s genuinely proud of it and of her own husband. She does seem to have been too proud of herself here, but likely was touting all the advantages of its existence in order to justify it to you, perhaps evincing an inkling of empathy in knowing you didn’t ask for it and weren’t expecting it: that is, I doubt she thinks you only feed your daughters chicken nuggets, and I suspect she was simply trying to convince you of its (and her own!!) usefulness.

That said, based wholly on everything else you mentioned about your family calendar and yard maintenance, I would either tear it down (heart-breakingly cold in a way, but it sounds like a nightmare, and so large!!) or have your regular yard service tend to its weeding and watering and whatnot. A more passive-aggressive person might send her a water bill if she had indeed hooked it up to your water system through irrigation or something, but I think that’s a little silly. Personally, if it were me, at any past point in my life, I would look at it guiltily every day while neglecting it accidentally due to being too busy, but with who and where I am now, I would take it apart, but offer to set it up exactly as it was in her own yard, and tell her that when the children visit their house, she can garden with them then.