I've been told this is not bannable, but spells work on a limited number of plants. I have also recieved the warning about not being able to place any more items. can anyone explain this warning and tell me the max number of plants each spell can affect?
Large spell area can affect up to 69 plants. If you have more than that, it will affect 69 and you'll have to cast a second time to affect those over the number.
The warning may be because you have too many items placed. Let's say you have a castle with 150 housing spaces. Every gardening plot you place counts as a housing item. Every seed you plant counts as a housing item. So if you have a 69 plot garden, that's 138 housing spaces used between the plot and the seeds. That means you only have 12 spaces left to place anything else.
So let's say you have a few pets and a few housing items placed around, and it's more than 12, you'll need to pick some of them up.
If you have enough gold, I'd recommend buying a Red Barn Farm. You should have no problem having enough space as it has 250 housing spaces, plus it counts as a gardening like for almost all (if not all) plants. If you already have one and you just have too many items placed, again just removing enough items is all that's needed.
If you've already checked out some of the tutorials on best practices for plot stacking, including how and where to place items to target the spells to ensure they reach all plots, then good luck!