How to implement an association with restrictions
There are a couple of ways you can go about this, but the most simplest way to enforce this would probably be to create a dimension combination table. Such a table is pre-filled with all valid possible combinations (by id) and then that is the source of your application's controls (such as dropdowns, etc) because as you filter on one dimension in that table, only valid entries for the remaining dimensions will be available.
For example, your dimension combinations table would have a plant_category_id column and a tag_type_id and maybe a second tag_type_id2 column (or if you normalized your Tags table, the column naming would be a little better in the dimensions combinations table).

The other thing you can do is implement a complex check constraint (possibly with a function) but this is probably more work than it's worth, and will depend on which implementation of MySQL and version you're using.

The third way is handling the logic in stored procedures that manage data access via the logic you want to implement, but probably would also be a lot of work, especially to maintain over time. So the first suggestion that makes it table driven would be my first pick.