One way that you can accomplish this would be to place all of your existing tree species into a subcategory in the tree category.
Then, create a drawing that you will Xref into your main planting drawing, and place the existing trees in that Xref. You can label these existing trees in the Xref.
In the main drawing, you can create a schedule, and as long as the "Include Xrefs" option is left unchecked, the existing trees will not show up in that schedule.
This also has the added benefit of splitting out the tree layers for those existing trees and tree labels onto separate Xref layers that you can then viewport freeze, or grey out as necessary.
Here's some additional information on using subcategories for plants:
https://www.landfx.com/index.php/docs/planting/item/209-project-plant-manager.html#categories_and_subcategories
Let me know if that's the kind of functionality you were looking for.
Thanks,
Amanda