hello and thanks for your participation in the community, i need the help

Ok, first off I live in deep south texas and have built a framework of landscaped timbers to make a flower bed along the font of the house. I plan to plant small trees and shrubs and some ground cover such as lantana.

I plan to bring in some soil to fill in behind the timbers, place weed netting and then cover with mulch.

Here is my question... do I need to remove the existing grass or can I just put the soil, netting, and mulch right on top over the grass? It is a pretty good sized area and I would rather not mess with it if I dont need to.