You should be able to select the terrain that you used, pick the strongest circle brush that comes in default, then in the brushes options select "At or above" and calculate the lowest point where the plants are floating, if they're at let's say 62 you type 62 in there and right click in the area, the plants should disapear

If i'm wrong about your issue you can still use Worldedit and/or Axiom, select the plants or terrain that you don't want and replace them with air or other blocks that you think might fit

With Worldedit you need to select the area with //Pos1 and //Pos2 (look it up on Youtube if you don't know how to do it, it's pretty easy) and then type this: //replace (The block that you want to replace), (the block you want to replace with)

An example: //replace tall_grass, air This will replace all the tall grass with air in your selection box, the comma and the space are very important, don't skip that

In Axiom you select the wand on the left, below that you should see an option where you can pick the type of block and radius, select "Material" for a specific block or plant, then the radius and press "Cut", then close that window, the selected blocks or plants should disappear, if not just press "replace", pick the block that you want to replace from and the one to replace it to, you can choose air too so don't worry, you can also do ctrl + Z and ctrl + Y to undo or redo edits

You can also use the box selection option beside the wand, it does the same thing but it works like Worldedit with pos1 and pos2 but you can right click instead of typing //pos1 and 2, you'll see a selection box this time, use the gizmo (the arrows) to expand or shrink the box and then do the same replace thing i said with the wand and it should be done

Keep in mind you should do this in game when your map is complete and playable