The plants from the jungle gallery can not be deleted once they are placed. Please fix this
If the plants are placed actors and Delete or Backspace does nothing, the level is probably treating them as locked or as instances that the editor refuses to delete in place. Jungle Gallery foliage from the starter content is often instanced foliage or a foliage actor, not a normal placed actor. Instanced foliage does not delete like a static mesh: you select the foliage actor, then in the Foliage tab, select the instances and press Delete there, or use the foliage edit mode and remove them.
If they are normal placed actors, check the actor selection. Click the plant, look at the details panel for a lock icon, and unlock it. Then try Delete. If the editor says the actor is referenced, or Delete is greyed out, the plant is probably in a sublevel or it is the source of a level sequence or a blueprint instance you are not allowed to remove from this level. Open the outliner, find the plant, and see if it is in a different level. Switch to that level and delete it there.
Another common case: the mesh is a foliage instance, so the selection is the FoliageActor. Select that foliage actor, open the Foliage tool, and delete the instances. If you only deleted the selection and the actor stayed, use Alt+Delete or the outliner context menu “Delete”. If it is a Blueprint actor, delete the instance from the outliner rather than the mesh, because deleting the mesh component can leave the actor behind and the actor refuses to go away while it is being edited.
If none of that works, the plant might be a level instance or a World Partition actor. In the outliner, right-click the actor and choose Delete. If the editor prompts that it is referenced, check level sequences and blueprints that point at it before you force the delete.