I have a crowded scene, now in hierarchy view, with prop town and people. how wipe them all away without removing one at a time?

On the Windows keyboard, you either hold down the Shift or Ctrl keys while selecting each model with the mouse on the Hierarchy panel.  Right-click the mouse button and select delete or hit delete on your keyboard.

Actually, you can't mass delete everything from the hierarchy. what you can do is select everything as hborre says except 1 thing (like the hedges). Parent them to that item by dragging and dropping then delete that item making sure the "delete attatched" check box is checked on the pop up window.

Interesting solutions. Thanks 

For people who have quite often many items to delete, you have a gem, written by Snarlygribbly: it's called "Scene Fixer", which provides many tools, and among them a multi-delete tool.
Head over https://cobrablade.net/snarlygribbly/poser.html and select the one that corresponds to your version of Poser: V12 & V13: first part, V11 second part. This is mainly due to the major change in Python version.

One tool that I quite often appreciate: the possibility to create hierarchical groups directly. First select the group under which you'd like to create a new one, then:

Result: