Code:

    @Override
    public void start(Stage stage) throws Exception {
        final TreeItem tn = new TreeItem("root");
        TreeItem tn1 = new TreeItem("node1");
        TreeView treeView = new TreeView(tn);
        tn.getChildren().add(tn1);

        final Button button = new Button("Apply graphic");
        button.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent t) {
                tn.setGraphic(new Rectangle(0, 0, 10, 10));
            }
        });
        VBox pane = new VBox();
        pane.getChildren().addAll(treeView, button);
        Scene scene = new Scene(pane, 300, 700);
        stage.setScene(scene);
        stage.show();
    }

Look at the attached movie.

I set new graphic (on movie, using binding, in the attached code, using setter). You can see, that change listener in movie notified, that rectangle became a value of graphic property of node (ROOT). But it will be applied, on when node will be expanded/collapsed.

To reproduce - click the button in the attached code, and expand or collapse a node.