I have a few stands of "Becky" Shasta Daisy that put on an outstanding display each summer. However this season I can see that a few of these plants are not as tall and vigorous as in the past. This is probably their 3 or maybe 4th season. I understand that these plants benefit from being divided every few years? What is involved in doing such a division for health reasons? Do I remove the 'weaker' centre section (in the fall?) leaving a "donut" or do I remove the centre section and then move the outside sections to the now vacant middle? I need your advice and experience.