tanecetum and veronica

Hi,
Could any of you tell me why tanecetum niveum "Jackpot" should be listed as zones 7-9 in one catalogue and 4-9 in another?
 
Also, would you propagate veronica prostrata "Trehane" by division or cuttings? Any hints?
 
                                        Thanks,
                                        Mary
                                        zone 6 MO
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