I'll start by saying I live in the woods and would hate to introduce an invasive species here. I so wanted to plant the Berberis thunbergii ' Rose Glow' and 'Lime Glow' but am now having second thoughts. Does anyone know if these are sterile or what conditions will encourage their spread? Are 'Royal Burgundy' and 'Crimson Pygmy' as likely or unlikely to spread? Thanks.