Lily-of-the-Nile, African Lily, Agapanthus— it’s a lovely tropical plant whatever you call it. The flowers resemble a cross between an allium and an amaryllis. In fact, it is a member of the Amaryllis genus. Until just recently, no agapanthus cultivar was reliably hardy in the ground below zone 7. This is starting to change, with cultivars bred for colder regions starting to come to market.
Still, if you want to grow this plant reliably in zone 6, a pot or planter is the best way to do it. Luckily agapanthus adapts readily to pots.