Hi Summerflowers
Lilies can stand very cold weather, so "cold hardiness" isn't the problem. The difficulty for cold zones is when does Fall dormancy occur relative to when the lily blooms.

Lilies need time after they bloom (which depletes the bulbs energy stores) to build back up through photosynthesis. If the leaves die back shortly after bloom that can't happen.

The orientals are the later blooming of the lilies Many bloom from late July up to late August and so are riskier for cold zones - where Fall comes early.

Lucky for you, Mona Lisa is one of the earliest blooming of the Orientals - blooming in mid June for most people. Since yours are blooming now you should have time for them to build up before they go dormant. (Just how early does Fall come where you live?)

The Lily Nook (a Canadian nursery) has a bunch of information pages on growing lilies in cold (i.e., early Fall) zones.

-Helen

I have been growing oriental lilies in zone 2b for many years. When I planted them, I planted a good 6 inches deep and planted the bulbs sideways. They bloom later, but have reliably come back and bloomed year after year.

Maybe post this question on the Far North Garden forum...there are lots of us cold zone gardeners there.

Good luck with your lilies,
Sierra