overwintering Delosperma in zone 5

I hope this topic is in the right section?
I am located in S. Ontario, Canada zone 5. We have irregular snow cover and winters tend to be wet. So far I have not been able to keep 'hardy' Delosperma.

Last winter I tried 2 in pots, and in fall sunk the pots in the ground to keep them warmer. We had excellent snow cover (unusual), but we also reached the limit of zone 5 (-29C) for 1 or 2 days (also unusual). One looked like it was eaten by rodents, and the other was barely alive.

This year I am trying some in various locations:

Identification of Rhodiolas used in Tibetan Medicine

I am currently helping Dr Tsering Norbu, Director, Materia Medica (Medicinal Plants) Department of The Dalai Lama's Tibetan Medical & Astrological Institute in Dharamsala, N.India with providing Scientific Identifications for hundreds of Himalayan species utilised in Tibetan Medicine. I would welcome any suggestions for the Rhodiolas which he provided images off from Spiti, borderlands of W.Tibet and Nepal.

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