The Royal Horticultural Society rates it as hardy in zone 4, but as always much depends on when you get your first and last frosts, the duration of extreme low temperatures, etc. Zone 5 is close enough to zone 4 that in isolated cold areas you could well be pushing the hardiness beyond its limits.

But that is probably, as it were, "cold" comfort, especially coming from someone in a climate that is the equivalent of your zones 9 or 10.

You may like to visit the link below to see a shot of 'Forest Pansy' that I took during our recent Southern Hemisphere autumn.

Here is a link that might be useful: Cercis candensis 'Forest Pansy' in autumn

I know I'm very late in replying to this, but... I've had a single Forest Pansy for 5 years here in Z5b. After last year's January combination of extreme cold, high winds and very low humidity, I was sure it would not survive - there was not a sign of emerging leaves until early June, but after that it put on tremendous growth, no doubt helped by a summer of frequent rain. Anything that survived last year I would say is fully hardy in Zone 5.

I am late too but I have tree pansies and have had them for
10 years. Last year the largest of the three started
struggling and I lost half of it. They are gorgeous.

I have one 3 yrs old in zone 6... its an incredibly beautiful tree. I am glad to know it is hardy to zone 5.
Our winters here the past few years have been milder than normal and almost feel more like zone 7. I have been wondering about my forest pansy and some other plants/trees etc that i have acquired the past few yrs and how they will fare when our typical winters resume.
i wonder why the 10 yr old tree mentioned began to struggle. that would be good general info to know.

I was first shown the Forest Pansy by a landscaper friend of mine here in Denver and fell in love. I started the search for one, but my two favorite (and very reputable) nurseries very vehemently advised me against it - same thing, that they would grow for a few years and then inexplicably have problems or just literally fall over. They said if I wanted to do it to make sure to find a sheltered spot and be ready to really baby it. I am more a fan of unfussy plants so I went with amur maples and a serviceberry instead. I'd say redbuds in our zone may be a roll of the dice - do you feel lucky?? :)