I would wait until you see some new growth. This can be slow to appear. Buddleia is a late starter.

I wait until I see small green leaves, then prune back the deadwood to the green. You can prune it back further than that if you don't want the bush to get too large. (Mine was over 6' tall last year, and it's only 4 years since it was wintersown!)
Wendy

I am also in zone 5a and mine die back to the ground in winter. What kind do you have?

As the others have said, prune it when the green starts to show. In sheltered areas which have southern exposure on our property sometimes they don't die right back to the ground, and in other areas such as those exposed to the northeast winds they die right to the ground. We have over 20 on the property and lots more in the holding beds right now.

The northeast ones are very late to emerge. One waited until July 1st last year but by September it was 5 feet tall and welcomed by the Monarchs. :O) Below is a photo of it with 5 Monarchs in late September. I also winter sow these as does Wendy and this one was just two years young.

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