this happened to me last year. I kept them alive all winter, staking
the spindly little things and putting them under lights. They took up
too much space in my dining room. I put them out at what I thought was
the right time in the spring but nighttime lows in the 40's brought on
the fall colors in May, but most of them bounced back and sprouted new
leaves midsummer. They're still in the nursery so we will see how the
do next year.
If I had to do it again I thought about keeping them alive for a month
or so, then allowing to go dormant and putting in the cellar, then
planting out next spring. Not sure if that's the best solution.
This time I saved my dried prunus pits and just planted out today . . .
we'll see how they go next year.