Was it in a pot before you planted it? If so, no need to prune it, but you need to pay close attention to the watering, since the roots are still confined to pot size and space. If it was freshly dug, meaning the roots were cut, then the top needs to be cut back as much as the roots were cut back. If it was a bare root plant, I would take about 1/3 of the top off. That is just my preference. Be sure to water it carefully to give it plenty, and not too much for a year or two so it can get established in it's new place. Good luck.
