I don't think any cover is required since they seem to take even temperatures into the low twenties with no damage to new buds. Mine have been growing evidently non-stop despite having two weeks or more of cold weather in the thirties or just barely above freezing and lows in the twenties many nights. I was a bit anxious seeing the early growth with our last very short period of weather weeks ago, and then seeing snow and very cold nights, but they are very tough when it comes to spring cold snaps. I wouldnt' swear they're still good into the teens for lows however, since mine have never gotten that cold in the spring.