Dave, I use these green velcro strips. They're about a half inch wide (maybe wider), and they're double sided (loop them around and one side of it sticks to the other side of the same piece). I got it at Home Depot... came in a rolled up single strip, maybe 25 feet to a roll (use scissors to cut to whatever length pieces you want). (I'm really not sure what the length was, but one roll was all I needed for 50 trees).
I've seen double-width wire twist ties too, encased in plastic with two wires inside. But I prefer the velcro.
Mike