When I was a kid, someone gave my dad a bunch of well dried 16' 4x4's. He decided to make fence posts out of them. They were heavy suckers and very hard to cut in half. We planted them and then tried to hang wire. It was impossible to get the staples in. We'd bend 3 for every one we got in. We finally gave up and pulled all the posts out. Dad found out later that they were black locust. The guy gave them to Dad because he couldn't get nails in them either.
Black locust is supposed to be the longest lasting post wood available in the US but you'd better get the wires up before they dry. Once they do, you'll never get the staples in.
