Black Locust Fence Post Advice

Here's what you need to know to prepare Black Locust logs for duty as highly durable fence posts. June 14, 2014

I have searched the posted responses regarding black locust and have not found clear answers to these questions: First, should black locust be harvested when the sap is down (the dormant season here in Indiana), for it to have better rot resistance as a fence post? Also, does it matter if the bark is left on when it is set as a post? Should the post air-dry prior to use? Does it last longer split or as a round post?

Forum Responses
(Forestry Forum)
From contributor W:
I believe it is the extractives in the heartwood that gives black locust its rot resistance. Therefore, I would suspect that it does not matter when the wood is harvested since the extractive level will not change. As to the bark, I bet it would be better to peel off the bark since most bark has little rot resistance and is a bug magnet. As to air drying, if you put an air dried post in the ground it will gain moisture equal to the ambient moisture in the soil, so an air dried post won't stay air dried-in-the-ground very long, especially in winter. Again, I suspect that it does not matter if it is air dried. However, if air drying results in a lot of splits and checks I bet that would be worse since that would create pathways for water and soil to migrate into the cracks and lessen the rot resistance.

From contributor Y:
I would say you need to remove the bark on green posts before you use them. I have seen a couple leaf out and grow. They make good posts and we have some here that have been in for close to 100 years.

From contributor J:
I can only help on the sap part of the question. The sap isn't really down during the winter, it's just not moving up and down. Some trees are best cut during winter because the bacteria won't cause the staining in the white woods when it's cold. If the logs are sawn very quickly after felling then placed into the kiln ASAP, then it's not an issue. I'm in Northern Indiana by the way, what part are you from?

From contributor C:
Peel the posts the day they are cut. One of the best ways is to lay them out about two deep and drive a tractor over them four or five times. This will slip the bark and make them peel very easily. 150 can be peeled by two people in two hours. Some posts are completely peeled and the otherís bark will come off in long strips. We used a flat screwdriver to get some tough strips started. Our order was for 700 posts.

The other thing that can be done is to char the bottom three feet. The charring should occur on wood that will be in the ground and about one foot out of the ground. Build a long fire, about ten feet long and three feet wide. Use a big green log as a fire backstop to lay the logs on. It doesn't take but a few minutes to char these logs. Logs should have dried a month or two after peeling. The charring will sterilize the sapwood and seal the pores to bacteria and fungus. It is a good bit of work, but the posts will last for a long time. If the posts are woods grown and the sapwood is only 1/4" thick or so, then peeling and drying is all that is needed.

From contributor W:
Black locust has one of the highest heartwood to sapwood ratios of any tree, since there is usually on a few annual growth rings that are sap. I guess the sap turns to heart very fast. That is one of the identifying characteristics that can be used to differentiate black locust from similar looking woods if you only have a wood sample.

From contributor A:
Regarding air drying black locust fence posts, keep in mind that the longer a locust post dries, the more difficult it is to drive a nail or fence staple into it.

From contributor D:
That bit about driving staples into hard locust is very true. Use the shortest fence staple you can get - 3/4''-1" work best. Also, putting a pole in the ground with the bottom of the tree in the bottom of the hole makes it last longer and sawn posts last longer than split posts, but 3"-5" rounds last longest.

From contributor B:
Also remember that black locust is very toxic to livestock and animals. It will poison cattle and horses very quickly.