I need these posts to last five years. Creosote?

My urban farming group is building 730 feet of fence. We want to use rough cut untreated wood posts, which will not include cedar, redwood, etc. They will be about six inches thick.

We need them to last for at least five years, though more would be better.

So;

It would obviously be a good idea to peel off the bark.

We could char them, but apparently this is complicated.

We could use a tar/ creosote type product on the base of the post. How much of this would ACTUALLY end up INSIDE a tomato growing ten feet away? As far as I can tell, the creosote should KILL things, so any toxins would not make it back.

We could use asphalt emulsion. Is this stuff toxic, or not?

We could use wood tar, but that is very quite expensive.

We could soak them in borax. Would this just leach way?

We could use linseed oil, but this is expensive. Would any other oils work?

Thanks for your suggestions.

Could you drive a metal stake or pipe down where each post is to be, and attach the base of the posts to this? If the stake protrudes a couple of feet out of the ground and you can attach with a couple of stout bolts or wire ties, the post shouldn't need to be in the ground at all. Place a tile or rock or something under the base of the wood....it's the point of soil surface contact that rots first. The second place a post will break down is at the top.....careful farmers often cap the ends with something....metal, paint, etc.

We could use a tar/ creosote type product on the base of the post. How much of this would ACTUALLY end up INSIDE a tomato growing ten feet away? As far as I can tell, the creosote should KILL things, so any toxins would not make it back.

We could use asphalt emulsion. Is this stuff toxic, or not?

We could use wood tar, but that is very quite expensive.

These are pretty much all the same: get enough PAHs (polycyclic aromatic hydrocarbons) on the wood so that decay will be greatly slowed down. Being big, greasy molecules, they don't move very fast very far, so your tomatoes should be OK.

Disclaimer: Other people on this board are scared away by PAHs, see this post

We could use linseed oil, but this is expensive. Would any other oils work?

Oils are just another variation on the PAHs, just not as drastic (or toxic). You want a "drying oil", one that will polymerize on the surface of the wood, making it more resistant to fungal attack. You can't expect it to last as long as creosoted railroad ties, but it should be good for 5 years in a dry climate.

We could soak them in borax. Would this just leach way?

You don't want to do this. In a dry climate boron has a tendency to accumulate and poison the soil.

I look around, I see ancient rough cedar posts were used all around here. No one spends a dime on fancy fences here-most were probably collected with a forest permit. But, I also see rough cedar posts stocked at our feed store. They aren't pretty, they are the real deal. No one treats them or even bothers to peel them and they outlast the farmer. I just reclaimed some that are probably older than me. Still serviceable. Would not pass any HOA standard ever written.

Edited to add: by cedar, I mean locally harvested juniper, which is also called cedar in the intermountain west. Not the PNW cedar.

juniper is indeed long lasting, especialy in the dry climates it tends to grow in.

To give a good answer to this we need to know about the climate and soil. Untreated pine poles will last 10 years or more in the well drained sandy soils of my families spead in michigan. But here on my homestead in the PNW on red clay with 6 months of rain with temps above freezing 6" treated poles are faling apart at 3-7 years. I'm hoping the red cedar I'm replacing them with will do better.

Gilbert have you read this thread ?

https://permies.com/t/22394/timber/charring-effective-treatment-ground-preservation

I did; but I was wondering if the fact that I am going to use green posts, which will get shrinkage cracks, would make a difference. I was thinking of charing and then applying some sort of tar, as the charred wood should absorb it.

Am I right in thinking that charring green wood will not get the same good results as with dry wood?

Hey Gilbert!

Glad you read the other post as I don't have to be so redundant. In time I will get around to writing "little gems" of folk wisdom that folks can use...one will be on this subject.

Green or dry makes not difference...

If you take a good rot resistant species like locust or osage you could, with proper treatment get 200 or more years out of them, depending on conditions. I was lucky to befriend a Dendrologist during a plane flight out west that studied decomposing wood for his PhD, (Carl has since passed away.) During the flight he share with me his studies and told me he had dead trees in the pacific northwest that had been 700 years old and now had been standing dead for another 300 to 500 years. I learned much about this topic and have been able to extrapolate much crossover info from folk style methods of preserving posts. You can also get "drill in" wood preserving capsules (probably over kill) and also a product like:

http://www.livingwithbugs.com/timbor.html

I would add borax with caution, as it is effective but has been shared can "foul" the soil around the post, so only treat the post by soaking in a bucket then letting dry.

Another treatment that helps in conjunction with the others to extend post longevity is painting the charr with "Pine Tar" like this product:

http://www.maritime.org/conf/conf-kaye-tar.htm

http://pinetarworld.com/

Also remember, MOST IMPORTANT, do not back fill with any soil!!! NO, NO, never...Use a large stone on the bottom of each hole (tamped in) as a plinth (or stones packed in on edge as the plinth,) then pack 30 mm and larger stone around this to grade level. All stones are packed in with them oriented on edge with long axis up. With the post in the center it should look like a "sun burst" of packed stone around the post. If the come past grade level and stick up out of the ground...all the better. With the above treatments your post will be around for a very, very long time.

Good Luck,

j

I have to ask, why will your posts not include cedar, redwood, etc.?

What are you planning to use - and, if I may ask, why?

Since, it seems to me, the first answer to achieving a long lasting fence post is to use a wood that will last a long time, rather than to avoid those woods and look for a way of treating another to make it last.

I missed it if anyone mentioned using black locust, which has quite the reputation for holding up in this sort of role.

Hi Peter,

If you follow my leads provided white pine, poplar, or cottonwood posts can last over a century before requiring replacement. That is the main reason to treat instead of the cost of shipping something your way. If you have a rot resistant species...all the better, but not absolutely required. Hope that helped...

Because I am using whatever we can find to cut up for free. We don't have much money.

In our area, that will be mostly weak woods, unless we can get our hands on some honey locust. (Not as good as the black, but OK.) The widely planted trees here are various pines, cottonwood/poplar, aspen, maple, russian olive, and siberian elm. (I don't have any data on Siberian elm. The others are all fairly week and rot prone.)

Gilbert,
I'm wondering what critter(s) you are fencing out? or fencing in?
John

I am fencing out deer, rabbits, and stray dogs, mostly. And I hope that it will be a partial barrier against people as well. Someone with a lot of determination could get through, but it would make most people think twice. Especially since there are windows overlooking the site, and climbing over would take some time.

The posts will be holding up wire mesh fencing, small at the bottom and big at the top, laid down across the ground for some distance at the base to deter digging.

As mentioned; backfilling with stone and coarse sand works well. The higher the clay content of the neighboring soil, the more backfill below the bottom of the post will be needed. As depends on amount of rain and typical relative humidity of the region.