what makes black locust last so long? Is it safe for gardens?

When folks ask me about what to use for a raised bed border, I always
say "stone".  And if that is in short supply, I have a long list of
what not to use:  railroad ties, treated wood, cedar, black walnut
....  and then there is the mystery jewel ....  black locust.  It
lasts so long that farmers called it "stone wood".  What makes it
last?  Is it safe for use in a garden?

Black locust is at least as long lasting in the ground as redwood. It contains high concentrations (up to 4% by dry weight, which is a lot!) of an anti-fungal compound called taxifolin, plus  flavonoids that also inhibit rot. I haven't seen evidence that this stuff leaches from the wood in harmful amounts, so I'd be comfortable using it in a garden. I've read that if children chew the bark it makes their tongue burn, but not much more than that. I'll bet redwood or cedar bark isn't exactly delicious either. The challenge would be finding enough lumber for a raised bed.

I've had solid-looking posts of black locust pointed out to me that were installed by the owner's grandfather 60 years before. The old story is that you put posts in the ground for 40 years, and then pull them out and put the other end in the ground for another 40. The trees do tend to sucker when they're cut, which may or may not be a problem--that's coppicing. The leaves are said to be good fodder, and of course, it's a nitrogen fixer. What's not to like?

My current raised beds are black locust.  Not lumber, but logs and branches from where a neighbor took down their tree.

So what you are saying is that black locust contains a lot of a strong, natural, fungicide?  But that it is pretty tightly locked up inside of the wood, so that it won't make the growies in the raised bed sad?

Here is a big gob of info on the topic:  http://www.lib.ncsu.edu/theses/available/etd-11242003-154755/unrestricted/etd.pdf

How do flavonoids inhibit rot?

I know that flavenoids are strong antioxidants. 

old timers where i am say that eastern red cedar lasts longer than locust when in contact with the ground?  observations/opinions?

Interesting post from Snyder (2003).  Limits to rot in wood are going to be interactions between wood and fungi.  Peek at page 12... and look for info on Phenolics (incl. Tannins).  I bet this is your angle...

3.3 Anti-quality Factors of Tannins

Several animal scientists explored the nutritional effects of tannins in detail (Kumar and Singh; Robbins et al., 1987; Van Soest, 1982). As expected from the ir strong efficacy as plant defense chemicals, tannins express a variety of toxic or anti-quality effects when presented in the diet of ruminant animals (Van Soest, 1982).

From what I have seen, black locust is very resistant to decay(aside from the heart of the log, most trees are susceptible there, cedar included).  It can last 150 years in the "clear shakes" form that people have used in roofing.  There is a variety of the black locust that grows long straight timbers(ship mast stuff) and shingles can be cut from that.  Or posts, which if sawn or rived could end up being free of heart(pith), and would likely last forever.  When the pith is included in the cut of wood, there is a chance it could rot from the inside out.  Usually with black locust the wood is so strong it can handle having some hollow parts inside making round fence posts made from 4" to 6" fine and ideal for garden posts, borders, etc. The typical black locust that "Farmer Joe" planted way back because of erosion would produce timber that would likely not be straight enough to remove pith effectively, so you just deal with it and let the center of the piece you are using rot from the inside if it wants to. 

Nice info!

I'm definitely not scared of tannins. You could say I eat them for breakfast...but it would be more accurate to say that I drink coffee.

Black locust has unusually high amounts of calcium oxalate. Apparently one can see crystals of calcium oxalate in thin sections of the wood. I suppose presence of oxalates might inhibit microbes as it would severely disrupt their uptake of iron.

I do know that during my time in the southern Appalachians, black locust was always used wherever rot resistance was needed and that included raised beds, composting bins and retaining walls.

I don't think you need worry about toxicity. The wood is pretty 'tight' and what fraction is going to get into your compost? I knew people using locust for compost bins and they never had problems getting a pile to heat.

Thomas

tc20852 wrote:
Black locust has unusually high amounts of calcium oxalate.

Oh, cool. Oxalate is great in terms of carbon sequestration. It's basically two CO2 molecules bonded together: not much energy to make, stable, not very acidifying for the amount of carbon.

If your goal is to help global warming, black locust wouldn't seem to be a good choice for biochar, then, because oxalate decomposes at the temperatures in question.

Good friends have raised beds from locust; twenty years old at this point and still going strong. My experience with red cedar (mine is not the same as what left coasters call red cedar though) is not as successful...

S

What is the best way to get black locust established from seed or sapling?

Mangudai wrote:
What is the best way to get black locust established from seed or sapling?

Whichever you have. All it needs is soil and light it will grow easily.

I grow seed in a nursery bed first, then plant it out the next year, so it is tall enough to be above the weeds.  They transplant easily and seem to grow super fast in their youth.

We had black locust at a previous house. It was a weed at the time. I spent a lot of time pulling the saplings to keep the small yard I had from being overrun in a black locust mono-culture. Worse than the lilacs.
Based on that, you should have no troubles getting them started.

Scott Reil wrote:
Good friends have raised beds from locust; twenty years old at this point and still going strong. My experience with red cedar (mine is not the same as what left coasters call red cedar though) is not as successful...

S

Any chance you can post some pictures of the twenty years old black locust raised beds?

I'll see if I can get an image from Anita...

S

Black locust might be one of those materials you ideally get from a neighbor who already has it growing....and then you don't have to deal with its downsides (vigorous proliferation and therefore equally vigorous containment [goats!]) at your place. 

Makes sense that it's so hard and dense it doesn't leach nasties very much, as opposed to cedar. 

I know that each situation is different, but Black Locust is a tree that i wouldn't want to be without. I have a lot of uses for B. Locust and the only thing i have to do is remove any trees that get into places where i don't want it to be. If it gets into the garden, pull it while small. If it is in the mow area mow it like a weed.

I don't think that B. Locust can be eradicated once established, but this also means that all i do is harvest it when i want to use it.

I guess that a person thinking of growing B. Locust should weigh the pro's n con's. Once established it is forever.  I love my B. Locust groves.

I'm very seriously considering a black locust tattoo, though I have been for about five years, so maybe it isn't that serious.  I just really have an affinity for it.  maybe it's my totem or something.  I'll gladly take the good with the bad.

working URL for "Evaluation of Robinia pseudoacacia L. as Browse for Meat Goat Production in the Southeastern USA" as of January 2021: https://repository.lib.ncsu.edu/bitstream/handle/1840.16/5769/etd.pdf?sequence=1&isAllowed=y

Toby Hemenway wrote:Black locust is at least as long lasting in the ground as redwood. It contains high concentrations (up to 4% by dry weight, which is a lot!) of an anti-fungal compound called taxifolin, plus  flavonoids that also inhibit rot. I haven't seen evidence that this stuff leaches from the wood in harmful amounts, so I'd be comfortable using it in a garden. I've read that if children chew the bark it makes their tongue burn, but not much more than that. I'll bet redwood or cedar bark isn't exactly delicious either. The challenge would be finding enough lumber for a raised bed.
...
The leaves are said to be good fodder

The leaves are what I want to ask about. This fall I made couple containers of liquid fertiliser (JADAM Korean technique), with the main component being black locust leaves. There are other plants in there, but it's mostly black locust leaves.

There is information on the web stating that: "The leaves are rich in tannin and other substances which inhibit the growth of other plants"
https://pfaf.org/user/Plant.aspx?LatinName=robinia_pseudoacacia

Upon "researching" further (meaning reading whatever I could stumble upon), I found that the main culprits for inhibiting the growth of other plants in black locust, are: robin, robitin, and robinine, which I first thought was flavonoids but in fact they are toxalbumins.

https://www.researchgate.net/publication/308086447_Toxalbumins
Toxalbumins are complex proteins found in certain plant species that are toxic when ingested, inhaled, or administered parenterally. The most common plants containing toxalbumins are Ricinus communis, Abrus precatorius, and Robinia pseudoacacia. Although R. communis and A. precatorius concentrate the toxin within their seeds, the toxic lectins of R. pseudoacacia are found in the bark, seeds, leaves, and roots of the plant. The toxalbumins are summarized in Table 1. The primary toxins in these plants are ricin, abrin, and robin. They are classified as ribosome-inactivating proteins (RIP) and will be described in further detail later in the chapter.
Robinia pseudoacacia aka Black Locust.


So my dilemma is how good does robin and other phytotoxins decay in fermentation or anaerobic decomposition in water or ground.
Is it safe for garden plants to use black locust leaves JADAM liquid fertilizer on them? Maybe it's safe for biochar and new ground preparation, or will it become safe after some time spent decomposing? Also, can the branches (when chipped) be safely used as mulch in raised beds?

If anybody can shed some light on this, I'd be thankful.
I'm quoting Toby, but really I am asking if anybody knows or has an educated guess on this subject.

Black locust (Robinia)  far far outlasts  eastern red cedar (Juniperus) in ground contact but can’t hold a candle to Osage orange (maclura spp).  Grandfather in law once said he’d sold hedge posts, guaranteed them for 50 years, and never had one returned). Don’t know why either lasts so long but Robinia wood hasn’t affected my garden raised beds. Hope it helps.