Toxicity of Black Locust

The leaves, bark and wood of Black Locust are poisonous to both people and livestock. March 31, 2008

I have a customer who wants locust fence posts, and claims to have horses that will chew on the fencing. He is concerned that the wood may be poisonous to them. I can't find any literature on how toxic the wood itself can be, but have found that there is some sort of poison between the bark and wood. This is now a growing concern to me because I am exposed to this wood everyday. I wear a respirator, but running a manual mill and having the dust thrown onto you, it only does so much.

Forum Responses
(Sawing and Drying Forum)
From contributor A:
Found this for you...
"Toxicity: Black locust is poisonous to all animals if ingested. Although fatal cases are rare, recovery may take several days or even weeks. There have been reports of children poisoned by chewing the inner bark or eating seeds. However, most reported cases involve horses that became ill after eating young shoots or chewing bark. Cows, chickens, and sheep have also been poisoned. The toxic young shoots appear to be desired by livestock, even if there is plenty of other forage available. It has been suggested that flowers are toxic as well. In some cases, it may be advisable to fence off black locust trees to eliminate access of grazing livestock to shoots, bark, and seeds. Care should be taken after black locust trees are cut to remove any shoots that are likely to sprout from the stumps."

From contributor T:
What I've found on this in the past is similar to what contributor A posted. If there's no hurry getting these posts in the ground, then I would debark them. This will not only remove the poisonous bark, but it will make the post last longer. I've literally seen them sprout when set in the ground. Some take this farther by charring the part of the posts that will be set in the ground.

From contributor C:

Below is content from the Purdue website. The link to the webpage is at the end of my post.

Robinia pseudo-acacia
(pea family)

TOXICITY RATING: High to moderate.

ANIMALS AFFECTED: Horses are particularly at risk, but all animals ingesting the plant may be poisoned.

DANGEROUS PARTS OF PLANT: Leaves, especially wilted leaves, young shoots, pods, seeds, inner bark.

CLASS OF SIGNS: Depression, poor appetite, weakness, paralysis, abdominal pain, diarrhea (which may be bloody) and abnormalities in the heart rate and/or rhythm. Death is possible.

PLANT DESCRIPTION: These moderate-sized trees with rough bark often bear two short spines at the base of each leafstalk (easiest to see on young leaves). Leaves are alternate and pinnately compound with oval, entire leaflets (fig. 48). The fragrant flowers are creamy white, sweet-pea-like, and arranged in long drooping clusters. The fruit is a flat brown pod which contains kidney-shaped beans (fig. 48A). Black locusts are common in well-drained woods, thickets, and waste areas, especially in the southeastern part of the state. They are often planted along highways and fencerows as ornamentals and for erosion control.

SIGNS: This discussion will center on the effects in horses, the species most likely to be poisoned by black locust. Horses may ingest the bark or leaves when hungry and no other forage is available, or if they are confined or bored in the vicinity of the tree.

There are several toxic components in black locust including the toxic protein robin, the glycoside robitin, and the alkaloid robinine. The toxins affect the gastrointestinal tract as well as the nervous system. Clinical signs can manifest as soon as one hour after consumption and can include depression, poor appetite, generalized weakness to paralysis, abdominal pain, diarrhea (which may be bloody) and abnormalities in the heart rate and/or rhythm. With sufficient amounts ingested, death may occur within a few days, although black locust is not always lethal. Some animals recover despite showing clinical signs, an indication of the dose-dependent nature of the toxin.

Honey locust (Gleditsia triacanthos, pea family) has been implicated in causing similar toxic signs, but the information on this is not clear. Prickly ash (Zanthoxylum americanum, citrus family) superficially resembles black locust in vegetative aspect and has been blamed for loss of sheep.

FIRST AID: If horses are observed eating black locust, contact a veterinarian immediately, since emergency measures to rid the gastrointestinal tract of toxin may be implemented. Beyond this, therapy is aimed at preventing further exposure and keeping other animals away from the trees, and treating clinical signs symptomatically. Recovery may take days to weeks. Be extra cautious around affected horses to prevent human injury, and these horses should not be ridden until all clinical signs have resolved.

SAFETY IN PREPARED FEEDS: Reports are not clear on this matter, but given the potentially toxic nature of black locust, it should never be allowed to contaminate feeds, especially those destined for horses.

PREVENTION: Do not confine horses in an area where black locust grows. If this is unavoidable, provide enough palatable feed so that the horses leave the trees alone. Some horses are wood and bark chewers, however, and for these horses is may be necessary to fence off the trees or utilize a different pasture to prevent toxicosis. Paints and sprays to prevent wood chewing may be tried, but long-term success with these treatments may be difficult.

From contributor P:
It looks like it isn't the post that is the danger, but you sure wouldn't want any to sprout!

From the original questioner:
Thanks to all for the great feedback. Think I'm going to let these logs sit and let the bark fall off even though they are going to be harder to saw as they dry up a little bit. These logs are a pain to stack without bark - they are extremely slippery!

From contributor L:
What about using black locust as landscape timbers around a vegetable garden? Can the poisons affect the veggies and my family when we consume them? I have lots of standing black locust, living and dead with bark, and with treated 6x6x12 going for approximately $25 each, I'd thought the black locust would make good raised bed timbers, but don't want to poison our garden produce!

From contributor P:
I am not aware of any ability of this wood to transfer a toxin via a garden plant or vegetable to a person in this way. Couldn't say the same for some preservatives, such as creosote and goodness(!) how many raised beds have been built up bordered by old railroad ties over the years?

From contributor L:
Thanks much. I agree and sawed about a dozen 12 foot long locust timbers today. Man! Hard stuff! Very pretty too.