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"Poisonous" does not mean deadly. Some manifestations of toxicity are subtle. The dose, as always, determines if a plant is safe source of nutrients or a toxic hazard.

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BRIEF: Are Leyland Cypress trees toxic to horses?

We just purchased some Leyland Cypress ((Cupressocyparis leylandii) trees to use as a break because they grow fast. We want to find out if they are toxic to horses and other animals. We have a farm west of Seattle, Washington and have a large variety of animals (ducks, peacocks, geese, emus, chickens, horses, donkey, goats, etc).


The cypresses probably have some thujone among their monoterpenes, so you probably wouldn't want to make it a major forage source. On the other hand, if these are baby trees, the animals are more of a threat to them than vice versa. Of course, when prepared from a different plant, a thujone-rich abstract became famous as absinthe... And this essential oil was once used as a worm medicine. That's why absinthe is sometimes called wormwood.