Among the many varieties of toxic plants throughout the United States, very few are a serious threat to horses. Most of these plants are either unpalatable—thus avoided unless there is absolutely nothing else to eat—or the amount a horse needs to ingest for it to be dangerous makes it nearly impossible to get sick. With that said, however, some plants can be lethal in smaller doses, and if your horse is hungry enough, the bitterness may not defer him!

Here are five of the most toxic plants to horses, the effects of which are either irreversible, quick to set in, or both.

The fatal five

1. Yew. This evergreen shrub has flat, needlelike leaves containing red or yellow berries. It is planted nationwide, but mostly in ornamental use. All parts of the plant except the berry contain taxine, which can cause respiratory/cardiac collapse. Even a single mouthful of a yew plant can be fatal to a horse within minutes.
2. Water hemlock. This perennial weed has long, toothed leaves with smaller white flowers forming at the ends of its branches. It is able to grow up to 6 feet tall and can be found throughout the United States. The most common areas for this plant are along streams and the more marshy areas of meadows. Water hemlock contains a cicutoxin alkaloid that detrimentally affects the central nervous system. About a pound of the stems and leaves can end up being a lethal dose.
3. Oleander. This evergreen shrub contains red, pink or white flowers that grow in large clusters toward the ends of its branches. Oleander has long, leathery-like leaves and is sometimes referred to as rose laurel. This plant is mainly found in hotter climates of the United States and is often used for landscaping. Containing the toxins oleandrin and nerlin, this plant disrupts the heartbeat, causing cardiac collapse. A horse need only to ingest 30 to 40 leaves for it to be deadly.
4. Red maple. Medium in size, this tree contains green leaves in the spring and summer that turn to a bright red in the fall. The bark is gray and smooth, and the twigs a more reddish-brown. Though native to the Eastern United States, it can actually be found planted most everywhere from coast to coast. Fresh, the leaves do little harm. Fallen and wilted, however, 1 to 2 pounds of the leaves can be a fatal dose, as the toxins rapidly take oxygen out of the bloodstream.
5. Bracken fern. This perennial fern has triangular leaves and grows up to 2 to 3 feet high. Bracken can be found throughout the United States, though it is partial to areas that are woodsy and moist. Containing thiaminase, this fern causes a vitamin B-1 deficiency in horses, which leads to neurological impairment. Though it can take ingesting hundreds of pounds over time, this plant can eventually cause death. 

This list is in no way exhaustive. Please keep in mind that if your horse has a satisfied appetite, it is rare that he will go near, or eat, these plants.