There are at least 200 plants/trees or scrubs that are toxic to horses at some level if they are ingested. Depending on the plant species, only a small amount need be ingested to prove fatal.  With other plants or grasses, a larger quantity would need to be eaten over a longer period of time  before toxic levels  would be reached. 

Horses, in general, will not eat toxic plants. But occasionally accidents will happen.  Accidentally poisonings usually occur when toxic plants are mixed in with processed hay. The few plants and trees that I have included in this list are generally thought of as "medicinal" to humans in herbal remedies.   BUT, they are not safe to give to horses.  These plants are considered toxic to some level if fed to horses and should be avoided at all costs.

Please note: I have first listed the medicinal properties associated with these plants as they pertain to humans, and I follow that with the the plants toxic attributes that can and will affect horses.

The caster bean as been used for its medicinal "oil".  Caster bean oil has been used for centuries. Heat is used to extract the oil from the seeds-which is "ricine" free. The oil that is extracted contains "ricinolein" which is relatively safe, but used as a very strong laxative. 

The "seeds" of the caster bean plant contain a very active poison called "ricin".  One caster bean seed can kill a small child.  Only a tiny amount of seeds need be ingested by a horse to prove fatal.  This usually occurs when the plant is harvested along with a hay crop. 

The signs of poisoning can occur several hours after the seeds are ingested.  Colic, decrease in urination, weakness, diarrhea, convulsions are all signs of acute poisoning. Ricin poisoning is usually fatal.

St. Johnswort Herb- "Hypericum perforatum"

St Johnswort is a very common medicinal herb in today's society. You can purchase this herb in Wal-Mart.  St Johnswort is now being used to treat mild forms of depression in humans. It is the #1 seller in the alternative market today. Must be a awful lot of depressed folks around!!

There are at least 25 species of St.Johnswort that can poison horses and it doesn't matter if the plant is fresh or dried.  New growth can be as toxic as older, more mature plants. The toxic element in the plant is " hypericin". Once ingested, the toxin is spread through the circulatory system to the skin, where it causes photosensitivity in non-pigmented skin areas. Animals with alot of large white areas can rub themselves raw from the irritation.These raw areas can become infected easily and are difficult to clear up.  Horses may also experience mild fever, and some diarrhea. If left unchecked, and the animal continues to eat this plant, blindness may occur.

American or European Mistletoe-both have been used medicinally as emetic and nervines.  American Indians used it to induce abortion. It has styptic type properties and will  help stop bleeding after giving birth.  Has also been shown to have anti cancer type properties. ONLY extremely knowledgeable herbalists should handle this plant.

The Mistletoe plant is a "parasitic" plant. It attaches itself to a host, usually an oak or elm tree.  Horses may poison themselves accidentally while browsing and eating the tree leaves of the host tree. Mistletoe contains toxic proteins and other unknown elements that are extremely dangerous to horses. After eating Mistletoe, there are few symptoms that one may see, such as mild colic or diarrhea, but death is quite sudden. The berries of the Mistletoe are also extremely toxic and can kill a child if ingested.

Milkweed has been used as a diuretic and purgative for centuries.  Useful for kidney problems, water retention, gallstone, and some forms of asthma.  Some American Indians rubbed the juice of the milkweed on warts, others drank it as a means of making themselves  sterile.

There are more than 36 species of Milkweed, and the narrow leaf variety is more toxic than the broad leaf varieties.  Milkweed contains galitoxin resinoid and a very strong toxic cardiac glycoside.  Milkweed is toxic in the fresh or dried form. Signs of poisoning are bloating, staggering, weakness, fevers and then death.  Symptoms may appear within a few hours of horses ingesting the plant.

Boxwood has been used as a diaphoretic and purgative over the years.  The bark and leaves are considered medicinal. 

The toxic alkaloids in this plant are not clearly identified, but are know to be highly poisonous to horses. A horse need only eat about a half pound of leaves to be fatal. Signs of poisoning are severe stomach/colic like symptoms with blood in the manure. The horse usually dies from respiratory failure within 5 hours. Death is swift.

The bark of the Black Walnut tree is used as an astringent, the leaves as a detergent, and the rind of the nut shell as hepatic. The bark is excellent for diarrhea and to stop the production of milk, also used as a mouthwash for cankers and sores. The green rind is powdered and applied to treat ringworm, and repel fleas.

The toxic chemical in Black Walnut is "Juglone".  This is a growth inhibitor and it is carried in the roots.  Plants that live around a black walnut tree are usually stunted and rather unthrithy looking. Some plants living nearby will not produce fruit or flowers. Horses are affected when black walnut shavings get mixed into their bedding material. Horses become ill by just standing in the shavings, they do not need to ingest the shavings.As little as 5-l5 percent of Black walnut mixed into regular bedding material can cause a horse to founder, produce leg swelling, depression and respiratory problems.

Actually all fruit trees are toxic in some manner to equines. The bark of the Wild Cherry tree was a favorite ingredient in cough and cold remedies. It has a sedative type action on the respiratory nerves. Peach and Apricot leaves are dried and powdered and are used to heal sores and wounds. The leaves and bark are used for chronic bronchitis.

All the above mentioned fruit trees contain cyanogenetic glycosides in the leaves and seeds. This glycosides increases if the leaves are wilted. Wild Cherry  has the highest level . Horse will eat fruit tree leaves readily, and drinking water after ingesting the leaves will quickly release the cyanide into the bloodstream.  Death is rapid with hardly any visible symptoms.

There seems to be a bit of confusion  pertaining to Horsetail grasses. One must always go by the botanical or Latin name to avoid confusion. Horsetail Grass-{equisetum arvense} is also called "Foxtails, Scouring Rushes, Pewterwort, Shavegrass and Bottlebrush. The confusion seems to arise from its folk name Foxtails. There is another weed called called Squirrel Tail Grass that goes by the folk name "Foxtail grass". Now, its botanical name is Hordeum jubatum.

Horsetail is used in herbal medicine to treat arthritis type conditions.  It is also useful for urinary treat problems.   Claims that horsetail contains silicic acid which helps aid circulation. A decoction applied externally will stop bleeding of wounds and help to heal them.

Horsetail Grass-Equisetum arvense, contains toxic aconitic acid, palustrine and thiaminase.  Horses are the most susceptible to horsetail poisoning-as these grass can easily be mixed in with harvested hay.  Toxic levels are higher in green plants than in older mature plants. Young horses are more likely to be affected than older horses. Hay that contains Horsetail grass would have to be fed for about two weeks before symptoms would appear.  Signs are weakness, staggering, muscle problems, diarrhea, and then death. Squirrel Tail grasses do not contain any certain toxic chemicals, but do contain  flowery seed heads that have tiny spikes or teeth. These spikes can pierce the skin and hook into and are very hard to remove. Horses grazing on this grass can get severe ulceration's on their mouth, eyes, neck and face which can get infected or abscess.