Twelve Poisonous Plants

Twelve Poisonous Plants

Twelve Plants Poisonous to Horses

Here we will discuss twelve plants that are poisonous to horses. You will learn how to identify them, how to identify toxicity in your horse and when you should call your veterinarian in.

Oleander is a shrub. It has leaves that are thin, narrow, waxy, and pointed. The blossoms are usually pink or white. Since California is a coastal state, it provides the perfect conditions for this plant to grow. All parts of the Oleander plant are toxic to many animals, including humans, and it only takes a small amount to be lethal. Signs of toxicity in the horse can appear within hours of ingestion.

Signs of toxicity in the horse can appear within hours of ingestion. Signs include colic, muscle tremors, labored breathing, ataxia, weak pulse, irregular heartbeats, and even sudden death.

If you suspect that your horse may have eaten oleander, it is critical that you call your veterinarian immediately. There is no specific treatment for oleander toxicity, but early intervention and supportive care may allow a horse to recover if a lethal dose was not consumed.

If you have any suspicious plants, take a picture or bring it in at your next appointment.


Yellow Starthistle is an annual invasive weed that blooms from mid to late summer. This plant has stiff green branching stems that can grow up to three feet tall. It has tiny grey hairs that grow all over its stems and leaves which sprout at the base of the plant. Only one flower bud adorns each branch. The bud is bright yellow and will have straw colored points that can be up to two inches long.

Eating large amounts of the Yellow Starthistle causes a chewing disorder that can lead to fatal neurologic disease. Most horses will not eat Yellow Starthistle unless they have very little else to eat. If your pasture is dry, it is important to make sure this weed is removed and that the horses have adequate roughage in their diet.

There is no specific treatment for its toxicity, but early intervention and supportive care may allow a horse to recover if only small doses have been consumed.


Fiddleneck (aka Rancher’s Fireweed or Tarweed) looks fuzzy because of its hairy stems and small clustered yellow blossoms. The name fiddleneck is due to the stems ending in a coiled cluster of flowers, like that of a fiddler's neck. It is a poisonous plant that is native to California. It likes sandy areas and open areas under 6,000 feet elevation and blooms from February to June.

The plant’s yellow blossoms produce toxic seeds that contain pyrrolizidine alkaloids (PAs), causing liver disease in horses and cattle. Clinical signs may not appear until weeks to months after the first ingestion, as the toxins accumulate in the liver over time. The most common signs of toxicity are appetite loss, chronic weight loss, and photosensitivity of non-pigmented skin. Severe liver damage may result in neurologic signs, including an awkward gait, head pressing, and aimless wandering.

A diagnosis can be made based on history, clinical signs, bloodwork, abdominal ultrasound, and liver biopsy. Due to the symptoms appearing so late after ingestion, treatment is primarily supportive to try to allow the liver to recover. However, severe signs of toxicity often cannot be reversed.

Seeing Fiddleneck in your pasture? Time to get weeding!


Johnson Grass is a weed that sprouts in unmaintained pasture and is often found along roads, as it survives easily in drought-like weather. The plant grows from 3 to 8 feet tall, with scaly root stalks and relatively broad leaves with a prominent mid vein. It is especially toxic while growing. Neuropathy and teratogenesis (fetal defects) are the most important risks.

Horses can show signs after a few weeks of continuous grazing, including gradual onset of ataxia, weakness, and dribbling urine. This can progress to flaccid paralysis of the tail and hind legs.

There is no specific treatment; however, with supportive care, some horses' condition may improve. The nerve damage is permanent and once ataxia occurs, the prognosis is poor. Prevention includes good pasture maintenance and monitoring.


The nightshade family contains many toxic plants, including horse nettle, black nightshade, bittersweet nightshade, some species of groundcherry, and even tomatoes and potatoes. The plants can contain toxins such as alkaloids, saponins, and nitrates. They mainly affect the central nervous system and the gastrointestinal tract. Clinical signs of poisoning include dilation of pupils, diarrhea, loss of appetite, and loss of muscular coordination. They are toxic to horses, sheep, and cattle.

While ingestion is usually rare, last year we saw a suspected case of nightshade toxicity. The horse required immediate and intensive care, but is on the road to a full recovery.

Here are articles that explain more about the nightshade family, including how to control it. We routinely walk our paddocks, looking for noxious weeds, and nightshade is one that we have found.

It is mostly prevalent during summertime, so beware while you are out enjoying a trail ride or horse camping during the warm weather.


Black Walnut can be found in shavings bags, and there only needs to be 20% to cause a problem to your horse. A horse can succumb to the toxicity in Black Walnut after only a short period of exposure. Issues that can occur include edema of the lower legs and lameness due to laminitis. If affected horses are removed from the source of the black walnut shavings early enough, and treated for laminitis, they can recover without the severe consequences of hoof deformity and coffin bone rotation.

While toxicity is most commonly caused by exposure to shavings, it is also toxic to consume any part of the plant. Be proactive and make sure these trees are cleared from your pastures and that your horse's feed is not contaminated. Some organic medications may have trace amounts of toxic plants, so be aware and check your medication before giving it to your horse.


Approximately 100 species of Lupine occur in North America, ranging from the dry hills and plains to moist mountain valleys up to 11,000 feet altitude. Lupines bloom in the spring and summer. Their blooms can be red, white, pink, purple, blue, or orange and they can grow up to 5 feet tall. Some varieties of Lupines are drought tolerant. They do not need fertilization, and they flourish in the sun, so they thrive in our Central California weather.

Although Lupines are beautiful and many enjoy placing them in their garden, they can be toxic to horses. A horse that has eaten Lupine can have serious reproductive failure. Common fetal deformities include twisted and deformed limbs, resulting from contracted flexor tendons, and abnormal development of the bones and joints.

It is, however, difficult to confirm that poisoning is the cause for a reproductive problem, as the effects of the plants are likely to be encountered long after the plants were consumed.


Poison Hemlock can be found in California along roadsides, ditches, cultivated fields, and waste areas. It is especially prominent wherever there is moist ground.

Poison Hemlock is a coarse, stiff, biennial or perennial plant, 4 to 6 feet tall. It’s smooth branched stems are hollow with purple dots, especially near the bottom of the plant. It has a fern like appearance with small 5-petaled flowers in a loose umbrella-like shape.

Clinical signs can develop within an hour of consumption. If a lethal dose has been consumed, death from respiratory failure occurs in 2 to 3 hours. Salvation, abdominal pain, muscle tremors, and incoordination will occur initially, followed soon by difficulty breathing, dilated pupils, weak pulse, and frequent urination and defecation. Pregnant animals that survive the acute toxicity may abort.

Since there is no specific treatment for hemlock poisoning, acutely poisoned horses should be given supportive treatment as necessary. Try to create a calm environment for the horse, and if the hemlock has been recently consumed, your vet may lavage the stomach and give activated charcoal to try to remove some of the plant from the gastrointestinal tract.

Poison hemlock is relatively easy to control with herbicides. Mowing and hand-weeding should occur well before flower production to prevent seed production.

Here are some great articles to read more about Poison Hemlock:


Milkweeds are erect perennial herbs that have either broad-veined or narrow linear leaves, arranged either alternately or in spirals. The color of the flowers may vary from greenish white to red. They often grow in open areas along roadsides and waterways, preferring sandy soils of the plains and foothills.

Acute death from milkweed poisoning results from the cardiotoxic effects of the cardenolides (a form of toxin). Milkweeds are most toxic during rapid growth and retain their toxicity even when dried in hay. Fatal poisoning of an adult horse may occur with the ingestion of as little as 1.0 kg of green milkweed plant material. In addition to the cardiotoxic effects common to most milkweeds, other body systems that can be affected include respiratory, digestive, and nervous systems causing dyspnea, colic and diarrhea, muscle tremors, seizures, and head pressing.

Signs of poisoning usually begin within 8 to 10 hours after ingestion, with the severity of symptoms depending on the quantity of plant consumed. In acute milkweed poisoning, the animal may be found dead without any prior symptoms.

Because milkweeds generally tend to grow singly or in relatively small strands, they can be controlled by digging out individual plants or selectively spraying the plants with a herbicide.


Oak trees can be found all over North America and can live for a couple hundred years. Ranging from large trees to shrubs, oaks have alternate, simple, toothed, or lobed dark green glossy leaves. The leaves may be deciduous or persistent depending on the species of oak. The fruit, an acorn, is a nut partially enveloped by an involucre of scales. Two common species of oak growing in western North America commonly associated with livestock poisoning are scrub oak and shinnery oak.

Oaks at any stage of growth are poisonous, but they are particularly toxic when the leaf and flower buds are just opening in the spring. The principal toxin is gallotannin. The tannins found in the leaves, bark, and acorns of oaks affect the intestinal tract, liver, and kidneys.

Initially, affected horses stop eating, become depressed, and develop intestinal stasis. Excessive thirst and frequent urination may be observed. The feces are hard and dark initially, but a black tarry diarrhea often occurs later in the course of disease. Teeth grinding and signs of colic indicate abdominal pain. Icterus, red-colored urine, and dehydration are additional signs encountered with oak poisoning. Horses may live for 5 to 7 days after the onset of clinical signs.

Horses should be removed from the oak and given supportive care by a veterinarian. This may include intravenous fluids to rehydrate severely affected animals and maintain kidney function. Horses that continue to eat have a much better prognosis.

Take a look at the following websites for more information on oak poisoning:


There are three main clovers we are going to discuss here:

Red Clover, White Clover, and Aslike Clover.

While Red and White are not poisonous themselves to horses, a distinct mold can grow on them this time that becomes noxious to horses. In a sense it is the mold, and not the clover that does the harm. The toxin in the mold can cause excessive drooling. Although rare, this toxin can be found in hay and cause it to become moldy faster. Luckily, most horses tend to not like eating the dried clover in hay.


Aslike Clover on the other hand, is a whole 'nother ball game. It can cause photosensitization that makes the skin appear severely sunburned. The liver can also be majorly affected by the Aslike clover. In this case watch your horse for loss of appetite, weight loss, jaundice, and colic. Severe symptoms can even lead to death.

While some clover is good, because it produces Nitrogen back into the soil, a lot can start to introduce issues. Good Pasture management can include using a gentle broadleaf herbicide to help keep these plants in control.


Western Jimson Weed is very prevalent in our area. It has a large trumpet-shaped white flower. The leaves are slightly dark green and oval-shaped and can have sharp white lines streaking the leaf. The entire plant will grow to be around 4 feet. It flowers from April to October here in California. This plant is part of the nightshade and solanaceae family. It is commonly found along roadsides and in sandy or gravel areas.

Other names include: Angel's Trumpet, Devil's Weed, Indian-apple, Jimson Weed, Pricklyburr, Sacred Datura, Sacred Thorn-apple, Sacred Thornapple, Thornapple (Spanish: Toloachè, Toloachè Grande, Tecuyaui, Belladona).

All parts of this plant are extremely toxic. The best way to prevent your horse from eating this plant is to be vigilant and make sure your pastures and riding areas have been removed of all weeds, either by hand or with a safe pesticide.



  1. Oleander Photo:
  2. Yellow Starthistle Photo:
  3. Fiddleneck Photo:
  4. Johnson Grass Photo:
  5. Nightshade Photo:
  6. Black Walnut Photo:
  7. Lupine Photo:
  8. Poison Hemlock Photo:
  9. Milkweeds Photo:
  10. Oak Tree Photo:
  11. Red Clover Photo:
  12. White Clover Photo:
  13. Jimson Weed Photo: Taken by Brianna Sweet

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