Growing Danger: Poisonous Plants to Avoid

Although your horse may enjoy a day in the field, lurking pasture weeds and some trees and shrubs can be a danger to his health. | © Amy K. Dragoo/

Your horse is where he should be on a summer day, grazing out in the pasture. But while he’s enjoying himself, could he be in danger? Scores of common pasture weeds, trees, and shrubs contain toxins that can make him sick or even kill him.

Here we spotlight some top risks, a rogue’s list of plants that could do serious harm to your horse should he eat them. They’re far from the only threats, says Cynthia Gaskill, DVM, PhD, clinical veterinary toxicologist at the University of Kentucky. The list of toxic plants varies from one region to the next, so it’s essential to be familiar with the ones that grow where you live. Your county cooperative extension service and state land-grant university are good sources for this information and can help you identify plants you’re not sure about. To find one in your area, go to

A few dangerous weeds in your pasture may be no cause for major alarm. Many toxic plants taste bad, and if your horse has plenty of good forage he likely won’t be interested in them. The risk increases if the plants are tasty, if there are lots of them or if grazing is poor. Young horses and bored horses are more likely to experiment with weeds or other unusual things. Danger levels also depend on the type of plant and often on its growth stage, the season, climate, weather and other environmental factors.

Since it’s often hard to say how much of a toxic plant a horse needs to eat before poisoning occurs, the best course is to keep these plants out of his pasture and hay.

Bracken Fern and Horsetails

Bracken fern (Pteridium aquilinum) grows 2 to 4 feet tall and typically spreads to form large clumps. Horsetails (Equisetum species) are odd-looking plants with jointed, hollow green stems and no true leaves.

Where you’ll find them: These plants are common in most parts of the United States (except deserts) in open areas and woodlands. Horsetails are often found along streams and in other wet areas.

Why they’re dangerous:Both plants contain a toxin that destroys thiamine (vitamin B1), which is essential for the nervous system. All parts of the plants are toxic, and they remain dangerous when dried in hay. If a horse eats enough over a period of time, the toxin builds up in his system.

Effects:Weight loss, depression, incoordination, muscle twitches, spasms and other signs. The poisoning can be fatal, but in the early stages it can be treated with intravenous thiamine hydrochloride.

Good to know:Horses are not likely to graze large amounts of horsetails or bracken fern unless no other food is available, although young fern shoots (which are especially toxic) may tempt them in spring. “Bracken fern or horsetail poisoning in horses is more likely to occur from ingesting hay contaminated with these plants,” Dr. Gaskill says.

Endophyte-infected Grasses

Varieties of tall fescue and perennial ryegrass that are infected with endophytes (fungi that live inside plant cells) can cause problems for horses.

Where you’ll find them: Mainly in the Eastern half of the United States (tall fescue) and the Pacific Northwest (perennial ryegrass).

Why they’re dangerous:The endophytes don’t affect the plants and can’t be seen, but toxins they produce can have a range of harmful effects in horses.

Effects:Endophyte toxins in tall fescue are especially linked to abortions and other reproductive problems in mares. Endophyte toxins in perennial ryegrass produce neurological signs—ryegrass “staggers”—with trembling, incoordination, and collapse. Mild cases can recover if moved off the infected pastures.

Good to know:Fescue and ryegrass varieties developed for turf are most likely to be infected; those developed specifically for forage typically have endophytes that do not produce toxins or are endophyte free. However, Dr. Gaskill says, “Native tall fescue that grows in unimproved pastures in the Southeast is typically endophyte-infected.” An unimproved pasture is one that has not been seeded with forage grasses and/or legumes.

Jimsonweed, a member of the nightshade family, is unpalatable so horses will avoid it unless no other forage is available. | Courtesy, Anthony Knight, BVSc, MS, DACVIM

Jimsonweed and Other Nightshades

Toxic plants in the nightshade family include horsenettles, black and bittersweet nightshades, and some garden plants, including tomatoes and potatoes. Jimsonweed (Datura stramonium) is one of the nastier family members. A bushy annual (a plant that completes its life cycle in a year), it has large leaves, pale trumpet-shaped flowers and spine-covered seedpods. It goes by many names, including thorn apple, devil’s trumpet and stinkweed (for its vile odor).

Where you???ll find them: These plants grow throughout North America in fields, along roadsides and in other open areas.

Why they’re dangerous:All parts of the plant are poisonous, containing atropine and related toxins. Nightshades as a group contain a number of different alkaloid toxins.

Effects: Rapid pulse, dilated pupils, dry mouth, diarrhea, incoordination and convulsions. Poisoning can be fatal.

Good to know: Most nightshades are unpalatable—jimsonweed tastes as bad as it smells—so horses avoid them unless no other forage is available. The plants may be baled up in hay, though, and consumed that way. If the plants grow in crop fields, the seeds may contaminate other grains.

While less severely affected horses can recover after consuming poisonous locoweed, any neurological damage may be permanent. | Courtesy, Anthony Knight, BVSc, MS, DACVIM

Locoweed and Milkvetch

Locoweeds and milkvetches (Oxytropis and Astragalus species) are leafy legumes, like alfalfa, with pealike flowers and seeds in pods. Not all plants in these families are dangerous; some (such as chickpea milkvetch) are used as cattle forage. “The risk comes with ingestion of endophyte-infected locoweeds,” Dr. Gaskill says.

Where you’ll find them: Various species grow throughout the West. Many prefer dry or gravelly soil.

Why they’re dangerous: Poisonous locoweeds contain swainsonine, a toxin that has neurological and other effects. Recent research has shown that swainsonine in locoweed is produced by an endophyte rather than the plant itself.

Effects: Depression, loss of muscle control, extreme nervousness, and odd behavior. Some milkvetches contain other toxins that produce severe respiratory distress. Horses may find locoweed and milkvetch tasty, and if they eat enough over time the results can be fatal.

Good to know:Less severely poisoned horses can recover, but neurological damage may be permanent.

Whorled milkweed, one of the most dangerous varieties of milkweed, poses a serious risk when baled in hay. | Courtesy, Anthony Knight, BVSc, MS, DACVIM


Milkweeds (Asclepias species), named for their thick white sap, are upright plants topped by clusters of small flowers. There are many varieties; the most dangerous include whorled milkweeds, labriform milkweed and wooly-pod milkweed.

Where you’ll find them: Various milkweeds grow throughout the United States.

Why they’re dangerous: The sap is toxic. Many milkweed varieties contain poisons that disrupt cardiac function. Whorled milkweeds also contain a neurotoxin.

Effects:Cardiac arrhythmia and heart failure; colic, incoordination, convulsions and respiratory failure from the neurotoxin.

Good to know:Horses are not likely to eat milkweed unless other forage is limited, but whorled milkweed baled in hay can pose a serious risk. Large groups of horses have died after eating hay contaminated with these plants.

Poison Hemlock and Water Hemlock

Water hemlock is more poisonous than poison hemlock; a small amount of the root is deadly to horses. | Courtesy, Anthony Knight, BVSc, MS, DACVIM

These unrelated plants look similar—thick branching stems, clusters of small white flowers in summer and leaves that smell like parsnips when crushed. Water hemlocks (Cicuta species) have narrow, serrated leaves; poison hemlock (Conium maculatum) leaves look more delicate.

Where you’ll find them:Poison hemlock and various species of water hemlock grow throughout the United States. Both plants like wet spots, water hemlock especially so.

Why they’re dangerous: Both plants are lethal. All parts of poison hemlock (leaves, stem, fruit and root) are toxic. Water hemlock is even more poisonous; even a small amount of the root (a tuber) is deadly.

Effects: Tremors, incoordination, rapid heart rate, depression, seizures, coma and death.

Good to know: Death can be quick, especially with water hemlock.


Ragworts (Senecio species) are weedy plants with clusters of small yellow flowers. There are many varieties, including tansy ragwort and common groundsel. Some are more toxic than others.

Where you’ll find them: Different varieties are found throughout the United States, as pasture weeds and along roadsides.

Why they’re dangerous:The toxins in ragwort damage the liver. Damage is cumulative and may be well along before you see signs.

Effects: Skin damage from photosensitivity, weight loss, depression and jaundice. Severe liver damage is not reversible.

Good to know: Ragwort remains toxic when dried in hay.

Red Maple

Red maple leaves pose a threat to horses especially when wilted; most poisonings occur in the fall or after storms when horses eat leaves from downed branches. | Courtesy, Anthony Knight, BVSc, MS, DACVIM

The red maple (Acerrubrum) is a tall shade tree with three-point green leaves that turn crimson and orange in fall. Spring flowers are red.

Where you’ll find it:Red maples can be found throughout eastern North America.

Why it’s dangerous: The leaves contain a toxin that damages red blood cells, although they are dangerous mainly when wilted or withered. Most poisonings occur in the fall or after storms, when horses may eat leaves from downed branches.

Effects: Depression, poor appetite, dark red or brown urine, colic and rapid heart and respiratory rates. Poisoning can be fatal.

Good to know: Other kinds of maples can also be toxic to horses, although fewer poisonings are reported.

Yellow Star Thistle and Russian Knapweed

An invasive species, Russian knapweed contains a neurotoxin that affects the brain and the nerves that control chewing. | Courtesy, Anthony Knight, BVSc, MS, DACVIM

Star thistle (Centaurea species) and Russian knapweed (information can be found under the scientific names Rhaponticum repens, Centaurea repens and Acroptilon repens)are tough-stemmed annual weeds with thistle-like flowers. Star thistle has spines;Russian knapweed is covered with fine hairs.

Where you’ll find them:These plants grow throughout much of the eastern, southern and western United States but appear more commonly in the West. Both are considered invasive species.

Why they’re dangerous: The plants have a neurotoxin that affects the brain and the nerves that control chewing.

Effects:Lip twitching, tongue flicking, involuntary chewing movements, drowsiness and difficulty eating and drinking. Horses, donkeys and mules seem to be the only animals affected.

Good to know:Horses can develop a taste for these plants. While they need to consume a lot over time to be poisoned, the effects are not reversible.


A highly toxic plant, yew can cause rapid death if a horse ingests even a very small amount. | Courtesy, Anthony Knight, BVSc, MS, DACVIM

Yews (Taxusspecies) are narrow-leaved evergreens. Japanese yew, with waxy red berries, is the most common variety in the United States.

Where you’ll find it: Japanese and English yews are widely planted as ornamentals. Ground hemlock, a native yew, grows in wooded areas in the northeastern United States and Canada.

Why it’s dangerous: Yew is highly toxic. Horses are often poisoned by discarded yew clippings, which they will eat willingly.

Effects:Rapid breathing, trembling, gastrointestinal distress, collapse and heart failure.

Good to know: A very small amount of yew can cause rapid death.

And More

All parts of the poison hemlock are poisonous. | ourtesy, Anthony Knight, BVSc, MS, DACVIM

The plant world is full of other dangers. Out in the field may be pokeweed and buttercups (mouth irritation and colic); St. Johnswort (skin damage from photosensitivity); white snakeroot (neurologic signs) and more. “There are many different types of toxic mushrooms, some causing liver damage, some affecting the central nervous system and others that affect the digestive system,” Dr. Gaskill says.

Sudangrass and sorghum can cause neurologic problems. Besides maples, toxic trees include oaks (especially buds and new leaves) and black walnut (especially wood shavings), black locust and buckeye. And yew is not the only toxic ornamental. Oleander is lethal, and rhododendrons, boxwood, privet and many other common landscape and garden plants are dangerous. (Want more? Dr. Gaskill recommends A Guide to Plant Poisoning of Animals of North America by Anthony Knight and Richard Walter.) Your horse may not be keen to eat these plants, but it’s best to keep him away from them.

Toxin-free Forage

Make sure dangerous ornamental plants, such as rhododendrons (shown above), are out of your horse’s reach. | © Dusty Perin

Take these steps to keep your horse safe from poisonous plants.

· Patrol your pasture for dangerous plants. Inspect 3 or 4 feet beyond the fence line, since horses can often reach that far. Look for downed maple branches after storms.

· Remove the toxic plants you identify. Dig up weeds, and cut down dangerous trees and shrubs (or trim them out of horse reach). If the plants are too numerous or deep-rooted to dig up, use an herbicide or a biological control. Your county extension agent can provide information on effective products for specific plants. Follow grazing restrictions on the product label, and keep horses off the pasture until the plants are gone.

· Keep pasture quality high. Good maintenance allows grass to flourish and crowd out weeds, which often thrive in harsh conditions. And since many toxic plants are unpalatable, horses will ignore them if they have good grass to munch.

· Make sure your horse can’t nibble dangerous ornamental plants. Put fallen branches, cuttings, leaves and discarded garden plants in a compost pile that’s out of his reach. Don’t let neighbors toss clippings over the fence into your pasture.

· Check his hay. Poisonous weeds can be baled with hay, and drying will not eliminate all toxins. Buy hay from a reputable supplier, Dr. Gaskill advises. “If that is not possible, it is best to feed small bales of hay so you can look at each flake before it is fed and make sure there are not lots of weeds present. It can be hard to evaluate large round bales of hay for weeds,” she says

It Could Be Poison If …

When plant toxins irritate a horse’s mouth, he may drool and slobber. | © Dusty Perin

Toxins act in various ways, affecting different body systems. Signs can be confused with other illnesses, and they may appear soon after ingestion or days later. Poisoning might be on your list of suspects if you see:

· Colic, diarrhea and other signs of gastrointestinal upset, which can appear when toxins irritate the intestines.

· Drooling and slobbering, which result when plant toxins irritate the mouth.

· Inability to swallow, which is a common sign of yellow star thistle poisoning.

· Red, cracked, blistered skin, a sign of photosensitivity (hypersensitivity to sunlight), which can be brought on by toxins in a number of plants. It typically appears in white areas.

· Hyperexcitability, depression, incoordination, tremors, convulsions, strange behavior and other neurological signs, which can be produced by plant toxins that affect the nervous system.

· Rapid pulse and labored breathing, signs of problems with the heart and respiratory system, which can be brought on by certain toxins.

Gradual poisoning can produce subtler signs such as exercise intolerance and weight loss. Call your veterinarian if you have any reason to think your horse may have consumed toxic plants. While many plant toxins have no known antidotes, prompt supportive care can make a difference.

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