The short answer: keep horses away from wilted red-maple leaves, yew, oleander, oak leaves and acorns, avocado plant parts, black-walnut material and shavings, rhododendron or azalea, onion and garlic, and overly large meals of high-starch concentrate. This is not a list of nine ordinary snack foods. Several hazards are trees or landscaping plants that can blow into a paddock, hide in hay, or become contaminated bedding. One is a feeding-management error rather than a poison.
If your horse may have eaten any item below, call your veterinarian promptly. Have the plant or product name, estimated amount, time of exposure, your horse’s body weight, and photographs or a sample available. The ASPCA Animal Poison Control Center also directs owners to its poison-control service. Do not give a home remedy or attempt treatment unless a veterinarian tells you to do so.
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“Never eat” is intentionally conservative. Toxicity depends on the species, plant part, freshness, amount, route of exposure, and the horse’s access to normal forage. A branch can fall into a field, a horse can reach through a fence, and a toxic plant can be baled into hay. Black-walnut shavings are a separate bedding hazard because a horse may ingest them while lying down or eating from the floor.
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| # | Hazard | How exposure occurs | Documented concern |
|---|---|---|---|
| 1 | Wilted or partly dried red maple leaves | Grazing, fallen branches, hay | Red-blood-cell destruction; possible secondary kidney damage |
| 2 | Yew | Chewed landscaping branches or clippings | Heart-rate and rhythm changes, collapse, sudden death |
| 3 | Oleander | Leaves, stems, or trimmings | Cardiac glycoside poisoning, irregular rhythm, possible death |
| 4 | Oak leaves and acorns | Browsing, fallen acorns, limited-forage fields | Tannin-related stomach upset and possible colic |
| 5 | Avocado plant parts | Fruit, leaves, stems, or seeds | Reported myocardial injury, mastitis, and colic in horses |
| 6 | Black-walnut plant material | Branches, bark, leaves, or wood in a turnout area | Laminitis |
| 7 | Black-walnut shavings | Bedding that contains black walnut | Laminitis; ASPCA reports cases with as little as 20% black walnut in shavings |
| 8 | Rhododendron and azalea | Browsing ornamental shrubs or clippings | Listed by ASPCA as toxic to horses; remove access and seek advice |
| 9 | Onion and garlic | Garden waste, kitchen scraps, or accidental feed inclusion | Listed by ASPCA as toxic to horses; treat any ingestion as a veterinary question |
1. Wilted or partly dried red maple leaves
Red maple (Acer rubrum) is especially dangerous after leaves wilt or partially dry. ASPCA describes destruction of red blood cells, with possible secondary kidney damage. Dark or red urine can be an early warning sign, but a horse may be seriously affected before an owner notices obvious changes.
Do not assume a fallen leaf is harmless because it is no longer green. Remove branches after storms, inspect fence lines, and check hay for unfamiliar leaves. Keep horses away from piles of pruned material. If exposure is possible, call your veterinarian immediately rather than waiting for urine discoloration or weakness.
2. Yew
Yew (Taxus species) is a landscaping and cemetery plant that can be fatal in very small ingestions. ASPCA reports changes in heart rate and rhythm, low blood pressure, collapse, and sudden death. Needles, twigs, and clippings should never be placed where a horse can browse them.
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Yew can be particularly difficult to manage because a horse may reach over a fence or consume a fresh trimming before anyone sees it. Remove the entire plant from the horse’s access and treat a suspected bite as an emergency, even if the horse appears normal.
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3. Oleander
Oleander (Nerium oleander) contains cardiac glycosides. ASPCA reports irregular heart rate or rhythm, electrolyte changes, and possible death after exposure. Leaves, stems, and discarded hedge trimmings all warrant the same caution.
Do not compost oleander where horses can reach the pile, and do not use its branches as temporary fence posts or shade. Because heart effects can progress quickly, contact a veterinarian as soon as ingestion is suspected and identify the plant by name or photograph.
4. Oak leaves and acorns
Oak (Quercus species) leaves and acorns contain tannins. ASPCA describes significant stomach upset and potential colic, with risk influenced by circumstances such as limited forage. Spring buds and small leaves can be relevant, as can a field where fallen acorns are abundant and grass is sparse.
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5. Avocado fruit, leaves, stems, and seeds
Avocado (Persea species) should not be used as a horse treat. Merck Veterinary Manual records horse cases involving myocardial injury, mastitis, and colic after avocado exposure. The relevant material may include fruit, leaves, stems, or seeds, so removing only the pit does not make scraps safe.
Keep avocado trees, fallen fruit, kitchen waste, and compost inaccessible. Because reports involve several plant parts and different clinical problems, tell the veterinarian exactly what was eaten and which part was involved.
6. Black-walnut plant material
Black walnut (Juglans nigra) can cause laminitis when horses ingest plant material. Branches, bark, leaves, and wood fragments may enter a turnout area after pruning or a storm. A horse may also nibble a living tree through a damaged fence.
Remove black-walnut trees or establish a barrier that prevents browsing; do not leave cut limbs in a paddock. Laminitis can become a prolonged, painful condition, so report suspected exposure promptly, even before obvious lameness appears.
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7. Black-walnut shavings used as bedding
Black-walnut toxicity is not limited to pasture plants. ASPCA reports laminitis in horses or donkeys exposed to bedding shavings containing as little as 20% black walnut. That figure is a reported exposure observation, not a universal safe threshold for every product, mixture, or duration.
Ask the supplier what wood species are in a bedding blend; a generic “hardwood” label may not answer the question. If black walnut is present, remove the bedding and prevent further contact while speaking with your veterinarian. Do not wait for a horse to become visibly lame before treating the exposure seriously.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.8. Rhododendron and azalea
Rhododendron and azalea are ornamental shrubs included on the ASPCA toxic-plant list for horses. Their presence around a barn, garden, or trail creates a browsing risk because clipping piles and hedge trimmings may be more accessible than the planted shrub.
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Do not rely on a horse’s usual avoidance of unfamiliar plants. Remove these shrubs from the horse’s reach, secure garden waste, and obtain veterinary guidance after any known ingestion. The official list is broad; it does not establish a safe serving size for a particular cultivar or plant part.
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9. Onion and garlic
Onion and garlic also appear on the ASPCA list of plants toxic to horses. Accidental sources can include garden pullings, kitchen scraps, compost, or a feed mixture contaminated with human food. The list entry is a reason to prevent access, not a basis for calculating a safe amount at home.
Keep scraps and compost behind a secure barrier and check that treats or supplements do not contain ingredients your veterinarian has advised you to avoid. If a horse has eaten onion or garlic, report the product, approximate amount, and timing rather than waiting for signs.
Too much grain is a separate, measurable feeding hazard
High-starch or high-sugar concentrate is not one of the poisonous plants above, but feeding excess concentrate can raise serious disease risk. Merck Veterinary Manual’s February 2026 full review says that more than 50% of ration dry matter from high-starch or high-sugar concentrates has been documented to increase the risk of laminitis, colic, and equine gastric ulcer syndrome in otherwise healthy adult horses.
The same review says not to offer more than 0.5% of body weight in grain-based concentrate at a single feeding. For a 500-kg (1,100-lb) horse, that equals 2.5 kg (5.5 lb) at one meal. This is a feeding limit, not a claim that every horse will become ill at exactly that quantity; ration composition, meal frequency, workload, and individual health matter.
Measure feed by weight, divide concentrate into smaller meals, and make diet changes gradually. ASPCA advises gradual transitions to help avoid colic or laminitis. Ask an equine veterinarian or nutrition professional to calculate the ration when a horse is overweight, insulin-dysregulated, prone to laminitis, or recovering from gastrointestinal disease.
Where accidental exposure happens
- Pasture edges: wind-blown branches, fallen leaves, acorns, and reachable landscaping plants can enter a field without being deliberately planted there.
- Hay: toxic leaves or small branches can be baled with forage. Reject a bale containing unfamiliar plant material rather than picking out a few visible pieces.
- Fences and tie areas: horses can reach shrubs or tree limbs over a rail, through a gap, or while tied near a trailer.
- Bedding and compost: black-walnut shavings and discarded garden trimmings create exposure routes outside the feed room.
- Feed rooms: open grain, kitchen scraps, and mixed treats make accidental onion, garlic, avocado, or concentrate exposure more likely.
What to do after suspected ingestion
- Move the horse away from the source without putting yourself at risk.
- Call your veterinarian promptly and follow the instructions given. The ASPCA Animal Poison Control Center is another resource identified in its horse-care guidance.
- Save a plant sample, photograph, feed label, or bedding label when it is safe to do so. Do not delay the call while trying to identify the plant perfectly.
- Record the approximate amount, time, route of exposure, and the horse’s weight and current symptoms.
- Monitor the horse continuously for changes such as weakness, collapse, abnormal heart rate, dark or red urine, lameness, abdominal pain, or reduced manure, and report changes immediately.
Do not induce vomiting, give activated charcoal, administer oil, or use another home treatment unless a veterinarian specifically directs it. Different toxins require different management, and an apparently mild exposure can worsen after a delay.
Quick Recap
Preventive pasture and feed checks
- Walk every paddock in spring, after storms, and during leaf and acorn drop.
- Photograph unfamiliar plants and confirm their identity before turnout; when uncertain, keep horses out and ask a veterinarian or local extension professional.
- Inspect each hay delivery for leaves, seeds, or branches that do not belong in the forage.
- Confirm the wood species in every bedding product, including blends and recycled materials.
- Store grain, treats, produce, and compost behind closed doors or secure fencing.
- Weigh concentrate portions and introduce ration changes over several days rather than switching abruptly.
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