Trees Toxic to Horses – A Practical Checklist

Knowing which trees poisonous to horses grow in and around your grazing is one of the most useful pieces of pasture management a horse owner can master. Most poisoning cases are accidental — a branch blown into a paddock during a storm, a yew hedge running along a fence line, acorns showering down after a hot summer, or garden clippings tipped over the fence by a well-meaning neighbour. The horse rarely goes looking for trouble. Trouble arrives, and the horse is hungry.

This guide covers the trees most relevant to UK and South African paddocks, ordered by how often they actually cause clinical disease rather than by how frightening they sound. Sycamore comes first, because atypical myopathy kills the majority of horses it affects and because almost every piece of standard advice about it — fence off the tree, mow the seedlings, spray them — turns out to be either incomplete or actively wrong. For the wider picture beyond trees, see our companion guide to plants poisonous to horses.

Trees poisonous to horses: at a glance

If you are walking a fence line today and want the short version, this is it. Speed of onset matters as much as toxicity — some of these give you days, and some give you no warning at all.

Sycamore poisoning and atypical myopathy

What the toxin actually is

The active toxin is hypoglycin A (HGA), a non-protein amino acid found in the seeds, seedlings and — in lower concentrations — the leaves of Acer pseudoplatanus, the European sycamore. Once eaten, HGA is converted in the body to methylenecyclopropylacetic acid (MCPA). As its CoA ester, MCPA blocks the enzymes horses use to extract energy from fat, which stops energy production in muscle fibres and in the heart. The related compound MCPA-carnitine is what laboratories measure in blood and urine to confirm a diagnosis. The resulting disease is atypical myopathy (AM), also called seasonal pasture myopathy.

One important nuance that gets lost in most warnings: the Royal Veterinary College notes there are more than 25 species of Acer, and not all of them contain HGA. Nor is every sycamore equally dangerous — HGA concentrations vary considerably between individual trees, and between seedlings taken from them. You cannot tell a high-toxin tree from a low-toxin one by eye, which is precisely why the standard advice is to manage exposure rather than to try to assess the tree.

When sycamore poisoning peaks

There are two peak periods in the UK. Seeds fall onto pasture through autumn and winter, and the germinating seedlings come through in spring. A heavy seed year is therefore followed by a heavy seedling spring, and both ends of that cycle produce cases. If you are reading this in late summer, you are roughly a month out from the start of the autumn risk window — which makes now, not October, the time to plan.

How little sycamore is dangerous

Research suggests as few as 50 sycamore seeds may be enough to kill an average horse. That figure is worth treating with care rather than quoting as gospel: susceptibility varies between individual horses, HGA content varies between trees, and it is genuinely not established how much material a horse has to eat before signs appear. Some apparently healthy horses grazing alongside affected ones have been found with high blood HGA and no disease at all. The practical reading is not “50 seeds is the limit” but “there is no known safe quantity”.

Signs of atypical myopathy

Onset is rapid and horses can deteriorate within 6 to 12 hours. Signs include:

• Muscle stiffness, trembling and reluctance or inability to walk
• A depressed horse standing with its head hung low
• Dark red or brown urine, from muscle pigment released into the bloodstream
• Sweating, breathing difficulty and a high heart rate
• Signs that look like severe colic
• Inability to stand in severely affected horses

The single most useful differentiator: the RVC points out that although affected horses show signs of severe colic, unlike a genuine colic case they usually still have an appetite. A horse that looks colicky but is happily eating in autumn or spring, near sycamores, should move sycamore poisoning straight to the top of your list. If you are unsure how a true colic presents, our guides to spasmodic colic and colic surgery are worth reading alongside this.

Reported mortality varies by source and by how early cases are caught. The British Horse Society cites mortality of around 56–61%; the RVC states the disease is fatal for around three quarters of affected horses. Either way it is a disease that kills more horses than it spares. The encouraging part is the other side of that statistic: horses that survive the first few days of treatment usually go on to recover completely, though it can take several months.

What does not work — and what actively makes it worse

This is where a lot of well-intentioned yard management goes wrong. The BHS guidance is specific:

• Harrowing increases the risk. Chain harrowing a sycamore-affected paddock disperses seed and seedling material across the whole field rather than leaving it concentrated under the tree. If you harrow as part of your routine — and there are good reasons to, as we cover in our guide to grass harrowing — keep sycamore-risk paddocks out of that rotation during the high-risk seasons.
• Spraying does not remove the toxin. Herbicides will kill seedlings, but studies have found HGA still present in stored seedlings 6 to 8 months after spraying. A dead seedling is still a toxic seedling. It has to be physically removed.
• Mowing only helps if you collect the clippings. Cut material left lying is still available to eat, and because mowing leaves the root in the ground the seedlings simply regrow.
• Fencing the drip line is not enough. The RVC notes that “helicopter” seeds can travel up to 200 yards. Paddocks with no sycamore in them at all can still be contaminated from a tree in a neighbouring field or garden.
• Watch natural water sources. HGA is water-soluble and can pass from plant material into water by direct contact. The BHS advises against using pastures containing a river or stream during high-risk seasons unless you can provide an alternative water supply.
• Do not make forage from contaminated pasture. Seeds and seedlings in bales still contained HGA after 6 to 8 months of storage, so affected fields should not be cut for hay, haylage or silage.

Practical pasture management that does work

• Physically collect and remove fallen seeds and seedlings — rake, hoover or hand-pull, and take the material off the field.
• Limit grazing time to under six hours a day where sycamores are near the pasture; length of exposure appears to matter.
• Keep ad-lib forage available. In 64% of AM cases reported to the Atypical Myopathy Alert Group between 2006 and 2019, the pasture had minimal grazing — hungry horses eat what is on the floor.
• Use hay feeders rather than feeding from the ground near sycamores, so horses are not hoovering up seeds with every mouthful.
• Check neighbouring land, not just your own, and talk to whoever owns it.
• Move the most susceptible horses off sycamore-adjacent grazing entirely in peak years.

Testing your pasture for HGA

The RVC’s Comparative Neuromuscular Diseases Laboratory offers HGA testing of seeds, seedlings and leaves directly to horse and yard owners, as well as blood testing for vets with a suspected clinical case. Samples do not need to be pre-booked. It is most worthwhile when you are taking on new grazing, assessing a rented field, or trying to settle an argument with a landlord about whether a particular tree needs managing.

Acorn and oak poisoning in horses

The autumn risk

Oak (Quercus robur and Q. petraea) produces acorns from late summer through autumn. Mast years — heavy crop years that arrive irregularly every few seasons — drop enormous quantities and produce a matching spike in poisoning cases. Search interest in acorn poisoning is one of the sharpest seasonal curves in equine health, climbing steeply from late August and peaking in October, which tells you fairly precisely when yards start finding problems.

The toxins are tannic and gallic acids, concentrated in green acorns and young leaves. They damage the lining of the gut and, in larger doses, the kidneys.

Why some horses are affected and others are not

Plenty of horses graze under oaks for years without trouble. The problem is that individual horses develop a genuine taste for acorns and will then seek them out and eat them in quantity, hoovering the ground and ignoring perfectly good grass. Once a horse has acquired the habit it is very difficult to break, and that individual becomes a recurring autumn risk. Ponies and youngstock appear more susceptible. Signs include:

• Colic, abdominal pain and reduced gut sounds
• Dark, bloody or tarry diarrhoea
• Loss of appetite and obvious depression
• Reduced urine output as kidney damage develops
• Collapse in severe cases

Managing oak in a horse paddock

• Fence off mature oaks well before the drop begins, not once acorns are already on the ground.
• Collect windfalls daily in mast years — a garden vacuum or a stiff rake is faster than it sounds.
• Provide plenty of alternative forage so acorns are not the most interesting thing in the field.
• Move known acorn-eaters to acorn-free grazing for the six to eight weeks of peak drop. This is the only reliable answer for a horse that has developed the habit.
• Remember that a windy night can restock a “cleared” paddock overnight.

Yew: the fastest killer

Why yew is uniquely dangerous

Yew (Taxus baccata) is the most acutely toxic tree to horses in the British Isles. Every part of the plant except the red flesh of the berry contains taxine alkaloids, which interfere directly with cardiac conduction. As little as a single mouthful of foliage can kill an adult horse, frequently within hours and often with no meaningful warning. In many cases the first sign is a dead horse in the paddock with foliage still in its mouth.

Where yew is encountered

Yew is widespread in UK churchyards, formal gardens, estate planting and old hedgerows. Two features make it particularly dangerous around horses: it is evergreen, so it is just as lethal in February as in July, and the clippings stay toxic as they dry and brown. Garden waste tipped over a fence is a well-recognised route of accidental poisoning, and dried yew is if anything more likely to be eaten than fresh.

Practical advice

Treat yew as an absolute exclusion. Fence it well beyond stretching reach — horses are astonishingly good at getting to the one thing they should not have — and make sure everyone with access to the boundary knows that yew clippings must never be dumped near grazing. There is no reliable treatment once cardiac signs are established, which makes prevention the entire strategy.

Laburnum: the overlooked garden tree

Laburnum (Laburnum anagyroides), the golden chain tree, is named on the British Horse Society’s poisonous plants list and is one of the most commonly missed hazards in this whole category — largely because people think of it as a garden ornamental rather than a paddock tree.

Every part of the plant contains cytisine, with the highest concentrations in the ripe seeds and seed pods. Cytisine acts on nicotinic receptors in a manner broadly similar to nicotine, overstimulating them and producing neurological signs. Onset is fast, though it can be delayed if seeds are swallowed whole rather than chewed. No reliable published figure exists for how quickly signs appear in horses, so treat any suspected laburnum ingestion as urgent rather than waiting out a supposed grace period. Signs include colic, drowsiness, frothing at the mouth, dilated or unequal pupils, incoordination, convulsions and, in serious cases, coma.

The practical risk is a laburnum in a garden that backs onto grazing, dropping pods over the fence in late summer. If you have one on a boundary, fence the fall zone rather than the trunk.

Cherry, laurel and other cyanide-producing trees

Stone-fruit family trees

The Prunus family — wild cherry, cherry laurel (Prunus laurocerasus), plum, peach, apricot and almond — contains cyanogenic glycosides in leaves, bark and seeds. In an intact, living leaf these compounds are held separately from the enzymes that release cyanide from them. Damage the tissue — by chewing, wilting, frost or pruning — and the two come together, releasing hydrogen cyanide.

This is why wilted leaves are considerably more dangerous than fresh ones, and why a storm-broken branch lying in a paddock is a more serious problem than the intact tree it fell from.

Signs of cyanide poisoning

Cyanide prevents cells from using oxygen even when the blood is fully oxygenated. Affected horses may show bright cherry-red mucous membranes, rapid distressed breathing, muscle tremors, staggering, convulsions and rapid collapse. Onset can be within minutes of a substantial dose. Treatment is a veterinary emergency in the truest sense.

Common UK exposure routes

The single biggest risk is cherry laurel hedge clippings thrown into a paddock — laurel is one of the most widely planted screening hedges in Britain and it generates a lot of clippings. Overhanging wild cherry after pruning or wind damage is the other recurrent source. Treat any storm-damaged Prunus branch as toxic and remove it rather than leaving it to be tidied later.

Rhododendron, azalea and privet

Rhododendron and azalea

Rhododendron (Rhododendron ponticum) and the closely related azalea contain grayanotoxins, which bind to sodium channels in nerve and muscle cells and prevent them resetting normally. Small quantities matter here — a few mouthfuls of leaves can cause serious poisoning. Signs include profuse salivation, repeated swallowing, colic, weakness, a slowed heart rate and collapse. R. ponticum is invasive across large areas of UK woodland and estate land, and as with laurel and yew, clippings are the most common route into a paddock.

Privet

Both wild privet (Ligustrum vulgare) and garden privet (L. ovalifolium) are toxic to horses, and privet is named on the World Horse Welfare and BHS hazard lists. Leaves and berries cause colic and, at higher doses, kidney damage. Its ubiquity as a suburban boundary hedge is exactly the problem: a great many horses at livery are grazing within reach of somebody’s privet.

Other trees and shrubs to fence off

• Beech (Fagus sylvatica) — often assumed safe, and largely is in small amounts, but beech mast eaten in quantity during a heavy mast autumn can cause colic and digestive upset. Worth knowing about if you have mature beech overhanging grazing.
• Horse chestnut (Aesculus hippocastanum) — despite the name, conkers and leaves are not safe. Reported effects are usually mild to moderate, involving colic, tremors and incoordination, but the sheer volume of conkers dropped near many paddocks makes it worth fencing.
• Box (Buxus sempervirens) — a small ornamental hedging plant containing alkaloids that cause colic, respiratory difficulty and seizures. Named on the World Horse Welfare list.
• Holly (Ilex aquifolium) — berries and leaves cause digestive upset. Rarely fatal, but the spines mean a horse determined to eat it will also injure its mouth.
• Black walnut (Juglans nigra) — uncommon in the UK but the classic bedding-associated cause of laminitis. Bedding containing as little as 20% fresh black walnut shavings is enough, with signs typically appearing within one to two days. The toxin is not juglone, despite that being the name repeated almost everywhere: juglone was tested directly and topical application to horses’ feet produced skin irritation but no laminitis (True and Lowe, American Journal of Veterinary Research, 1980), and heartwood extracts that contain no juglone still cause the disease. The responsible compound has never been identified.
• Red maple (Acer rubrum) — wilted leaves destroy red blood cells, causing severe anaemia and dark urine. Not native to the UK but planted ornamentally, and a significant risk where present.
• Box elder (Acer negundo) — another introduced Acer whose seeds contain hypoglycin A, causing the same disease as sycamore. This is the species responsible for most North American seasonal pasture myopathy.
• Black locust (Robinia pseudoacacia) — rare in UK paddocks but present on some woodland edges; bark and leaves cause colic and depression.

Which trees are safe for horses?

Fencing everything off is not the goal — trees provide shade, shelter and windbreak, all of which have real welfare value, and a bare paddock has its own problems. Species generally regarded as safe around horses include willow, ash, hazel, hawthorn, birch, rowan, field maple, poplar and lime. Willow and hazel in particular are frequently browsed deliberately and are sometimes planted as enrichment.

Two caveats worth holding onto. First, “safe” means non-toxic, not indestructible: horses will strip bark off almost anything, which kills the tree and can cause impaction if enough is eaten. Guard the trunks of anything you want to keep. Second, safety lists are species-specific, so identify properly before you rely on one — field maple is fine, red maple is not, and they are both maples. If you are planning planting or a paddock redesign, our complete guide to paddock care covers the wider setup.

How to recognise tree poisoning in a horse

Signs vary by toxin, but certain patterns should prompt an immediate veterinary call and point you toward a likely culprit:

• Sudden severe weakness or inability to stand — suspect sycamore or yew.
• Colic signs but the horse is still eating — strongly suggests atypical myopathy rather than true colic.
• Dark red or brown urine — muscle breakdown; sycamore, or red maple if the horse is anaemic.
• Acute colic with bloody diarrhoea in autumn — suspect acorns.
• Bright cherry-red mucous membranes with rapid breathing — cyanide, usually after wilted cherry or laurel.
• Convulsions and dilated pupils in late summer — consider laburnum pods.
• Sudden death in the paddock — assume yew until proven otherwise and check the entire boundary before turning anything else out.
• Unexplained laminitis in a stabled horse — check the bedding for black walnut shavings.

Tree poisoning is not a wait-and-see situation. Many of these toxins act within hours, and once cellular damage is established it is difficult to reverse. The phone call to the vet should happen on suspicion, not after signs peak.

Preventing exposure on UK and SA properties

Walk the boundary, and walk it after weather

Walk every fence line monthly with a proper identification guide, noting overhanging branches, leaning trunks, hedge species and ornamental shrubs within reach. Then walk it again after every significant storm — wind is what puts most toxic material into paddocks, and it does so overnight. Pay particular attention through seed-drop and seedling-germination seasons.

Keep forage constant

This is the single highest-value preventive measure and it recurs in every serious piece of guidance on the subject. Horses with constant access to clean roughage do not generally go looking for toxic trees. Hunger and boredom drive most accidental ingestion, and the 64% figure from the Atypical Myopathy Alert Group makes the link explicit. Avoid stripping paddocks to bare ground, and see our guides to hay alternatives and feeding basics if grazing is short. Sensible turnout management matters here too.

Talk to the neighbours

Most clipping-pile poisonings start with somebody who simply did not know. A polite conversation, or a printed sheet listing yew, laurel, rhododendron and laburnum as absolute no-gos, prevents a genuinely large share of these cases. This is worth doing proactively at a livery yard where several gardens back onto the fields.

Check your bedding source

Buy shavings only from reputable equestrian suppliers. Black walnut contamination is rare in UK supply chains, but if an unexplained acute laminitis coincides with a change of bedding, the bedding is the first thing to investigate.

Don’t forget the other pasture hazards

Trees are only part of the picture. Ragwort poisoning remains the best-known plant hazard in British grazing — see our guide to removing ragwort — and buttercup poisoning catches out owners of overgrazed fields. Grass sickness is a separate and still poorly understood pasture-associated disease worth being aware of.

South African-specific risks

South African paddocks face an overlapping but distinct list. Syringa (Melia azedarach), an invasive introduced tree found across much of the country, is a serious hazard — its berries are highly toxic and it grows readily along fence lines and disturbed ground. Oleander (Nerium oleander) is a widely planted ornamental containing cardiac glycosides comparable in severity to yew. Lantana and bracken are both common and both toxic.

Sycamore itself is present in introduced parkland planting in the Western Cape and on the Highveld but is uncommon in working paddocks, so atypical myopathy is far less of a concern than in the UK. The priority for SA owners is identifying introduced ornamentals around livery yards, roadside boundaries and old homestead gardens — most indigenous species such as karee and sweet thorn are low-risk, but the imported garden plants are where the danger sits.

What to do if you suspect poisoning

1. Call your vet immediately. Describe what the horse may have eaten and when. Do not wait for signs to worsen — with several of these toxins, waiting is the decision that costs the horse.
2. Remove the horse from the source, and move the others too. If one horse has found it, assume the rest of the group has had access.
3. Get the horse off the pasture while it can still stand — but do not exercise it. Older advice (including an earlier version of this page) told owners to leave a weak horse where it was and bring the vet to it. That is the wrong way round. Blue Cross advises calling the vet and then moving the horse out of the field to a well-bedded stable, and Redwings makes the practical point that it is far easier to move a horse that can still stand unaided — many cases need hospitalisation and intravenous fluids, and horses often deteriorate over the first 24 to 48 hours. Walk it quietly and only as far as it has to go, never force a collapsed or recumbent horse up, and do not ride, lunge or otherwise work it.
4. Keep the horse warm, calm and with access to clean water. Do not feed until the vet has advised.
5. Save a sample of the suspected plant. A photograph plus a sealed bag of leaves, seeds or bark helps enormously with confirming the toxin.
6. Brief the whole yard. Other horses may have been exposed, and managing the unaffected ones is usually what prevents the second and third case.

Plant poisoning is, in most cases, preventable. The owners who avoid it are not lucky — they are systematic. Walk the boundary, fence off the high-risk species, physically remove the material rather than spraying or harrowing it, keep the forage flowing, and act on suspicion rather than waiting for proof.

Frequently Asked Questions