The equinox has been and gone, the mornings smell different, and the ground is suddenly covered in things your dog wants to put in their mouth. Autumn is the best walking season in Britain and also the one with the longest list of small hazards underfoot. Here is what actually matters, what the real risk level is for each, and what to do if your dog gets hold of something they should not have.
The short answer: what is dangerous to dogs in autumn?
The autumn hazards worth knowing are conkers, acorns, wild mushrooms, fallen fruit, antifreeze and blue-green algae, plus the seasonal spike in grass seeds and, later in the season, fireworks and chocolate. Most of them cause problems in two ways: toxicity, or a physical blockage in the gut. Conkers and acorns are the two you will meet on almost every walk from now until November.
Conkers
Horse chestnuts are the classic autumn hazard and the one that sends most dogs to the vet this month. They contain aesculin, which is toxic to dogs, though the more common problem is mechanical: a conker is exactly the wrong size and shape, and it can lodge in the gut.
Signs to watch for: vomiting, drooling, being off food, a tender abdomen, restlessness or repeated unproductive retching. These can appear anywhere from an hour to a couple of days later.
What to do: ring your vet the same day if you know or suspect a conker was swallowed, even if your dog seems fine. Do not wait to see how it goes, because a blockage is far easier to deal with early. The spiny green case is worth avoiding too, for mouths and paws.
For a dog who picks up everything, the honest solution under a horse chestnut tree is a lead and a decent recall, not vigilance. It is a fortnight of the year. Take the long way round.
Acorns
Less famous than conkers and, if anything, more of a nuisance because oaks are everywhere and acorns are small enough to be eaten by the handful. They contain tannins, which irritate the gut and, in quantity or over a sustained period, can affect the kidneys and liver.
Signs to watch for: vomiting, diarrhoea, lethargy, abdominal pain, reduced appetite.
What to do: a single acorn in a large dog is usually a minor matter, but call your vet for advice rather than assuming. Repeated grazing under an oak is the bigger worry, and it is worth changing the route if your dog has developed the habit. Young oak leaves that fall are mildly toxic too.
Wild mushrooms and toadstools
Autumn is peak fungi season and this is the hazard with the widest range of outcomes. The great majority of British wild mushrooms are harmless. A small number are seriously dangerous, and you cannot tell which is which by looking, which is the entire point.
What to do: treat any mushroom your dog eats as a vet call. If you can do it safely, photograph the mushroom, or pick one and bag it. Identification changes the treatment completely and a photograph of the intact cap, stem and gills is genuinely useful to a vet. Do not wait for symptoms, because with the dangerous species symptoms can be delayed by many hours while damage is already happening.
Mushrooms in your own garden are worth clearing on sight, especially where a dog mooches unsupervised.
Fallen fruit
Windfall apples, pears and plums under garden trees are a genuine autumn problem, and there are two separate risks. The stones and pips in plums, damsons, cherries and apples contain cyanogenic compounds, and a plum stone is also a perfect blockage. Fermenting fruit on the ground turns into alcohol, and a dog working through a pile of rotting windfalls can get genuinely drunk, which in a small dog is serious.
What to do: clear windfalls regularly, fence off the tree if that is easier, and call the vet if a stone has been swallowed or if your dog seems unsteady, disoriented or unusually sleepy after being out in the garden.
Antifreeze
This one gets less attention than conkers and is far more dangerous. Ethylene glycol, in antifreeze and screenwash, is sweet-tasting and lethal in very small amounts. The season starts now, as cars get topped up and garages get tidied.
Signs: early on it looks like drunkenness, wobbliness, vomiting and excessive thirst. The dog may then appear to improve, which is the dangerous part, before kidney failure develops over the following days.
What to do: this is a genuine emergency. Go to a vet immediately, do not wait, and do not wait for symptoms. Treatment is very effective early and very poor late. Clean up spills on the drive at once, store containers out of reach, and be aware of puddles of coloured liquid under parked cars.
Blue-green algae
Blooms peak in warm weather but persist well into autumn, and after a wet, mild spell they can still be present on ponds, lakes and slow rivers. It can look like a blue-green scum, a paint-like slick or clumps in the water. It is rapidly and often fatally toxic, and a dog does not need to drink much. Swimming and then licking the coat is enough.
What to do: keep dogs out of any standing water that looks discoloured, scummy or has a warning sign. If your dog has been in suspect water, rinse the coat thoroughly and contact a vet straight away.
Grass seeds, and why autumn still counts
Grass seed season is usually associated with summer, but long, seeding grass on verges and field edges stays hazardous well into autumn. The seeds work into paws, ears and armpits and migrate. Check paws, ear openings and between the toes after walks in long grass, and get any unexplained limp, head shaking or persistent licking of one spot looked at.
The rest of the autumn list
Fireworks. Not a poisoning risk but the biggest welfare issue of the season, and preparation in September works far better than reassurance in November. Start early.
Chocolate and dried fruit. Both ramp up from late October. Theobromine in chocolate and grapes, raisins, sultanas and currants in everything baked. Raisins are unpredictable and dangerous even in small amounts.
Rodent bait. Rodenticide use rises sharply as mice move indoors for the winter. Assume sheds, barns and outbuildings may have bait in them.
Dark walks. The clocks go back on 25 October and most weekday walks move into the dark. Being seen is its own safety issue, and we covered it in our guide to walking your dog in the dark.
Mud and wet coats. Not dangerous, just relentless. Drying a wet dog properly and a waterproof coat that actually works take most of the misery out of it.
What to do in the moment
If you think your dog has eaten something they should not have: call your vet straight away, take the packaging, the mushroom or a photograph with you, and do not try to make your dog sick at home. Home remedies for inducing vomiting can cause serious harm, and the decision belongs to a vet who knows what was eaten and when. Your vet would far rather take a call about a conker that turns out to be nothing.
What actually helps on the walk
Most of this season comes down to seeing what your dog is doing and being able to interrupt it. A lead you trust under the chestnut trees does more than any amount of watching. Our collars, leads and harnesses are the ones we would reach for, and if your dog is a committed scavenger, a walking bag that keeps high-value treats to hand is what makes "leave it" work in the moment it matters.
Autumn really is the best season to be out with a dog. A short list of things to walk around is a small price for it.
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Frequently asked questions
Are conkers poisonous to dogs?
Yes. Conkers contain aesculin, which is toxic to dogs, and they are also the right size and shape to cause a blockage in the gut. Signs include vomiting, drooling, being off food, a tender abdomen and repeated retching, and they can appear from an hour to a couple of days after eating. Contact your vet the same day if you think a conker has been swallowed, even if your dog seems well.
Are acorns poisonous to dogs?
Yes. Acorns contain tannins that irritate the digestive system and, in quantity or eaten repeatedly, can affect the kidneys and liver. Signs include vomiting, diarrhoea, lethargy and abdominal pain. A single acorn in a large dog is usually minor, but call your vet for advice, and change your route if your dog has taken to grazing under an oak.
What should I do if my dog eats a wild mushroom?
Treat it as a vet call straight away, without waiting for symptoms. Most British wild mushrooms are harmless but a few are seriously dangerous and they cannot be told apart by sight. If you can do so safely, photograph the mushroom or bag a sample, since identification changes the treatment. With the dangerous species, symptoms can be delayed for hours while damage is already occurring.
Why is antifreeze so dangerous to dogs?
Ethylene glycol in antifreeze and screenwash tastes sweet and is lethal in very small quantities. Early signs look like drunkenness, wobbliness, vomiting and excessive thirst, and a dog may appear to improve before kidney failure develops days later. It is a genuine emergency, so go to a vet immediately rather than waiting to see what happens.
Can dogs eat windfall apples and plums?
The flesh of an apple is fine, but the pips and stones of apples, plums, damsons and cherries contain cyanogenic compounds, and a plum stone can cause a blockage. Fruit fermenting on the ground also produces alcohol, which can make a dog genuinely drunk. Clear windfalls regularly and call your vet if a stone has been swallowed or your dog seems unsteady.
Is blue-green algae still a risk in autumn?
Yes. Blooms peak in warm weather but can persist into autumn, particularly after a mild, wet spell. It may appear as blue-green scum, a paint-like slick or clumps in the water, and it is rapidly toxic in small amounts. A dog does not need to drink it, as licking a wet coat after a swim is enough. Keep dogs out of discoloured or scummy standing water and contact a vet immediately if they have been in it.