Why a Bowl of Grapes Can Kill a Dog That a Bowl of Onions Usually Won't

Grapes and raisins are the dangerous one: a dog that eats them can go into kidney failure from a small amount, while a dog that eats an onion usually gets an upset stomach unless it has eaten a lot. Grapes and raisins contain tartaric acid, the suspected toxin, and dogs cannot process it, so it damages the kidneys. What frightens vets is that the toxic dose varies from dog to dog and can be very small. Some dogs are barely affected and others are severely ill within hours, and nobody can tell beforehand which a dog will be. Every grape, raisin, currant or sultana ingestion is treated as a potential emergency. Most of the rest of this list is much less dramatic, but a Scottish vegetable and fruit garden is likely to hold several of these plants at once.

If a dog has just eaten any of these, call your vet or a poisons line straight away rather than waiting to see what happens. Don't try to make the dog sick at home: vets induce vomiting by injection, usually within the first few hours, and getting it wrong at home can do more harm than good. Take a sample of what was eaten, or the packet if it was dried fruit, so the vet knows what they are dealing with.

Severity at a glance

Why a Scottish garden is likely to have several of these

The plants aren't more toxic in Scotland. What changes is which crops a typical Scottish garden holds. Rhubarb needs a spell of winter cold to break dormancy and crop well the following season, which an ordinary Scottish winter provides, and garlic needs one to two months at 0 to 10°C to form a good bulb. Onions and leeks are grown widely too. Scotland is also a major producer of certified seed potatoes, and the tattie patch is a familiar sight on allotments. A dog owner with an ordinary Scottish vegetable plot is therefore likely to be growing two or three of these plants.

Grapes are the exception. Outdoor vines are only reliably viable in Britain south of a line between Pembroke and the Wash, so a garden grapevine is unusual in Scotland, and the realistic route to exposure is dried fruit: fruit cake, mince pies or mixed fruit going on the compost heap. Blackcurrant and redcurrant bushes are a different plant (Ribes, not the grapevine genus Vitis). The dried "currants" in baking are Zante currants, made from grapes, and they carry the grape risk.

The crops, in order of severity

Grapes, raisins, currants and sultanas: severe, unpredictable, always an emergency

Signs include vomiting, diarrhoea, a painful abdomen, low energy and reduced appetite, and in a severely poisoned dog kidney failure can develop over the following one to three days. Don't wait for symptoms before calling: left untreated, the poisoning can kill. Treatment is most effective when given quickly (stomach emptying by injection, activated charcoal, a fluid drip and repeated blood tests). No variety is safer than another, and grapes off a vine are no different from raisins in a fruit cake.

Onions, garlic, leeks and other alliums: dose-dependent, serious in quantity

Every allium, including onion, garlic, leek, chive and shallot, contains disulphides and thiosulfinates that damage red blood cells. When enough cells are damaged the dog develops anaemia. All parts carry the risk, raw, cooked or dried, and cooking doesn't destroy it. Unlike grapes, the risk rises with the amount eaten. Small amounts usually cause vomiting, diarrhoea or a sore stomach, and these signs can appear within a day. The anaemia (breathlessness, lethargy, pale gums) tends to come later, from one to five days after eating, so a dog that seems fine the evening it raids the onion bed isn't necessarily clear. With treatment most dogs recover well and fatalities are rare, but a large amount, such as a significant quantity of home-grown garlic or a whole leek, needs urgent veterinary attention.

Rhubarb leaves: serious, easy to avoid

Rhubarb contains soluble calcium oxalates, which bind calcium in the blood and crystallise in the kidneys. The effects include vomiting, drooling and tremors, and can progress to kidney failure. The leaves are the main hazard, for dogs as for people, while stalks are what gardeners cook with. Rhubarb is a permanent low-maintenance perennial in many Scottish gardens, so the plant in the ground is not the problem: it is the leaves, pulled at harvest and left on the compost heap, which is often the actual point of contact. For the growing technique, see forcing rhubarb.

Sweetcorn cobs: not poisonous, but they block the gut

Corn is not toxic to dogs, but the cob and husk don't digest, and a swallowed cob can lodge in the stomach or small intestine and cut off the blood supply. Vets at the pet charity PDSA treated 28 cases of swallowed corn on the cob in a single year, and one dog lost around half of its small intestine. Repeated vomiting, lethargy and weakness after a dog has had a cob are reasons to go to a vet at once. Bin cobs and husks in a lidded bin, not on an open compost heap, and watch for a dog carrying off a cob from a sweetcorn row. Kernels cut off the cob in a small amount are not the concern.

Potato plants and green potatoes: usually mild, worse in quantity

Given how standard the tattie patch is on a Scottish plot, so it is worth knowing on almost any allotment. A ripe cooked potato is a safe treat in moderation. The risk sits in the green parts: foliage, stems, sprouts and any tuber turned green by light. They belong to the nightshade family and share its toxic principle, solanine, and raw potato in general can cause significant stomach upset in a dog. A dog that snatches a green potato or chews foliage will typically show vomiting or a sore stomach, with more serious signs linked to a substantial quantity of the greener, more concentrated parts rather than an incidental mouthful.

Tomato plants and unripe green tomatoes: usually mild, same family as potato

Tomato plants and unripe fruit contain tomatine, another solanine-family glycoalkaloid. Signs described for the plant include drooling, reduced appetite, stomach upset, weakness and, in more severe cases, a slowed heart rate and dilated pupils. Ripe tomato flesh is different: tomatine drops sharply as the fruit ripens, and a ripe tomato is considered a safe occasional treat. The risk in a vegetable garden is a dog working through the plant, or eating fruit off the vine before it has coloured up.

Apple, cherry and plum trees: stems, leaves and seeds, not the ripe flesh

All three are common in Scottish gardens, and their stems, leaves and seeds contain cyanogenic glycosides, which release cyanide once broken down. It is most concentrated in wilting material, such as a fallen part-dried branch or a pile of prunings. Signs include bright red gums, dilated pupils, difficulty breathing, panting and shock. The fruit flesh isn't the hazard. The risk is a dog chewing fallen branches or prunings, or a mouthful of pips and stones, particularly windfalls left to rot under the tree. Stone fruit pits need to be chewed for the cyanide in the kernel to be released and it takes many to cause signs, so the more commonly reported problem is a stone lodging in the stomach or intestine, which can need surgery.

Slug pellets: a hazard from protecting these crops

Potatoes, alliums and rhubarb are among the crops gardeners protect with slug control. Metaldehyde pellets, historically the most common chemical control, have been withdrawn from outdoor use across Great Britain, partly over harm to pets and wildlife. Ferric phosphate pellets are the organic alternative most commonly sold now. If a dog has eaten any slug pellets, treat it like the crops above: contact a vet or a poisons line rather than waiting, and take the packet so the active ingredient is known.

Keeping garden and compost waste away from dogs

Many of these risks come from garden waste rather than the growing plant: rhubarb leaves pulled through spring and early summer, onion and potato peelings, sweetcorn cobs, and windfalls and prunings under apple, cherry or plum trees into autumn. An open compost heap a dog can reach is often the real point of contact. A closed bin, or a heap fenced off from the dog's usual run, removes most of the risk without changing how you grow.

Common questions

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