Harmful Properties

All plants contain harmful properties to some level, and it is that degree which determines the level of toxicity if eaten.  

For example, it is probably fine to feed your tortoise a plant that contains oxalic acid, providing it is only a small portion of its daily intake.  However, if you offer three or four different plants at the same feed, and they all contain high levels of oxalic acid and you follow this regime long term, then there is an increased risk of your tortoise developing dietary related disorders.  

We've all heard of the exception to the rule when people say such things as ‘My tortoise eats buttercups and nothing has happened to him’.  That may be true, but when there is a reference of just one tortoise dying after eating Buttercups (which is on record), then you have to consider whether that particular plant is something you would like to risk offering to your tortoise.   

Some of the harmful properties found in plants are included on this page, together with brief information on the effects they could have if ingested.  The information provided is to encourage you to use your own initiative in knowing what some plants contain and which are safe to feed to tortoises, and which might not be safe.


Alkaloids are organic compounds containing nitrogen found naturally in many plants.  Some of the best known harmful alkaloids are morphine, quinine, strychnine, nicotine and cocaine.  There are some alkaloids that are harmless but others like those found within the Apocynaceae, Papaveraceae and Solanaceae families are harmful.  Alkaloids often have a bitter taste which helps to protect the plant from being eaten, but hungry animals will browse and may accidentally eat plants containing high levels of alkaloids.


These are chemical alkaloids which are mostly found in three plant families:  the Boraginaceae, the Compositae and the Leguminosae.  Pyrrolizidine alkaloids are not toxic until they are eaten in quantity when they will cause damage to the liver and eventual liver failure.  They may also cause malformation in offspring.  The early signs are not immediately apparent.  Although scientific research has been mainly been carried out on cattle and horses, we can only assume that it could also affect tortoises, so until we have firm evidence on safety we recommend not feeding.


Coumarin is found in many plants, such as Sweet Woodruff, Mullein, Sweet Clover, Tonka Beans, Mayweed, some members of the Prunus genus such as Cherries and many others.  It has appetite suppressant properties and can be moderately toxic to the liver and kidneys.  It is a sweet smelling chemical compound that is used as a flavour enhancer in pipe tobacco and some alcoholic drinks.  Furanocoumarins, which are present in the Wild Parsnip, and Giant Hogweed are particularly toxic.  

Although Coumarin in its natural state in plants has no anticoagulant properties, when it comes into contact with certain fungi it is broken down into dicoumarol, an anticoagulant that inhibits synthesis of vitamin K in the liver, and whose derivatives are used in the production of warfarin. The presence of Coumarin in plants also encourages them to soak up nitrogen in the form of nitrates in the soil.  While small amounts of Coumarin in a plant are unlikely to do any damage, the level of Coumarin present increases as the plant grows older, and so it is wise to feed only the young leaves of these plants, and only sparingly.

Cyanogenic Substances

Cyanogenic substances in plants are those that are capable of producing cyanide.  This substance can be a compound within the plant that is safe until attacked or eaten, at which time the sugar is partitioned off and cyanide is released.  It can be found in plants of the Rosaceae family, the older and drying leaves and fruit of plants such as Almond, Apple, Plum, Apricot, Peaches and Cherries, and also the Euphorbiaceae family which includes spurge.

Hydrogen Cyanide and Cyanogenic Glycosides

Hydrogen cyanide (historically called Prussic acid and also Hydrocyanic acid) is a product of chemical compounds (cyanogenic glycosides) that are naturally present in some plants. It is produced from these glycosides in combination with an enzyme and water. The glycosides and the enzyme are contained in the same plant parts, but they are in separate cells, and it is not until the cells are ruptured and the water is added that Hydrogen cyanide produced. So although cyanogenic glycosides are not toxic as such, if the plant is damaged or stressed in any way (for example, by eating, picking, cutting, drought or frost) then that releases both the glycosides and the enzyme, and the toxic compound is produced (see also the section on Glycosides below).


Glucose is a product of plant photosynthesis and an excess is stored as glycogen to be used by the plant to give energy.  Sugar that is ingested as fruit: apples, plums, berries, tomatoes (yes, tomatoes are fruit),  ferments faster than that from a natural diet, causing high levels of endotoxins, compromising the normal gut flora which in turn may lead to liver abscesses and possibly prove fatal.  Some tropical species of tortoise, such as Red-foots, have adapted to have fruit as part of their diet, but fruit should be avoided for all Mediterranean and grazing species such as Sulcata and Leopard tortoises.


A plant glycoside is a chemical compound that hydrolyses (is broken down by water) into sugar and one or more other substances.  They are not always toxic, but particular glycosides can cause problems if eaten.

a)   Cardiac Glycoside
Cardiac glycosides are steroid glycosides found in a number of plants including the Foxglove (Digitalis), from where we get today’s powerful heart drug, Digoxin, which is used to treat heart failure.  Other plants containing cardiac glycosides include Oleander, Rhododendron, Azalea, Kalanchoe and Milkweeds.  Ingestion can cause diarrhoea, vomiting, irregular heartbeat and eventual death; so remove plants containing cardiac glycosides from in or near a tortoise enclosure.
b)    Glycoalkaloids
Glycoalkaloids are bitter tasting and can cause problems in the nervous system.  For example, the glycoalkaloid solanine, which is present in the Solanaceae family of plants such as Potatoes, Tomatoes, and Deadly Nightshade, can if ingested cause vomiting, diarrhoea,  and severe heart problems.
c)   Goitrogenic Glycoside (Goitrogens) (Glucosinolates)

Goitrogens compromise thyroid activity by interfering with iodine uptake and this could cause liver and kidney lesions. Thyroid hormones are associated with skin cells, growth and metabolism so any interference with this can lead to problems with shedding and cause slow growth.  Goitrogenic glycoside can be found in plants of the Brassica and related families, including Cabbage, Kale, Spinach and some hot peppery-flavoured plants like Wild Mustard, and can also contribute towards the formation of kidney and bladder stones.

d)   Ranunculin
Ranunculin is another glycoside which can be found within the Ranunculaceae family; for example, Buttercup, Delphinium, Wood Anemone and Clematis, and can cause mouth and throat irritation due to the acrid taste, colic and diarrhoea.  While some people say their tortoises eat Buttercups and Clematis with no ill effects, they are not plants that should be offered on a regular basis.

Protoanemonin is an unstable compound derived from Ranunculin and found in plants from the Ranunculaceae family including the genus Helleborus spp. It has antifungal and antispasmodic properties but if overfed can cause vomiting, diarrhoea, convulsions and paralysis; so best to prevent your tortoise from eating too many plants from the Ranunculaceae family.

e)   Saponins
Saponins are a group of steroid glycosides which are capable of causing anti-nutritional effects.  They are found in many plants including Alfalfa, Vetch, Spinach and trees such as Maples and Horse Chestnuts, as well as most legumes and soy products.  Cold-blooded animals such as fish, insects and reptiles are particularly sensitive to saponin glycosides, and high levels can speed up haemoglobin degeneration, irritate mucous membranes and act as an appetite suppressant.  Saponins often have a bitter taste.  Although all saponins are not bad it would be best to feed with caution as only a small part of a tortoise's diet.


Grayanotoxins are a group of toxins present in Azaleas, Rhododendrons and many other members of the Ericaceae family. They are present in all parts of the plant and in fact, are even present in nectar found in the flowers and can contaminate honey produced from the pollen of these flowers.
Grayanotoxins block normal function of the muscles in people and animals, including the heart, and can impair nerve function and cause diaphragm paralysis and hallucinations.  Even a small nibble on the leaves, wood or flowers, can cause symptoms to occur.  Physical symptoms of grayanotoxin poisoning occur after a period of minutes up to three hours or so.  In humans, the initial symptoms are excessive salivation, perspiration, vomiting, dizziness, low blood pressure, weakness and pins and needles in the extremities and around the mouth.  In higher doses, symptoms can include loss of coordination, severe and progressive muscular weakness, and severe heart problems such as complete failure or tachycardia.  Poisoning from grayanotoxins is rarely fatal in humans but can be lethal for other animals, and we know of at least one case where a tortoise that nibbled a few Azalea flowers suffered severe convulsions and came very close to death.

Oxalic Acid

Oxalic Acid is a naturally occurring chemical substance present in many plants which can be safely eaten.  But it is harmful in high levels as it binds with calcium to prevent the absorption of this much-needed nutrient, and because of this, it can result in calcium deficiency in a relatively short period of time.  Oxalic acid can be found in plants such as Spinach, Chard, Good King Henry, and Beetroot, which are all members of the Amaranthaceae (formerly Chenopodiaceae) family, as well as Parsley, Rhubarb and even Dandelions (which have a low level in young leaves but can have a higher concentration in older leaves). A low intake of oxalic acid doesn’t give rise to any problems, but it is best to avoid feeding large quantities of plants that are high in Oxalic Acid.


Calcium oxalates is a calcium salt of oxalic acid and often takes the form of tiny sharp colourless crystals that can cause irritation to the mouth, throat, tongue and digestive system.  It is the substance that makes antifreeze toxic and is the main contributor to the formation of bladder and kidney stones.  Plants containing high amounts of oxalates are fine to feed to tortoises providing they are fed in small quantities and as part of a varied diet. Several foods known to contain high amounts of oxalates should not be fed at the same time.

Phytic Acid

Phytic Acid, which is present in legumes like peas and beans, and also in cereals and nuts, works in a similar way to Oxalic Acid in that it binds to minerals like calcium and prevents the tortoise from absorbing them.

Raphides are bundles of needle-shaped crystals, usually of calcium oxalate, that develop as metabolic by-products in plant cells for defence against being eaten, for calcium storage and for structural strength.  If an animal eats plant material containing large amounts of raphides, the soft tissues of the tongue and throat may be damaged.  Although the effect of raphides on reptiles is unclear, it is known that mammals who eat a plant containing raphides can suffer painful oedema and a sense of burning in the mouth and throat.  For that reason, we do not recommend feeding plants that have a high raphide content, but the exceptions to this are plants like Fuchsias where the raphides are so deeply embedded in the cells that they are not easily released when eaten and therefore unlikely to cause damage.


Tannins are complex astringent compounds that give plants vibrant colour and bitter taste.  They are used for treating animal hides, in medicine, and also used as a stabiliser in pesticides.  Tannins can inhibit the absorption of iron (and potentially of zinc and calcium too), leading to anaemia if too much is ingested.  They are found in legumes, tree bark, Grapes, Strawberries, Pomegranates, Acorns, Nuts, and many other plants.  Tannins can inhibit herbivore digestion by binding to plant proteins that have been eaten and making it more difficult for the animal to digest that food.

© The Tortoise Table, 2017