Article 22 – ‘Toxic Bonsai I’

Toxic Bonsai (Part I)

In the early days of bonsai horticulture tree varieties included the Juniper JuniperusSpruce Picea Pine Pinus and Larch Larix. As time progressed, more species were added for example, flower and fruiting varieties Wisteria Floribunda and Azalea genus Rhododendron, in the genus Prunus plum, cherry, peach and apricot. Today many other species from around the globe have now become part of bonsai horticulture including those which may be considered as common, the European Horse Chestnut Aesculus hippocastanum the unusual Acacia genus Acacia and the exotic, the Madagascar Euphorbia milii described below.

Many bonsai enthusiasts either have traditional collections predominantly coniferous, whilst others prefer deciduous and some will have an assortment of species. These artistically shaped miniature trees portray delicate, graceful and rugged forms and although their beauty is beholding, all is not what it seems. These little adaptations are able to produce toxins just as their full-sized counter parts can, which have the power to incapacitate all fauna including humans even to the point of being lethal.

All flora have developed ways to defend themselves, from the production of toxins in their leaves, fruit and seeds to the emittance of gas and or extremely sharp thorns which deter most from ravaging their foliage. The European Horse Chestnut Aesculus hippocastanum a large deciduous tree has greenish-yellow to white flowers and fruit contained in spiny capsules. In the UK at autumn time children collect the fruit capsules and remove the seeds from within, thread them on lengths of string and participate in an old traditional game of ‘Conkers’. A game dating back to 1848 where turns are taken in striking each others ‘conker’ until one breaks; yet the players who take part in this ritual, are probably unaware that these ‘conkers’ or seeds are poisonous.


Acacia genus Acacia, of which there are approximately 160 species of trees and shrubs within the pea family Fabaceae are native to Africa and Australia. Those of the African savanna have an abundance of thorns for protection, but also use poison in their leaves as a second line of defence. When the tree is disturbed it pumps poison into its leaves releasing ethylene gas from the pores. This gas release if detected by other acacias in near proximity sound the alarm alerting them to a potential threat and in so doing they too inject poison into their leaves.


The Madagascar Euphorbia milii commonly known as the Crown-of-thorns – a shrub with flower clusters and red, petal-like bracts has thorns that are poisonous. These thorns if damaged secrete a latex sap called urushiol and when in contact either on or under the skin can cause a sumac rash a very serious allergic reaction; a form of dermatitis.


Perhaps the question on most peoples lips is why would you want to have a tree that is poisonous? The question can be answered in two ways (a) why do people keep exotic pets, arachnids such as the Mouse Spider Missulena, which is every bit as dangerous as it looks or a Burmese python Python bivittatus that can be extremely aggressive. Pterois, or lion fish with its showy pectoral fins and venomous spiky fin rays, a popular choice among marine aquarists. It is really down to human nature, some are content to have the mundane household ‘moggy’ (cat) as a pet, whilst others want something which can be seen as diverse. (b) It is possible that somebody was given a bonsai as a gift without the purchaser bothering to research the tree’s attributes, it just looked nice. Furthermore, most bonsai contain toxins of one sort or another in their defence that vary in type and amount.

There are countless species of flora that exist on the planet including wild or natural varieties and hybrids, the latter pioneered by Gregor Mendel whom is credited with starting the hybrid plant revolution with his genetic studies of peas in the early 1900s. And least we forget those that have been genetically modified to produce more flower and fruit and combat insect infestation. To describe the peculiarities and properties of flora is a monumental task and narrowing it down specifically to bonsai, would amount to a PhD thesis. Therefore, the discussion will be further refined focussing on bonsai that can be found in most collections either common or unusual. The complete topic will be in four parts this post being part I and arguably the most efficient way in undertaking this, is to list them in alphabetical order.

Acer – Family: Aceraceae. The toxins are found within the leaves which increase as they wilt and die. It is also found in the seeds although the content is less. The acer although not harmful to domestic pets and humans is potentially fatal to equines if ingested as the poison damages red blood cells, diminishing their ability to carry oxygen. Death can occur from between 18 hours to 10 days.

AppleMalus Spp. The seeds are mildly poisonous and contain a small amount of amygdalin a cyanogenic glycoside that play important roles in many plants including apple varieties. However, the amount of cyanogenic glycoside contained within the seed is not considered dangerous to humans nonetheless, ingesting a large quantity can provide severe side effects.

Alder BuckthornRhamnus frangula L. This tree or shrub found among hedgerows, along roadsides and in woodlands, has a number of toxic chemicals, including emodin that are within its bark and purple-black berries or fruits. Emodin is a purgative resin, which is also found in rhubarb and also produced by many species of fungi. If ingested the symptoms are: collapse, convulsions, diarrhoea, gastroenteritis, haemorrhage and vomiting.

Azalea – genus Rhododendron Family Ericaceae. Is a common species that appear in bonsai collections but, it is poisonous. The toxins Grayanotoxin and arbutin glucoside are found in all parts of the plant the flowers, leaves and nectar – the latter often referred to as ‘mad honey’. They can cause nausea, vomiting, weakness, dizziness, breathing difficulties, low blood pressure, reduced heart rate and irregular rhythm, which could be life threatening.

Beech – genus Fagaceae. Indigenous to Europe, Asia, and North America produce a triangular shaped fruit called beechnuts in the autumn. These nuts often used as a food source are high in tannins having a strong bitter taste and are toxic to both canines and humans especially children if consumed in large quantities. The European beech Fagus sylvatica, is believed to be more toxic than its the American relative, Fagus grandifolia. Symptoms include; vomiting, diarrhoea, abdominal pain, nausea, fatigue and dilated pupils – mydriasis.

BirchBetula Spp. Has more homeopathic properties as opposed to toxins nevertheless, pollen from the silver birch, Betula pendula, is the second most severe allergen for people as it can travel many kilometres via the wind. It is able to cause hay fever, conjunctivitis and severe respiratory problems with disease to the lungs and asthma. Severe cases of pollen infection do require medical attention.

BoxBuxus sempervirens. A common species found in bonsai collections is one to handle with caution as it is poisonous to humans. The leaves produce an alkaloid buxine which causes nausea, vomiting and diarrhoea and respiratory paralysis in humans and livestock. Contact with skin can cause irritable rashes and when pruning the clippings should be handled with care.

Cherry (Wild) – Prunus Spp. Wild Cherry trees produce fruit that are reddish black in the summer, which can be consumed. However, the wild cherry twigs and leaves contain the chemical prunasin, a cyanide that when ingested, can be fatal. Prunasin breakdown and cyanide release occurs when the tree becomes stressed and an indication of this is when the leaves begin to wilt.

Chestnut (European horse) – Aesculus hippocastanum. As described above.

CotoneasterCotoneaster Spp. Are grown as bonsai mainly for their display of coloured berries ranging from bright orange to red to purple. This species is said to be a high risk in the toxicity range, because their leaves, berries and flowers all contain cyanogenic glycosides. These toxins if ingested are converted to cyanide during digestion causing serious effect on the heart, liver, kidney and brain. For children the risk is higher than in adults, although much depends on the amount consumed.

CitrusCitrus Spp. Citrus oil is a concentrate of the fruit produced by the tree and also a protective barrier found on the leaves, which can be activated by a gentle rubbing with the fingers. The scent of the oil is pleasant but the taste is bitter, leaving a nasty after taste due to Coumarin a fragrant organic chemical compound in the benzopyrone class. Although citrus oil is not harmful to humans, felines are more susceptible to citrus poisoning, which can result in diarrhoea, vomiting, liver damage or even death.

Douglas FirPseudotsuga menziesii. A native of north America has smooth grey bark when young that are covered with numerous resin blisters, which should not be ingested. The leaves needle like in appearance have two whitish stomatal bands on their underside, that are pores to allow the exchange of gas. If the leaves are damaged they emit a sweet fruity-resinous scent. Ingesting needles can result in vomiting, anorexia, abdominal pain, and lethargy. Other trees with similar attributes are: the Balsam Fir Abies balsamea, Blue Spruce Picea pungens, Red Spruce Picea rubens, White Spruce Picea glauca, Scots Pine Pinus sylvestris and Red Pine Pinus resinosa.    

DogwoodCornus Spp. A genus comprising of approximately 60 different varieties known for their brilliant floral displays in spring time. The fruits of many dogwood varieties are rather tart and unpalatable due to the amount of Tannins, but can be consumed if cooked. However, fruit of the dogwood in the sub-genus Swida are toxic and should be avoided. Dogwoods are prone to attack by insects and fungal disease for example. Botryosphaeria Canker a dark yellowish pitch that oozes from dogwoods and Phytophthora a reddish orange sap oozing from the tree as a result of destroyed tissue. Dogwoods infected with this disease should be kept away from pets, children and other plants.

Dieffenbachia – Family Araceae. A native from Mexico, West Indies and Argentina is widely cultivated as an ornamental houseplant and although not considered by some traditionalists as bonsai material, it is found in some collections. Dieffenbachia is poisonous, it contains Raphides needle-shaped crystals of calcium oxalate crystals and if the leaf or its residue is ingested it causes a burning sensation and erythema a redness of the skin or mucous membranes, caused by increased blood flow. Dieffenbachia can cause other symptoms including numbness, oral irritation and localized swelling.

ElderSambucus racemosa. Elder is cited as a poisonous plant because the bark contains calcium oxalate crystals and the leaves and unripe fruits and seeds cyanoglycoside sambunigrin. But, ripened fruit when subjected to a cooking process reduces the toxins. Elder suffers from Hyphodontia sambuci or Elder Whitewash a basidiomycete fungal pathogen forming a thin white, pruinose (flour-like dusting) on the limbs and branches. The pathogen should be avoided as the spores are easily carried by a gentle breeze.

ElmUlmus. The elm has no toxins to speak of that are a danger to humans or domestic pets but its seeds, leaves and bark should not be ingested as a precaution. Because it is possible that the tree may be infected by Ascomycetes a pathogen relatively common to this species. Ascomycetes not only infest and destroy, they also produce secondary metabolites that are poisonous.

Eucalyptus – Family Myrtaceae. The leaves of this tree contain an oil that if treated and diluted can be safe for adult humans. But untreated oil is extremely toxic and ingesting a small amount (3.5 mL) can have fatal results. Symptoms of eucalyptus poisoning may include stomach pain, a burning sensation, dizziness, muscle weakness, small eye pupils, suffocation, nausea, vomiting, and diarrhoea.

Ficus – Family Moraceae. Is a genus of approximately 850 species that include trees, shrubs and vines collectively known as fig trees. Common species used in bonsai Ficus microcarpa and Ficus benjamina are quite popular especially as a beginners tree. However, they are poisonous due to the milky white sap containing Furocoumarins psoralens and ficin that oozes out when pruned. This sap causes Dermatitis and allergic reactions for example, itching of eyes, coughing and wheezing, skin irritation with redness and stinging.

Forsythia – Family Oleaceae (olive family) is also a popular choice in bonsai and there are eleven species predominately native to Asia. The species Forsythia suspensa is considered a major herb used in Chinese medicinal practices as it is non-toxic. But for safety reasons, one should not consume any part of the plant that is not edible.

Firethorn Family Rosaceae. A large shrub with sharp thorns related to the Cotoneaster (Cotoneaster is thornless). This a popular choice for bonsai due to its bright red and orange fruit that are toxic. The seeds of the berries contain cyanogenic glycosides as do almonds, apples, cherries and plums that can cause gastro-intestinal problems when ingested raw. They are only edible if crushed and washed under running water.

In toxic bonsai part II we continue the discussion on species ‘G’ to ‘M’ until then BW, N.

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