2021 Plant Talk 8: Poisonous Plants

June 25, 2021

Plant Talk 8 Poisonous Plants

Hello Plant Enthusiasts!

What’s Blooming

Avens (Geum spp.), Bee Balm (Monarda spp.), Bellflowers (Campanula spp.), Butterfly Bush (Buddleja davidii), Carolina Bugbane (Trautvetteria caroliniensis), Common Hedgenettle (Stachys officinalis), Day Lily (Hemerocallis spp.) Dahlia spp., Daisy (Leucanthemum vulgare), Elecampagne (Inula helenium), Elderberry (Sambucus canadensis), Evening Primrose (Oenothera spp.), Gayfeather (Liatris spp.), Lamb’s Ear (Stachys byzantina), Lucifer’s Tongue (Crocosmia x crocosmiiflora) Gladiolas (Gladiolus spp.), Hydrangea spp., Marigolds (Tagetes spp.), Motherwort (Leonurus sp.), Mullein (Verbascum spp.), Phlox sp., Poppies (Papaver spp.), Sages (Salvia spp.), Scarlet Rose Mallow (Hibiscus coccineus), Skullcap (Scutellaria lateriflora), Sourwood (Oxydendrum arboreum), Spilanthes (Acmella oleracea), Stoke’s Aster (Stokesia laevis), Sumac (Rhus spp.), Thimbleweed (Anemone virginiana), Trumpet Creeper (Campsis radicans), Queen Anne’s Lace (Daucus carota), Vervain (Verbena spp.), and Zinnia spp.

Ready for Harvest

            Lots of fruits such as Blueberries (Vaccinium spp.), Goumis (Elaeagnus multiflora), Mulberry (Morus spp.), Raspberries (Rubus spp.) and Strawberries (Fragaria spp.).

            Lots of wild greens such as Chickweed (Stellaria media), Lambsquarters (Chenopodium spp.), Pigweed (Amaranthus spp.), Quickweed (Galinsoga spp.), Violets (Viola spp.) Wild Carrot (Daucus carota) and Yellow Dock (Rumex spp.).

            Some cultivated greens especially include Lettuce (Lactuca sativa), Spinach (Spinacia oleracea) and Swiss Chard (Beta vulgaris ssp. cicla). The Brassica family is ripening in an array of forms including Arugula, Cabbage, Kale, Kohlrabi, Mustard, Turnips and Radishes for instance.

            i have recently picked my first Hot Peppers (Capsicum spp.) as well.

A photo album of most of the plants mentioned above and below can be seens at the following link


Introduction to Poisonous Plants

Relatively few poisonous plants are largely responsible for many people’s aversion/fear of nature for example Poison Ivy/Sumac/Oak (Toxicodendron spp.). These plants are so important to people that they have their own collection of literature (Anderson, 1995; Day, 2008; Hauser & Epstein, 2008). Frank Cook would speak of referring to this plant as sister ivy or sister oak considering the term poison to be pejorative and provocative towards the plant thus potentially even inciting more ire. Either way, this is one to be aware of, as is the fact that the toxin manifests as an allergy by which only some people are affected. Those familiar with areas where Mangoes (Mangnifera indica), Brazilian Pepper (Schinus spp.) and Cashews (Anacardium exelsum) grow will possibly know of potential severe allergic reactions to these foods and additional members of the Anacardiaceae as well. Interestingly, the non-poisonous red fruited Sumacs (Rhus glabra, R. typhina and spp.) share the same family but represent widely distributed culinary use all over the temperate world and no toxicity that i know of. Surprisingly, the pulp around the seeds of the rather unrelated Gingko has a similar compound to the urushiol in Toxicodendron spp. However, the seed inside of the Gingko is considered a delicacy when cooked though possibly poisonous in a minor way as well (Cox & Ruter, 2013; Crane, 2013; Singh et al., 2008)!

Plant allergens have their own specific literature (Huntington, 1998; Mills & Shewry, 2004; Ogren, 2015). The BBC has featured the evolving research around Peanut allergies in particular “Peanut Allergy Treatment in Sight” and “Tiny Doses of Peanut Protein offer Hope to Allergy Sufferers

When covering a subject like the use and possible ingestion of plants clearly a class concerning the main prevalent poisonous plants and their effects on various forms of life is in order. The phenomenon of plant poisons offers an excellent venue to examine plants at the family level. A relatively few families in the temperate world tend to be have a majority of poisonous members such as the Arum (Araceae), Buttercup (Ranunculaceae), Bittersweet (Celastraceae), Euphorb, (Euphorbiaceae), Lilies (Liliaceae) sensu lato and Nightshades (Solanaceae). We have already covered the fact that new descriptions for Lily like plants (Lilioid) have broken them into many separate families. Edibility versus poisonousness does seem to trend out along SOME of these new family lines. For instance many plants formerly in the Lily family are now placed in the Melanthiaceae and Convallariaceae and both new families contain several deadly poisonous members. However, even families that tend to be predominantly poisonous have some exceptions especially depending on proper preparation.

For other families it is not nearly as cut and dry. The Carrot family (Apiaceae) contains a multitude of edible plants. It has many famous well used culinary herbs and spices in particular. Which ones can you think of? However, it also contains some of the most toxic plants in the U.S.A. and the world including Poison Hemlock (Conium maculatum) and Water Hemlock (Cicuta spp.) (Schep et al., 2009; Vetter, 2004). Poison Hemlock looks very similar to Queen Anne’s Lace (Daucus carota) and some Biscuitroots (Lomatium spp.) among other relatives. Water Hemlock can look similar to a number of other plants in the family as well like Angelica spp. Several other members of the Apiaceae can also cause skin irritation due to furonocoumarins  including Celery (Apium graveolens) Cow Parsnip/Giant Hogweed (Heracleum spp.) and regular Parsnip (Pastinaca sativa).

The Mint family (Lamiaceae) is probably one of the safest groups in the world. However, several members can be toxic in high dosages or in the case of pregnancy including Ale Hoof (Glechoma hederacea), Perilla (Perilla frutescens), Germander (Teucrium spp.) and Pennyroyal (Hedeoma pulegioides) (Duke & Foster, 2014; Frohne & Pfander, 2005; Kingsbury, 1964). According to Dr. Jim Duke’s Phytochemical database American native Mountain Mints (Pycnanthemum spp.) have fairly large amounts of Pugelone depending on species which is a   compound known to be part of the challenge related to Pennyroyal. Therefore, they should possibly be avoided during pregnancy as well. It would be good of course to review all items one might potentially ingest while pregnant with a special lens towards safety… A number of books are geared towards teaching children about poisonous plants too (Coil, 1991; Levy, 2019; Schuh, 2018).

Big families such as the Aster (Asteraceae) and Bean (Fabaceae) have a broad spectrum of plants. Some are used for food, some for medicine and some are poisonous. Several sources list plants by family which may aid in recognizing these trends (Frohne & Pfander, 2005; Kingsbury, 1964; Muenscher, 1940; Westbrooks & Preacher, 1986). Other books specifically address the interface between plant poisons and drugs (Blackwell, 1990; Wink & Van Wyk, 2008).

A classic statement of toxicology attributed to Paracelsus centuries ago is that “the difference is the dosage”. Therefore, even some poisonous plants may provide powerful medicines through the right means of extraction and appropriate levels of intake. Mayapple (Podophyllum spp.), Poke (Phytolacca americana) and Foxglove (Digitalis spp.) are examples of such a phenomenon. Amount of intake, frequency of use, method of exposure, personal biochemistry and site ecology may all influence the effects of certain plants. The fields of genetics and biochemistry are starting to reveal that plant populations can vary greatly in their phytochemistry on a level within the species (Dhar et al., 2006; Gonick, 2014; Jarić, Mitrović, & Pavlović, 2015; Jia & Zhao, 2009; Ogunwenmo, Idowu, Innocent, Esan, & Oyelana, 2007; Schlag & McIntosh, 2006; Searels, Keen, Horton, Clarke, & Ward, 2013).

Can you think of some reasons why this might be?

Some plants are toxic to livestock but not to humans and vice versa. This phenomenon largely has to do with levels of typical intake and differing digestive regimes. Humans have of course historically learned plant uses from other animals which is a practice in one way labeled bioethnopharmacognosy. However, this is NOT a very dependable and fail-proof way to test if something is wholesome for human intake! Plants that are poisonous to horses have their own literature (Allison, 2011; Burger, 1996; Davidson & Braid, 2001). Such is also the case for plants poisonous to more typical pets (O’Kane, 2011; Smith, 2003).

A number of books look at human poisoning by plants from an historical perspective including how humans can be poisoned by the intake of animal products like milk containing poisonous compounds and how people have used plant poisons intentionally through history (Brown, 2018; Stewart, 2009, 2016; Stuart, 2004).

How to deal with poisons is beyond the scope of this class currently. However, milk and activated charcoal are two often recommended items worth being aware of.


Numerous compounds are responsible for the phenomenon of plant poisoning. The most prevalent amongst them are various types of alkaloids. Elpel (2004, p. 203 2014, p. 219) has a great write up in the back of Botany in a Day on various plant compounds discussed below. Other compounds typically cited include amines, glycocides, oxalates, polypeptides, resins, saponins, minerals and compounds causing photosensitivity (Kingsbury, 1964).  Many other plants also contain compounds that don’t fall into the above listing but may still cause health problems i.e. hydroquinone in Cocklebur (Xanthium sp.) and thiaminase in Bracken Fern (Pteridium aquilinum) (Kingsbury, 1964). Lisa Ganora (2009) is the author of a relatively accessible and affordable text on the subject of phytochemistry. Of course there are a number of college type textbooks as well (Alamgir, 2018; Bruneton & Hatton, 2008; Hoffmann, 2003; Malviya & Rawat, 2020; Yarnell, 2019).

Plants containing pyrrolizidine alkaloids typically are located in the Bean, Borage (Boraginaceae) and Aster families (Biller et al., 1994; Nelson et al., 2007). Common knowledge of these compounds is relatively recent. Therefore, traditional internal use of plants such as Comfrey (Symphytum officinale) which has such compounds is now often discouraged especially for any prolonged period. Other species in the Comfrey genus may have even higher amounts of these compounds.

Oxalates are compounds typically found in the Amaranth (Amaranthaceae incl Chenopodiaceae) Buckwheat (Polygonaceae) and Wood Sorrel (Oxalidaceae) families. Purslane (Portulaca oleracea) in the (Portulacaceae) also contains soluble oxalates. These compounds have the potential to leach calcium from the body if not prepared in the appropriate way. One method of preparation is through boiling or blanching as the acid goes into the water. Some individuals are particularly susceptible to the formation of stones made of calcium oxalate as well. Care should be exercised in not consuming excessive amounts of members from these families in a raw form. However, researcher Dr. John Kallas (2010) debates the role of oxalic acid in causing health issues in people regarding kidney stones in particular.

Plants in the Araceae family contain calcium oxalate crystals that may cause an INTENSE burning sensation upon ingestion! Fish Tail palm (Caryota sp.) fruits also contain calcium oxalate crystals as i had to learn the hard way in a tropical locale once. i was going off the idea espoused by Frank Cook that all palm fruits are edible… Remember that nature is not a fan of absolutes like “all” and “never”. The kernel inside even the Fish Tail is edible apparently once the pulp is removed. However, that pulp can even be caustic topically and should be removed with care.

Important glycocides including goitrogenic substances that may inhibit proper thyroid function are present in some members of the Mustard (Brassicaceae) and Amaranthaceae families. Ranunculin is a glycocide in the Buttercup (Ranunculaceae). Cardiac glycocides are the most powerful. They are typically found in species from the Lily sensu lato, Foxglove (Scrophulariaceae) and Dogbane (Apocynaceae) families. The Apocynaceae now includes the formerly distinct Milkweed family (Asclepiadaceae). One side note is that many members of the Scrophs as traditionally described including Foxglove have been moved to the Plantaginaceae and Orobanchaceae families. Hedge Hyssop (Gratiola) is an outlier from the Lamiaceae that also contains cardiac glycocides.

Cyanogentic glycocides are primarily present in members of the Grass (Poaceae), Bean (Fabaceae), Flax (Linaceae), Mochatel (Adoxaceae) and Rose (Rosaceae) families (Elpel, 2013; Kingsbury, 1964). Small amounts of cyanide occur in the seeds of many members of the Rose family but are normally not an issue at typical rates of consumption by humans. The potential for poisoning of livestock by consumption of the wilted foliage of wild black cherry (Prunus serotina) is well known (Kingsbury, 1964). Cyanogentic compounds also occur in Hydrangea and Elderberry (Sambucus spp.) (Nelson et al., 2007). A form of cyanide has also been reported in  the exotic invasive Garlic Mustard (Alliaria petiolata) from the Brassicaceae (Cipollini & Gruner, 2007).

Saponins are toxins that have also been used to make soap. A diverse array of plants from several families contain such compounds. A list according to (Kingsbury, 1964) follows.

Alfalfa (Medicago sativa) and Rattle box (Sesbania spp.)  (Fabaceae)

Beech (Fagus grandifolia) (Fagaceae)

Bouncing bet (Saponaria spp.) and Corn Cockle (Agrostemma githago) (Caryophyllaceae)

English ivy (Hedera helix) (Araliaceae)

Poke weed (Phytolacca americana) Phytolaccaceae

 Ceanothus (Rhamnaceae), Clethra (Clethraceae), Aesculus (Sapindaceae) and Yucca  (Agavaceae) all have saponins as well. Elpel includes Lychnis (Caryophyllaceae), Symphoricarpos (Caprifoliaceae) and Cyclamen (Primulaceae) as containing saponins too (Elpel, 2013). Steroidal saponins are also used medicinally in plants like Ginseng (Panax spp.) and Sweet Tea Vine (Gynostemma pentaphylla) (Chang et al., 2005; Cui et al., 1999; Razmovski-Naumovski et al., 2005; Schlag & McIntosh, 2006).

Poisoning by the mineral uptake of plants is another potential cause for concern. Excessive amounts of copper may be taken up by plants in areas where a Bordeaux fungicide have been applied for many years. Note that Bordeaux is a mixture even employed by organic farmers as well. Also cadmium may be taken up where triple superphosphate has been applied (Kingsbury, 1964).

Excessive nitrates may especially be taken up by members of the Amaranthaceae, Asteraceae, Brassicaceae, Poaceae, and Solanaceae. Out west many plants are known to take up harmful amounts of elements like selenium and one should be especially aware in places with serpentine soils. More can be seen on this in the class covering Phytoremediation.

Plant Poisons That Can Act Topically

Some plants can act topically to cause irritation or photosensitization as mentioned above for the Apiaceae. The photosensitivity can be caused through their effect on the liver including members of Buckwheat (Polygonaceae), Grass (Poaceae), Pea (Fabaceae) and Verbena (Verbenaceae) families. Notable genera in this regard include Clover (Trifolium), Knotweed (Polygonum), Oats (Avena), Verbena (Lippia) and Vetch (Vicia) (Kingsbury, 1964). 

(Hardin & Arena, 1974) compiled a more comprehensive list of 78 plants that grow in the United States which may cause dermatitis. Those that grow in Appalachia and are considered especially important by me are included below.

Giant Hogweed (Heracleum mantegazzianum) Apiaceae

Posion Ivy/Oak/Sumac (Toxicodendron spp.) Anacardiaceae

Spurge (Euphorbia spp.) Euphorbiaceae

Stinging Nettle (Urtica dioica) Urticaceae

Trumpet Creeper (Campsis radicans) Bignoniaceae

Wood Nettle (Laportea canadensis) Urticaceae

Both types of Nettles are choice edibles that can be picked with gloves and deactivated by drying or cooking. The sting of Urtica is also considered to be of medicinal value as well (Duke & Foster, 2014). One of my all-time favorite ethnobotanical memories is witnessing Frank Cook “urticate” the aching legs of our friend and teacher the legendary ethnobotanist Jim Duke at a Warren Wilson College herbal symposium in 2009.

Another source lists even more plants that can irritate the skin (Nelson et al., 2007). In this reference the sources of phytodermatitis are broken down in five categories: mechanical irritants, chemical irritants, allergens, phototoxins and pseudophytodermatitides. The main culprits are the Araceae, Asteraceace, Apiaceae, and Euphorbiaceae.


Toxins from mushrooms and microbes are beyond the scope of the current discussion. However, a couple fungal organisms that act on plant materials do bear mentioning. Ergot (Claviceps spp.) is a commonly occurring toxic fungus on members of the Poaceae (Elpel, 2013; Stewart, 2009). Aflotoxin is a commonly occurring fungus on several kinds of nuts. A number of books deal with the toxins of plants and also cover fungus as well (Blackwell, 1990; M. R. Cooper & Johnson, 1997; Kingsbury, 1964; Turner & Aderkas, 2009; Turner & Szczawinski, 1995).

Medicine to Poison

            A classic statement attributed to Paracelcus as stated above is that the dose makes the poison. However, some classic medicines and foods have subtle poisons that allow them to get accepted by various cultures potentially to their detriment. Common name confusion can further cloud understanding. The plant sometimes called Red Sage (Lantana camara) Verbenaceae is one example. It is potentially toxic when taken internally which is a common practice in the Caribbean (Morton, n.d.; Pour et al., 2011; Pour & Sasidharan, 2011; Sharma et al., 1981; Wolfson & Solomons, 1964). Many other plants may act in this way including members of the Aristolochiaceae family due to the presence of Aristolochic acid (Hashimoto et al., 1999; Schaneberg et al., 2002). i hope to expand this section in the next iteration. The members of the Asteraceae with pyrrolizidine alkaloids mentioned above would fit into this section as well i.e. Eupatorium spp., Ligularia sp., Packera,  Senecio spp, and Tussilago sp.. (Chen et al., 2010; Colegate et al., 2018; Prakash et al., 1999; Robertson & Stevens, 2014). Toxins from some commonly consumed Cycads are another example.

Miscellaneous potentially toxic plants

Many other potentially toxic plants that have not been mentioned occur in temperate regions. Horsetail (Equisetum spp.) Equisetaceae, has sometimes been recommended for internal ingestion medicinally but may cause upset due to the presence of silica (Abbe, 1985; Rogers, 2014).

A small fraction of people are sensitive to Pawpaw (Asimina triloba) Annonaceae, either through exposure to the skin or ingestion of the fruit (Kingsbury, 1964). The Annonaceae also has some pretty severe compounds that have been researched thoroughly and may be toxic after extended intake or exposure but at the same time are being researched for use against cancer (Araya et al., 2002; Champy et al., 2005; Harper et al., 1947; He et al., 1997; Lannuzel et al., 2002; Levine et al., 2015; McLaughlin, 2008; Mukhlesur Rahman et al., 2005; Pomper et al., 2009; Potts et al., 2012; Ye et al., 1996; G. Zhao et al., 1992; G. X. Zhao et al., 1994, 1996).

Members of the Ericaceae including Laurels (Kalmia spp.) Pieris (Pieris spp.) and Dog Hobble (Leucothoe spp.) are known to be toxic to livestock. Many Ericaceous plants contain sodium channel inhibitors (Nelson et al., 2007). Poisoning may also occur from the consumption of honey that contains nectar from members of the Ericaceae. However, Doug Elliot (2011) says that honey his bees made from Rhododendron was unusually clear and very bitter thereby precluding ingestion. Carolina Jessamine (Gelsemium sempervirens) in the Loganiaceae is a plant that has been shown to cause toxicity in humans both from flowers and honey containing nectar (Hardin & Arena, 1974).

Some plants that are commonly consumed have poisonous parts. Examples include Elderberry (Sambucus spp.), Pokeweed (Phytolacca americana), Rhubarb (Rheum sp.) and members of the Solanaceae family. A full treatment of poisonous and edible fruits can be referenced for further clarification (Hardin & Arena, 1974).

Food allergies are also common to a subset of people. Plants causing allergies include many nuts, Soybeans (Glycine max), species in the Rosaceae and gluten containing members of Poaceae such as Wheat, Barley, Rye and Spelt. i have also met people occasionally with more rare plant allergies such as Cacao, Ginger, Sesame, Strawberries, Nightshades and others…


One must be careful when analyzing the poisonous potential of plants. The devil is in the details. The poisonous nature of plants is often related to a multitude of factors. The information presented here represents a broad overview simply designed to raise the awareness of potential toxins. Several plants and especially plant families mentioned can represent choice members of a healthy diet. Much toxicological research has also focused on the effect to livestock versus people. It is not possible for me to always differentiate between these two classifications at present. Easy to attain books concerning poisonous effects of plants on humans solely can be referenced for further study on that subject (Alber & Alber, 1993; Dauncey, 2010; Hardin & Arena, 1974; Nelson & Balick, 2020; Schmutz & Hamilton, 1988; Scott & Thomas, 2000; Wyk & Wink, 2015). A treatment of over 1,200 poisonous plants of the world and their relative toxicity may give further insight (Wink & Van Wyk, 2008).

A few books list specifically the poisonous plants of the eastern U.S. (Blackwell, 1990; Pammel, 1911; Westbrooks & Preacher, 1986). A publication detailing the most common plants causing poisoning in the southern United States has also been written listing 56 species and 26 families (Kates et al., 1980). The major families by far followed by number of species listed were the Pea (Fabaceae) (9), Heath (Ericaceae) (8), and Aster (Asteraceae) (6).

A number of texts cover the whole United States and beyond (G. E. Burrows & Tyrl, 2001, 2013; Dauncey & Larsson, 2018; Kinghorn, 1979; Lampe et al., 1985; Leikin & Paloucek, 2007; Tornio, 2017; Wagstaff, 2008).

Regional texts are also available for the following areas listed below:


            Rhodesia/Tanzania (Shone, 1900)

South Africa (Steyn, 1934; Watt & Brandwijk, 1962; Wyk et al., 2002)


            China (X. Y. C. Bian, 2018; Z. B. Y. Bian, 2015)

            India (Chopra, 1965)

            Japan (Eldridge, 1876; Hirono, 1993; Murata, 2019; Poisonous plant of Japan, 2012)

            Pakistan (Askari, 2010)

Australia/New Zealand

            (Covasevich et al., 1989; Everist, 1981; McKenzie, 2012; Seawright et al., 1985; Williams, 2012)

            (Connor, 1977)


            Jamaica (Adams et al., 1963; Lowe et al., 2004)


            Britain (Bevan-Jones, 2009; M. Cooper, 1984; Gillam, 2008; Johnson, 1861)

            Europe (Woodward, 1985, 1986)

Middle East

Iraq (Al-Rawi, 1988)

North America/USA

Alaska (Extension, 2012)

Arizona (Schmutz et al., 1974)

California (Fuller & McClintock, 1987; Wiltens, 1999)

Canada and N USA (Ammirati, 1985; Fyles, 1920; Mulligan & Munro, 1990; Thomson & Sifton, 1922)

Colorado (Durrell & Newsom, 1939)

Hawaii (Arnold, 2019; Scott & Thomas, 2000)

Illinois (Evers, 1972; Tehon, 1946)

Oklahoma (G. Burrows & Tyrl, 1996) 

North Carolina (Hardin, 1961, 1966)

Pennsylvania (Ditmer, 1965; Hill, 1986)

USA (Northwest) (Tapson-Jones, 1995)

USA (Southeast) (Powe, Jr. et al., 2005; Schwaner, 1974)

USA (Southwest) (Schmutz & Hamilton, 1988)

USA (West) (Agriculture, 1985; Weathers & Cheeke, 1998)

Virginia (Massey, 1963)

A number of resources also cover poisonous plants of Florida and the Caribbean as well (Adams et al., 1963; Campbell, 1983; Dahlgren, 1944; Lockey et al., 1992; Lowe et al., 2004; Morton, 1971; Nellis, 1997; West, 1984).

Hopefully this overview has given a sense of some of the toxins out there and the major families containing them as well as a plethora of resources for further study.

For the next class we will cover major Woody Plant families it will be posted around July 10

Below are items to think about/comment on. Please write me directly at marc@botanyeveryday.com or leave information in the commentary under this class.


- Make a list of the poisonous plants around you and share that info with some people.

- Attend a workshop or a class and write up a brief description of plants or information learned.

- Try to make a list of the plants for your county using the USDA website search at  www.plants.usda.gov       

- Post any clear photos of question plants to Facebook or send in an email.

Praises to all that have donated to the cause. i encourage everyone reading this to donate as they are able financially, commentarially, or energetically... Your contributions greatly help me continue this crucial work of ethnobotanical research and education. Please let me know your thoughts in general and anyway i can help this class serve you best.

Thanks, marc

Literature Cited

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