Toxic Plants in Yards and Fields

Poisonous plants are found in many yards and gardens either as weeds or cultivated for ornamental purposes. In several posts I will feature species that  grow either in my yard (or once did) or nearby in unmanaged areas. All of  these are common in warm-temperate to tropical locales in much of the world.

This post is a follow-up to an earlier description of toxic members of the parsley family (Apiaceae).  To read this, go to the Plant Category elsewhere on this website (see ‘Hold the Parsnips’) where you can read about deadly relatives of the parsley plant {e.g., poison hemlock (Conium maculata) and water hemlocks (Cicuta spp.)} that have potentially lethal alkaloids throughout most of  the plant organs.

 Note that many toxic plants seen in residential yards apparently are purchased and cultivated by gardeners without knowledge of their poisonous properties. Based on my visits to garden shops, no effort appears to be made by many vendors to alert purchasers that their offerings are highly toxic or even lethal to humans or pets.

 Reference is made below to some of the chemicals that contribute to toxicity but no attempt is made to technically characterize these compounds. However it’s worth noting that several classes of compounds are particularly common in plants and are responsible for much of the poisoning of mammals that is known. The largest class consists of alkaloids, nitrogen-containing organic compounds that tend to be basic or alkaline in reaction rather than acidic; there are about 3000 types of alkaloids, mostly found only in vascular plants.

 A second class consists of glycosides which are carbon-based organic compounds in which one or more sugar molecules are bound to a carbon atom. Again, these compounds are principally produced by higher (vascular) plants. In a sub-category of glycosides called glucosides, the attached sugar molecule(s) is specifically glucose rather than one of the many other sugars. Alkaloids generally occur as byproducts of metabolism and are believed, in many if not most cases, to benefit the plant by inhibiting attacks by herbivores, especially insects but also  rodents and birds. However, in only  a relatively few species has this  inhibiting effect been demonstrated and shown to be of probable survival benefit. 

 Here are several tropical (in one case, warm-temperate) plants that are lethal if ingested but sport spectacular flowers that keep them common in yards, parks and roadsides of warm regions of the world.

Flowers and leaves of Thevetia peruviana

Flowers and leaves of Thevetia peruviana.

 Thevetia peruviana, (native to Tropical America}, is grown extensively in Florida in appreciation of its large, pink to peach-colored flowers.  The species has several common names including lucky nut and yellow oleander. A member of the Apocynaceae family to which belong many species that produce toxic chemicals, T. peruviana is known to be highly toxic in all its parts. The main toxin, thevetin, is a cardiac glycoside ( a glycoside that is especially damaging to the cardiovascular system). The presence of thevetin characterizes this genus of about 9 species as does the copious white latex  (milky sap) that is released when the plant is cut or injured. One or two seeds of “lucky nut” have been known to be lethal to children who consumed them and 15 to 20 grams of  leaves fatal to horses. The seeds have been used for insecticides, fish poison and even homicide. Many deaths have resulted from (mis)use of the plant in folk medicine.

 

Latex containing alkaloids leaking from cur leaves of T. peruviana.

Latex containing alkaloids leaking from cut leaves of T. peruviana.

Several other glycosides also are present in Thevetia as are other compounds that contribute to the plant’s toxicity. Although  its highly toxic sap discourages most herbivores, at least one, a caterpillar that ties together thevetia leaves and feeds on the tissues, has shown up in my yard. Apparently this herbivore is able to detoxify the alkaloids as does the monarch butterfly which feeds exclusively on the tissues of the glycoside-containing milkweed plant (Asclepias spp.)

 

Mature fruits of Thevetia turn hard and black.

Mature fruits of Thevetia turn hard and black.

 

Thevetia pods broken open to reveal 4 large seeds.

Thevetia pods broken open to reveal seeds (best seen in photograph below).

Four large very toxic seeds freed from the hard pod.

Nerium oleander, like Thevetia, is a latex-containing shrub of the Apocynaceae. It is grown in the far-southern U.S. and California for its flowers, which are large and pinkish to reddish, or whitish.  N. oleander is an Old World species found in warm, dry regions such as surround the Mediterranean Sea.  It is commonly seen along rivers such as drain the Italian peninsula as well as along the Jordan River and Sea of Galilee.

Just like Thevetia, all parts of the oleander  are highly toxic and contain cardiac glycosides (in oleander, neriin and oleandrin) which resemble digitalis, a glycoside derived from the foxglove plant that in low doses has proved valuable in treating heart problems but may be lethal at higher doses. Consumption of a single leaf of oleander has been reported as sufficient to kill an adult human and 15 to 20 grams of leaf material will kill an adult cow. Honey made from the nectar is toxic and death has occurred from roasting food on the branches of oleander. Even inhaling smoke from burning oleander branches has caused significant injury.

 

Row of oleander plants along roadside in central Florida.

Row of oleander plants along roadside in central Florida.

Oleander is one of the most widely planted ornamental shrubs in the southern coastal United  States, on the Caribbean islands, and widely in the Old World. It continues to be used in highway beautification projects by government agencies, and by landscapers and gardeners. This deadly plant is unharmed by most insects particularly in the New World, but apparently herbivores from its native range that are able to detoxify the alkaloids have reached the New World. This includes  the beautiful black, blue and white day-flying moth ( Syntomeida epilaius) which is preceded by spiny orange caterpillars (seen here in my yard but not yet photographed), a scale insect, and a second moth so far restricted to far southern Florida.

The showy, long -flowering and fragrant oleander flower.

The showy and fragrant flower of the long -flowering oleander.

Datura and Brugmansia

 The Solanaceae (the potato or nightshade family) is a large family of plants (2000-3000 species) with both food staples, (tomato, aubergine, potato, etc.) and  highly toxic species (e.g., tobacco, belladonna, deadly nightshade). Even the edible staples however, may contain toxic alkaloids. For example, solanine accumulates in potato tubers in the presence of light, causing greening of the tubers, which might provide protection from herbivores when dug up, but also confers an associated poisoning risk to unwary human consumers. Other alkaloids  occur in edible species as well but at least  there appears to be no basis for worry about consuming the leaves or immature fruits of species like the tomato which contain tomatine, a rather innocuous alkaloid at concentrations found in garden plants.

 The toxic alkaloids in plants of the Solanaceae occur particularly in leaves and seeds but also in all parts of the plant including pollen. Some alkaloids like atropine and hyoscyamine, and other members of the tropane group of alkaloids are well-known for having, in low doses, medical  uses, in somewhat higher doses hallucinatory effects, and, in high doses, toxic or even lethal effects, 

 The genera Datura and Brugsmansia of the Solanaceae are closely related and sometimes Brugmansia is absorbed into a larger Datura genus. However the two are easily distinguishable as the latter is larger both in flower size and in height (reaching 3-4 m) thus gaining the common name of tree datura, while the latter will usually be under 1 m in height. In addition, at least two annual species of Datura are native to the temperate U.S. and southern Canada. Members of the Datura genus reproduce readily by seed while Brugmansia  spp ,which originate  in South America but  are known now only in cultivation, are propagated asexually by cuttings. In the U.S. at least, Brugsmania plants seldom produces seeds.

 Perhaps best known to wanderers in urban wastelands and untended areas, D. stramonium (jimson weed  or thorn apple) reaches way into the northern states. Like the other species, all parts of the jimson weed are narcotic and toxic. In the north, the temperate Daturas typically do not last beyond the first frost but their spiny pods full of viable seeds may still be attached to the dead plants.

Datura stramonium. From N.L.Britain & A. Brown (1913), An Illustratwd Flora of the Northern United States and Canada. Charles Scribner & Sons.

Datura stramonium. From N.L.Britain & A. Brown (1913), An Illustrated Flora of the Northern United States and Canada. Charles Scribner & Sons.

 Brugmansia plants put on a spectacular flower display and as result are widely planted in sub-tropical and tropical regions around the world. The pendent, strongly scented flowers may exceed a foot in length and be white, pink or yellowish depending on species. The genus apparently originated in the Andes from Columbia to Peru although  B. suaveolans, one of the commonest in cultivation, probably is native to the South American  lowlands. At least 7 species are recognized. 

Brugsmansia growing in my yard. Flowers are almost intoxicatingly fragrant.

Brugsmansia growing in my yard. Flowers are almost intoxicatingly fragrant.

Well known as toxic and narcotic, B. suaveolans has been used by native American peoples as a hallucinogen alone or mixed with other plant extracts. The similar D. metel  is native to tropical Asia. In India where it is known as dutra, it has been valued since prehistory while in China it is featured in early writings. It can also be smoked and used to stupefy enemies!

 

Closer view of Brugmansia.

Closer view of Brugmansia flower.

Columbian Devil’s Breath from the borrachero tree (the Columbian name for Brugmansia) is a drug whose active ingredient is the alkaloid scopolamine. In low doses scopolamine is reported to temporarily block memories from forming and to induce a kind of hypnosis. As such it was once investigated by the CIA for use as a truth serum but its strong hallucinogenic properties and toxicity effects discouraged its adoption.  Reportedly even powder made from the plant, if inhaled, has strong hallucinogenic effects. Although at low levels, scopolamine has some medical uses in the U.S., in doses only modestly higher, it is lethal.

 Subsequent posts will present additional toxic species.

 

 

 

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