Tuesday, June 01, 2010

Living with poisonous plants

Yesterday I was interviewed by a Chinese language newspaper (Nanyang Siang Pau) on the subject of poisonous plants. There has been a spate of emails and sms messages saying that this or that popular house plant is poisonous and even cancer-inducing. As a result worried people have been throwing out their house plants. After the two- hour interview, I wrote a summary in English and sent it to the reporter to help her. Since I do not read Chinese, I have no idea what will finally appear in the Chinese press, but I thought my English summary might be of interest to blog readers.

Plants cannot run, fight or hide, so they make themselves inedible with poisons, otherwise they would get completely eaten up and become extinct. Animals that live on plants have some degree of resistance to plant poisons, but humans mostly lack such resistance. Salad plants have been selected and grown for their lack of poisons but such plants cannot survive on their own without human protection.

Some plant poisons act by contact. The most notorious are those of the poison ivy family which cause irritation in contact with bare skin. People learn to avoid touching these plants after one experience.

The most common plant poisons are tannins. Tannins react with proteins and are used to convert the perishable protein in animal skins into tough durable leather. Tannin will similarly react with the skin of the mouth and throat. Plants that contain tannins in large amounts are impossible to eat. Just try raw bananas or raw persimmons! When fruits ripen, their tannin content is deactivated to allow animals to eat the fruits and disperse the seeds. In dilute amounts, tannin is what gives taste to beer, wine and tea.

The next most common poison is calcium oxalate in the form of sharp microscopic crystals in the plant cells. When these cells are broken in the mouth, the crystals embed themselves in skin of the mouth and throat and cause swelling. The aptly named ‘dumb cane’ and other members of its family such as the popular ‘money plant’, as well as the ‘ZZ plant’ Zamiocalcus zamiifolia (marketed as a Chinese good luck plant), contain such crystals. Kids that put a leaf containing oxalate crystals in their mouths will learn never to put unknown plants into their mouths ever again, but it may be hours before the pain and swelling passes.

Then there are the plants that contain cyanide compounds, especially those in the tapioca (cassava) family that includes the ‘sweet leaf’ (sayur manis or Sabah vegetable). These poisons are totally inactivated by cooking. There was a case in Taiwan of death from eating raw ‘sweet leaf’, that caused a panic and a sharp drop in consumption of this popular vegetable when reported in the press. The person who consumed the raw leaves had done so deliberately in the belief that it would help her lose weight.

Finally there are the plants that contain alkaloids and other chemicals that can cause convulsions, breathing problems, nervous disorders, hallucinations or kidney failure. These plants are used to kill e.g. to tip arrows and darts for hunting and warfare, for execution (e.g. the famous case of Socrates who was executed with a drink of hemlock juice), for murder (when disguised as food) or suicide. Also to kill pain and give pleasure (e.g. opium). Many of these plants are used as medicines in small dosages.

Cases of accidental fatal poisoning by eating poisonous plants are extremely rare. The one case that everybody quotes is the deadly nightshade in Europe that has juicy delicious-looking but poisonous fruits. Because tomatoes resemble the deadly nightshade and belong to the same family, it took a very long time before Europeans would accept tomatoes as food after their introduction from the Americas.

In brief, fatal poisoning by accidental consumption of poisonous plants is almost impossible because poisonous plants are so unpleasant to eat and the human mouth is so sensitive to unpalatable substances. All the worry about poisonous plants is unnecessary.


AaronVFT said...

Thanks for the valuable information, Dr. Ng.

I agree that humans should not be so scared of plants since they won't hunt you down. I've received emails about disposing dumbcanes and ZZ plants because of death caused by the plants. These emails caused some of my relatives to panic but I just delete them right away.

Usually pets and small children are the victims since they are curious by nature and like to explore their surroundings. If we put the plant somewhere harder to reach for the child, then accidental poisonings will not happen. Carelessness in usually the cause of plant poisonings.

Well, I'm living with lots and lots of dumbcanes and a wild oleander that pop right out of a pot.

Thanks again for the useful information.

Autumn Belle said...

I am so glad that the press has taken the initiative to give some clarification regarding the poisonous plants emails and smses. I too have personally received many such emails. It is sad that some people become so worried that they throw the houseplants away or abstain from gardening. Oh, what a lot of magic and beauty of nature they will be missing.

It is alright if we are careful when handling/dealing with poisonous plants. Let the caterpillars eat the poisonous oleanders and vinca so that they'll become the butterflies that pollinate the flowers that turn into deliciously healthy fruits for human beings to eat. Even the birds that eat those caterpillars will not die. So, we shall continue to grow many types of plants.

This is a great educational post.

AaronVFT said...

Hi Dr. Ng,

Thanks for your directions to the Sungai Buloh nursery. I have found the purple passion flower and tulip gentians!

Unknown said...

Dear Dr Ng,
I stumble upon your blog by accident while doing some research on plants for my Resident Association for our replanting project. May I know how to get in touch with you, can I have your email address? My resident association need some advice on plants and trees for our replanting project.
I am also looking forward to your new book coming out soon.
Thank you.
Regards, Catherine Cheng, PJ

Dr Francis Ng said...

My blog was too complicated for the newspaper, so the reporter asked me to provide some blunt statements. I guess this is necessary considering the nature of the stories being spread. My statements were:

There is no truth whatever in stories of plants attacking people, releasing poisonous gases and causing cancer.

Epipremnum aureum, the 'money plant' has been grown by millions of people all over the world as a house plant for over 100 years. It has not been known to kill anybody.

Zamiocalcus zamiifolia is relatively new in cultivation, but it does not kill people or poison the soil.

I challenge those who spread such stories to say where their information comes from, giving the name of author, title of the original scientific/medical report, and date and place of publication. If these stories have any truth, their originators should be proud to identify themselves to the whole world.

Dr Francis Ng said...

Hi Catherine
My email is tropicalplantman@gmail.com

ts said...

Hi Dr Ng,

I just got my friend to purchase your new softcover book from Malaysia. Glad you came out with it as the hardcover was over budget for me.


Dr Francis Ng said...

Hi Siyang
You may be the first buyer. I checked in bookshops last weekend and did not see the book there. Hope you enjoy it.

antigonum cajan said...

I do not comprehend the silly fears about poisonous plants. My collection of over one hundred species has over twenty percent of them.

Since I have no children, I will not eat then, wear gloves,
me pet is not dumb, what is there to fear?

prince_rayme said...

good evening sir,
im just read ur entry about avocado, at the same time i like to know about avocado.
sir, did you know people that planting avocado in malaysia? especially in sarawak ?
i need to know them for me completed my research about it.
thank you sir.

MARIE HairStudio said...

I'm Angelina 19, from malaysia, now sitting for STPM.
I would like to study horticulture as i am interested in growing flowers and plants. However, many people said girls are not suitable to study this factulty. How do you feel? May I know what university shall i go to further my studies next year? Can you give me a suggestion? hope u can reply me asap. =)
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Yow Chuan said...

I remember reading about seeds from the Pinang tree containing alkaloid.

Dr Francis Ng said...

The nuts of the pinang palm Areca catechu, sliced and wrapped in the leaves of Piper betel for chewing, used to be a most common form of addiction in Asia. It is still common to chew betel in India and Nepal and also in Taiwan. Betel nuts contain chemicals that have pleasurable effects on people, somewhat like tobacco and coffee. In normal concentrations they would not be classified as poisons.

Anonymous said...

There is a popular trend nowadays to "juice" vegetables upon the belief that nutrients would be obtained faster and in a higher quantity this way. More attention should be highlighted in the danger of this (as in the case of the girl attempting to lose weight) . Wonderful piece. Thank you.

Ann, Chen Jie Xue 陈洁雪 said...

"plants that contain cyanide compounds, especially those in the tapioca (cassava) family ", there is a folklore in Sarawak that if you plant your tapioca up-side-down, the plant will be poisonous and people will die.
I wrote my book, WWII, with a chapter on this. Further reading tells me people who are allergic to latex may be allergic to may be allergic to tapioca.

I enjoyed reading this post.

Ann Chin