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.

Camellia japonica flowering in KL

 

Camellias flower all year on the highland resorts of Malaysia but not in the lowlands. I have tried for years to grow camellias in Kuala Lumpur, in sun as well as in shade, but even if they formed flower buds, the buds would dry up and abort without blooming. It has been terribly frustrating. Finally, one plant has flowered, after three years in a pot, under the shade of a langsat tree. The first flower, about 3 months ago, might have been a fluke. Now the plant is bearing its third flower, indicating that some barrier has been broken. Now we know it can be done, we can try harder to add camillias to our lowland gardens.

Dipterocarpus alatus--iconic giant street tree of Ho Chi Minh City

 
 
 

I visited Ho Chi Minh City recently and was amazed to see giant trees of Dipterocarpus alalus growing as street trees in the older parts of the city. I estimate their age to be 80 to 100 years, which means they would have been planted during French colonial rule. Dipterocarpus alatus belongs to Dipterocarpaceae, which is the dominant timber-tree family of SE Asia. Dipterocarpus alatus itself is native to Burma, Thailand, Laos, Cambodia, Vietnam and Philippines.

In Malaysia and Singapore two other members of the family have been planted as street trees in the past 10 - 20 years. These are Hopea odorata (also commonly planted in Vietnam) and Shorea roxburghii. Both are doing well as street trees and will eventually become giants.

Which passionflower is this?

 
 
I bought this plant in a Sungei Buloh nursery two years ago and it has been flowering in the Secret
Garden of 1 Utama. Could this be Passiflora amethystina? It does not fruit and the flowers are not perfumed. The leaves are deeply trilobed.

Is this a cycad?

 
 
 
Does anybody recognize this plant? I bought several small seedlings about two years ago from a retail outlet located in the shopping mall of 1 Utama and planted them in the Secret Garden on the roof. The plants grew and produced bulbous stems topped with closely clustered pinnate leaves. They look like cycads but I have not been able to identify them. The species has meanwhile disappeared from the market. This seems rather typical of the Malaysia horticultural market--somebody brings in one batch of plants (in this case probably as seeds), sells the lot, and moves on to something else. Some introductions persist while others fade out. This one may persist.

What monocot is this? Agavaceae

 
 
 

This plant has been in the Secret Garden of 1 Utama for several years. It came as tufted plants with stiff sharply pointed leaves. I planted one tuft in the cactus bed with other plants of arid climates because it looked like an agave. I planted another in a wet place. The plants thrived in both places but surprisingly it was the plant in the wet place that flowered. It produced a long slender inflorescence bearing a series of flowers, but failed to set fruits. The identity of the plant is a mystery. A friend has suggested Bromeliaceae, the pineapple family, but I think it is closer to Agavaceae the century-plant family.

Which Crinum lily is this?

 
 
This crinum lily was growing in a friend's garden at about 1000 m elevation at Genting Highlands near Kuala Lumpur where the climate is noticeably cooler than in Kuala Lumpur. I transferred some bulbs to the Secret Garden of 1 Utama (climatically in the lowlands, at about 100 m elevation), where it grows and flowers regularly, but whereas in the highlands the flowers open fully and face sideways, in 1 Utama they open less widely and face downwards. I thought this might be Crinum kirkii but the inflorescence stalk is long and carries the flower head way above the rather short cluster of leaves.