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How to identify a tropical plant

October 19, 2012

It can be quite a challenge to identify a tropical plant. What tools can help you fish out the correct international scientific name out of Earth’s pool of 60o+ plant families and 300 000 plant species?

With experience you can simply make a qualified guess about a plant’s family affinity, which is crucial to know when selecting which literature to proceed with. If you do not have that experience, there is a computer program which may help you identify the family: the DELTA Intkey.

Ordinary identification keys ask you one question at a time, and you can select between two alternatives. The problem arises when the key asks for a character that is not available (you hold a flower in your hand and the key asks if the fruit is fleshy or dry), or when the language is so technical you do not understand the question.

The DELTA Intkey allows you to pick and answer any question, thereby narrowing down the families from an overwhelming 601 to a range of 1-8. Reading about the remaining families in any book (although Mabberley’s Plantbook is the most comprehensive) makes you soon realize what you may hold in your hand. Be warned that many questions are very technical, even taking cellular and biochemical features into account, but gardeners may skip these and focus on simple questions such as number of stamens (may reduce the number of families with 2/3), shape of the flowers, if the leaves have a stalk (petiole), presence of a leaf rosette, succulence, if it is a herb, tree or liana etc. It is important to take notes while going throw the questions, because at times the program is reset and you lose all your work. With a more pedagogic approach the key could become a very powerful tool.

Once you know the plant family, you should proceed with identifying genus and species either using the printed Flora of Thailand series (which I hope will become available on-line in the future) and/or the Flora of China key which is available on-line.

Specialist or general plant literature will also help you, such as scientific articles (pdf) or books on gardening, fruits and vegetables, trees and orchids.

Never use a dictionary to identify a plant. A vernacular name is never precise and may refer to 20 other totally unrelated plants, one famous example is that of ‘sala’ under which Lord Buddha was born.

A Dokmai Dogma reader sent me this picture of a common weed in Chiang Mai, found also at Dokmai Garden. Although my gut feeling said ‘Acanthaceae’ simply because I have seen many similar flowers in that family, I thought I should give the DELTA Intkey a try. Answering 12 questions about the plant I narrowed down the number of plant families from 601 to 5, all having at least some species with four stamens in two pairs of unequal length (didynamous). Since mints (Lamiaceae) usually have fragrant leaves, bignonias (Bignoniaceae) usually have flat seeds with wings, snapdragons (Scrophulariaceae) often have many tiny seeds and many gesnerias are hairy (Gesneriaceae) all that remained was Acanthaceae.

Many visitors to Dokmai Garden passing our Justicia adhatoda in the same Acanthaceae family wonder if that is an orchid. Orchids do not have free stamens like in the five families mentioned above. Instead, orchids have their stamens and pistil fused into one unique structure named ‘column’. In addition, orchids are ‘monocots’ having parallel leaf veins while the ‘dicots’ above have netlike veins, and the seeds of orchids are dustlike.

Using Flora of China I ended up with Asystasia gangetica subsp. micrantha. An important feature of the genus is the presence of hooks (retinacula) in the dry four-seeded fruit capsule. The plant has a squarish stem with root-making nodes. The size mentioned in Flora of China is modest compared to its prolific growth here in Thailand and Hawaii (up to 3 m trailing over other vegetation).

Since it is a pantropical weed its origin is obscure; it could be anywhere in tropical Africa or Asia. English vernacular names of this species often include ‘violet’ which is misleading because this is not a violet (Violaceae) at all. How about ‘boomerang weed’, because no matter how many times you weed it and throw it away, it always comes back?

Text: Eric Danell

Photo: Ricky Ward

5 Comments leave one →
  1. October 20, 2012 8:03 AM

    It is important to know that you need to download and install the program first, before playing around.

  2. October 21, 2012 7:19 PM

    Hi Eric, been reading your blog for some time now and appreciate your writing on the vast number of plant related topics.
    Having been involved in helping out a little with teaching how to use tropical plant keys, I thought I’d mention as a worthwhile resource the Australian Tropical Rainforest Plants key (, which is a LUCID key not unlike the Delta key you were talking about. It does away with dichotomous couplets and has higher power in identifying sterile specimens. As far as I know, it is also probably the most complete tropical rainforest plant key (understanding of course that the southeast asian tropical flora way outstrips Australia).

    • October 22, 2012 9:33 AM

      Thank you very much David for this link and comments. I shall try it out. I think this is the way to go, and future biologists will simply carry an ipad to the forest.

      Cheers, Eric

    • October 22, 2012 9:48 AM

      Now I tried the key – it is indeed an even more pedagogic tool than the original Delta key. I hope huge amounts of money will be directed at creating such keys for national parks as well as countries and floristic regions (including tropical gardens: ‘key to the 5000 most common tropical gardening plants’).

      Thanks, Eric

      • October 23, 2012 5:01 AM

        Hi Eric, Yes, I think it is awesome too. I would love to see a LUCID key for Southeast Asia in the future. That would be great. But I imagine such things will have to progress in stages. Maybe someone could start with just trees and shrubs. This was how the Australian one started out as well.

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