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The art of selecting a suitable tree

May 11, 2011

Yesterday we received 44 mm of rain, and during the period of May 7-10 88 mm. Although most of the blossom is gone, the landscape is glistering green, spare loads of native orchids and emerging gingers. Some newspapers try to make a normal storm something exceptional. Such storms occur every year, and a few broken trees are normal. The challenge is, how do you avoid a broken tree on your house?

Select a suitable species. Many of the downtown trees in Chiang Mai are not suitable for a city. The beloved African tulip tree, Spathodea campanulata, and the red flame tree, Delonix regia, are both quite brittle and should be planted where falling limbs can not damage cars or buildings. A very poor choice is the exotic ton pradu baan (Pterocarpus indicus). Our experience here at Dokmai Garden is that it is prone to insects and fungi. Visiting guests from other parts of northern Thailand including Chiang Rai have similar experience. House owners often want something ‘quick and fast growing’, and here lies the paradox: fast growing trees often have a low density wood, making them more vulnerable in storms.

Another obstacle to consider is that your new tree may develop shallow roots, uplifting pavements and buildings. Examples of such trees are the popular Bucida molinetii, rain tree (Samanea saman) and cheesewood (Alstonia scholaris). The latter one is too often planted in Chiang Mai, I believe solely because it is cheap to buy, the decisive factor when most of the Chiang Mai gardeners make a choice.

Figs (Ficus spp) and willows (Salix spp) are gorgeous to admire from a distance, but they are experts in finding their way into water pipelines if planted too close to buildings.

You may also want to consider falling fruits and their smell of acetic acid when decaying, and heaps of small leaves when placing trees nearby a swimming pool. For shading a swimming pool, I should suggest large-leaved and evergreen species such as toddy palm (Borassus flabellifer), dwarf coconut (Cocos nucifera), breadfruit (Artocarpus altilis), banana (Musa spp) and Macaranga siamensis.

More suitable species along roads and nearby houses would be the hardwoods, but they do grow slowly, that is why they are so hard. One option is to stay away from most house builders’ instinct, i.e. to wipe out everything alive and then plant new things. It is most clever to leave a few old trees, which already constitute screens and shade, and they will in many cases improve the garden climate for your new tree seedlings.

So what would be good trees in a city? Cassia (golden and pink shower) are quite strong if left untouched by borers, but they are deciduous and so not the best shade trees. Mai dhaeng (Xylia xylocarpa) is very hard and valuable, and develops a dome-shaped crown for deep shade. With water it stays evergreen. Star apple (Chrysophyllum cainito) has a hard wood and beautiful evergreen foliage, but you may not want the fruits. Members of the Lagerstroemia genus would be ideal, really hard woods and large beautiful blossom, and so they are indeed frequently planted downtown and along highways. Dipterocarps, such as the famous Dipterocarpus alatus along the old road between Chiang Mai and Lamphun, are apparently very strong, but maybe too huge? Hopea odorata in the same family is very strong too, and will provide blossom with honey fragrance. Bon naak (Mesua ferrea) and magnolias (Magnolia spp) have hard woods and beautiful and fragrant blossom. I personally like the evergreen and sturdy mango (Mangifera spp) and litchi (Litchi chinesis) very much, but some do not like decaying fruit or even birds which are attracted to the fruits.

Select a healthy specimen. By investing in a good book on tropical or native trees you can come up with a list of some wanted species. Most house builders want a garden with big trees immediately, so they happily invest 5000 – 200 000 Baht/tree just to get something big. The sales person usually gives you a 6 months long guarantee. You might be lucky, but in many cases your trees die over a period of 2-4 years and you have to start all over. The reason is that a tree is not a dead object, it is alive and prone to stress and disease when roots are cut and kept above ground for months or years. Any damage of the bark and/or its root system might invite fungi which kill the tree over a period of a few years. The rot may cause heavy limbs to fall down. Even small trees, 3-4 meters tall, may have severe problems with dying limbs due to insufficient and cut root systems. To save such a tree you sometimes need to chop it down, and allow growth of root shoots. At Dokmai Garden we have compared jacarandas from seed with 1 meter tall saplings, and the seedlings overtook the saplings.

Some people judge the leaves when evaluating the health of a tree. A hole in a leaf simply means an insect had a munch, it tells you nothing about the fundamental health. In fact, a hole means the tree was not dipped in hazardous chemicals, a good sign! Instead of holes in the leaves, look out for barkless sections, mushroom fruitbodies, nails, dead branches and sap oozing out of the bark. If you see any of these symptoms, do not buy!

My strongest advice is: save your money and frustration, buy some 50-200 Baht seedlings which are young and healthy, and be proud like a parent when they grow. Most trees here seem to reach sexual maturity (blossom and fruits) after 3-4 years, have a decent size after 5-6 years and look old after 12 years.

Monitor and care for your trees. People would happily pay a million Baht for a car or a swimming pool, but look desperate if a bag of superior soil costs 160 Baht. Still, by digging a decent sized hole (50 x 50 cm in surface, and 40-50 cm deep) and mixing the soil with good compost (the preference depends on the tree species) you will have your mature garden much quicker! A sapling in a one litre container planted in a hole the size of the pot, surrounded by a concrete-like soil packed by the building machines, may struggle for years.

Remember to shade your seedling, sapling or tree, and water generously if planted in the dry season. ‘Generously’ is subjective. I have seen gardeners water newly planted trees the size of an elephant with a bucket. Such a tree may demand 200 litres a day. However, if the soil is poorly drained, the watering will kill it too.

Even if you buy a young hardwood and plant it in a generous hole with perfect soil, you should keep monitoring its growth. Frass on the ground and occurrence of holes may indicate borers, and such beetle larvae may kill limbs or the hole tree. Use a wire to kill the larva and coat the bark with permethrine (originally from Tanacetum plants) to repel repeated attacks. Dead limbs and parasitic plants should be removed to avoid unpleasant surprises such as a crushed car the morning you planned to go on vacation. The use of tree surgeons is common in the west, but in Chiang Mai they are rare and people tend to send up uneducated (cheap) people to whack about in the tree. Better do it yourself if you can not find a professional, and use a pole saw, not a machete.

Most welcome to Dokmai Garden to look at some of our 1000 plant species to inspire your own garden. We no longer sell plants as we focus on education and conservation.

Eric Danell & Ketsanee Seehamongkol

Some people simply prefer buying an already established garden!

2 Comments leave one →
  1. May 12, 2011 2:22 PM

    With ten years of living in the north of Thailand and studying and growing trees here I would strongly endorse young Eric’s observations.
    One thing that puzzles me though is his comments about supposedly shallow rooted trees. Alstonia scholaris is one he mentions. There is a fine row of this fast growing tree in front of the Chiang Mai Municpal Stadium where the surface roots are hemmed in by concrete kerbing and yet the trees appear to be thriving. Also the Rain Tree Samanea saman is often seen with out roots bulging out of the ground but sometimes the contrary is true. In Vietnam I have seen big Dipterocarpus alatus as street trees which sometimes destroy the surrounding pavement.
    Two trees I would add to his list. Bischofia javanica loves water and grows very fast giving much shade with shiny dark green leaves. I gave some seed to a nusery at Lamphun a few years ago and the local people went crazy with admiration of the tree. The other is Drypetes roxburghii ( Ma Ong Nok in Thai) , also evergreen and not unlike the Indian Mast tree. This was once common across the Chiang Mai plain but the only trees grown in government nurseries have been from seed I collected in recent years. Hopefully Kham Tien plant market will get the hint and start growing them. One lovely local tree they do have is Dillenia indica, ( Saan Yai in Thai). And if you want to grow local, please plant Dipterocarpus turbinatus (Yang Daeng) not D. alatus.
    If you are keen and need help getting trees ask Eric to give you my phone number.

    • May 14, 2011 9:26 AM

      Dear Ricky,

      Absolutely, we should be happy to forward any requests. We kept your Bischofia here for sale but people hardly buy any trees at all. They want cacao, coffee and lime or ‘summer flowers’ like petunia.

      Cheers, Eric

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