Yesterday we had the honour of hosting lecturer Ricky Ward on the topic of plants useful to prevent soil erosion. The discussion was active and so many interesting topics came up. Afterwards we drank coffee and then we made a tour in the garden to see some of the plants Ricky had mentioned in his talk.
These were some topics treated:
1. British man Ralph Angell reported that his new garden association in northern Chiang Mai has difficulties attracting members. It seems the foreign settlers here do not share the interest in gardening to the same extent as the people of their home countries. Gardening is a major (40%) spare time activity in the west, but apparently not so among westerners in Chiang Mai. Still, we know many examples where westerners due to lack of knowledge spend tens or hundreds of thousands of Baht on plants which are doomed in this climate. Why they do not seek knowledge in advance is difficult to understand. We concluded that we go on with our different activities anyhow because many members is not a goal per se. To us it is the exchange of experience that is important.
2. We discussed the canal road project, which seems unnecessary and luxurious when compared with the urgent needs of repair of many smaller roads in Hang Dong, Chiang Mai and in the rest of Thailand.
3. As to the flooding of the more southern provinces like in Ayutthaya, we fear total death of most trees and garden plants. Many of the original tree species, adapted to swampy conditions, have been eradicated when land was cleared for rice and settlements, such as Dipterocarpus turbinatus. We decided to promote propagation of local Thai trees and then ship seedlings southwards.
4. Ricky explained that indigenous tree species would be the best suited to prevent soil erosion, as they are adapted to local conditions. Unfortunately many of these tree species are hard to obtain, and due to lack of knowledge there is also a very limited demand. We concluded that thanks to Ricky’s own work and connections we should try to make many of these tree species available via our network of gardeners. When we walked about in Dokmai Garden Ricky demonstrated many species useful to prevent soil erosion along streams, such as Dracontomelon dao, Lagerstroemia calyculata, Bischofia javanica, Tetrameles nudiflora, Salix tetrasperma, Crateva magna and Hopea odorata just to name a few.
We discussed bamboo, and Ricky’s opinion was that many have too shallow root systems, and their dense mat of roots does not allow many tree seedlings to germinate or develop in a satisfying way. Some bamboos like Dendrocalamus sericeus do grow along streams though. Trees are better to bind the soil deep down, and figs (Ficus spp) are among the best. We also discussed vetiver grass (Vetiveria). Ricky’s and Dokmai Garden’s hands on experience is that this grass may only be suitable in sunexposed situations with access to irrigation. In shady and/or dry areas it tends to disappear. A suggestion is to plant vetiver grass as a quick solution for areas where irrigation is available, and at the same time also plant trees of the genera mentioned above which can replace the vetiver grass over time. Teak was concluded to be a poor tree in this respect, due to its affinity for dry soils.
On a sloping bank, sometimes 2-3 meters above the water level, one should not plant streamside plants, but consider such an area as a dry area. Streblus asper, Bauhinia variegata and Spondias pinnata would be fast growing and drought tolerant native options. It is also utterly important to leave at least a five meter broad barrier of trees along a bank of a river or quarry. The reason is that the roots of the trees grip each other. A single row of trees is better than nothing but usually not sufficient enough.
Ricky also urged any serious gardener to get the book ‘A field guide to forest trees of northern Thailand’ by Gardner et al. (2007). It has colour illustrations and names also in Thai.
5. The practice of spraying roundup instead of mowing is an appalling and obsolete practice promoting soil erosion. We strongly advice everyone to stay away from routinely using such chemicals.