The photo to the right shows one of two Yellow Flame Trees (Peltophorum pterocarpum) planted in front of the provincial hall of Nonthaburi, quite obviously because this is the provincial symbol tree for Nonthaburi.
I had compiled the complete list back when I researched to create the Wikipedia articles on the provinces, and the main sources were the province descriptions from the Ministry of Interior (the PDF files likes in the green box of this page), a list from panmai.com (flower and trees), and also somewhere in the TAT website. In most cases these sources all list the same plants, but in some cases there are problems, interestingly only for the trees. For example for Samut Songkhram most have Casuarina equisetifolia as the provincial tree, but the TAT listed Barringtonia asiatica instead. It is of course possible that a different tree has been assigned to the province lately and not all lists have been updated already. An authoritative list would of course be fine to settle the last doubts.
For most flowers and almost all of the tree I did not find any explanation on why that specific plant was chosen. But from the few where I know the rationale I am presenting four examples.
- The provincial flower of Surat Thani is the Rafflesia, which is an obvious choice since this flower is the most extraordinary flower of all Thailand, and is found most often and easily accessible in this province. I first heard about this flower when I researched that province article, but it took me some years till I was able to see one in nature.
- The provincial flower for Ubon Ratchathani is the lotus (Nymphaea lotus), which is also quite clear as Ubon means Lotus. Just not sure if it is in Sanskrit or on Pali, the languages which are the origin of many ceremonial names are words in Thailand.
- Another obvious choice is the Siam Tulip (Curcuma alismatifolia), the provincial flower for Chaiyaphum. I visited the Siam Tulip fields in Pa Hin Ngam national park in this province, which is clearly one of the attraction of this province.
- The provincial tree of Chumphon is the cluster fig (Ficus glomerata). The Thai name of this tree is Maduea Chumphon (มะเดื่อชุมพร), and it is one of the possible explanation for the origin of the name of the province.