Try Glyptostrobus (Chinese Swamp Cypress).

Resin

The first thing that comes to mind is one of the new Bald Cypress hybrids, which are hybrids between the American Bald Cypress(Taxodium distichum) and Montezuma Cypress(T. mucronatum). Some have been named, like 'Nanjing'. They seem to have inherited the ability to grow in shallow water from their Bald Cypress parent and are evergreen or nearly so in mild climates(sub-tropical and tropical), like the Montezuma Cypress parent. These hybrids were developed by and in conjunction with the Chinese government, so you might be able to track them down in Asia. I have heard they are apparently growing well in Hainan, China, which is at the same latitude or even a bit further South than you.

A probably much harder to find Bald Cypress would be a Southern Florida ecotype of the American T. distichum, which is virtually evergreen, grows well in standing water, and grows well in the tropical climate of South Florida.

Another conifer which might work for you, but it's not quite evergreen, is the Chinese Swamp Cypress (Glyptostrobus pencilis). It grows natively in swamps in sub-tropical SE China, and since you are so far North in Thailand and possibly high up the mountains(?) you should probably be able to grow it well.

The only other thing I can think of is that there are dozens, if not hundreds of tropical swamp adapted trees which you could plant. A few like Ploiarium alternifolium have showy flowers.

I forgot to add that the common name for Ploiarium alternifolium is Riang Riang and it's native to swamps of SE Asia.

Glyptostrobus is definitely the better to try; it is an endangered species in the wild, and planting it will aid its conservation. No such benefits with an artificial hybrid. Go for diversity over institutionalised uniformity.

Resin

Coconut trees grow well at the edge of paddy fields and thrive in periodically flooded land. Members of Myristica (nutmeg) and Mangifera (mango) genus grow in tropical swamps; I doubt if any large tree will grow well in a perpetually flooded paddy field.

John

It sounds like what you are looking for is small evergreen trees or tall shrubs to produce a hedge-like effect.

This has got to be the most interesting tree question I've ever seen on the tree forum. It's really none of my business, but Emily, what are you doing in Thailand or perhaps better question, how did you find Gardenweb? And how do the other posters know what trees to grow in rice paddies? I feel utterly provincial! Recently, I haven't looked beyond re-situating my stewartia....
Thanks for posting.

If Emily is looking for a hegde, Pandanus is a possibility. I am including the URL for a picture of a Pandanus hedge along a stream bank in a paddy field which is cultivated (flooded) year round in Kerala, India.

Idaban, I grew up in Kerala, India - 9 degress lat. See the slideshow below for pictures I took when I visited there recently. If you live in Kerala you are never more than a mile away from a flooded paddy field.

John

Here is a link that might be useful: Paddy field trees

Thank you to all for all your advice. I'll check out the trees recommended.
They have to be 500-700cm trees to screen the view of houses in the distance so shrubs are out however, I am planting Pandanas in some areas including on the riverbanks. Thanks John for the pics.
Re Idabea, well the internet is a wonderful place. I stumbled across GardenWeb a year of so ago. I've always asked obscure questions and always got informative, useful answers from the amazing members with a wealth of plant knowledge that blows my mind. I've had the great fortune to stumble into a landscaping job here in Thailand. I usually work in the deep south on the coast so all plants have to be high humidity and coastal hardy. Chaing Rai is very different and I've never had to create rice paddies before!
Thanks again,

Emily

23' is barely into the realm of trees. You are definitely talking about large shrubs to small trees. And bushy growth with many stems coming from near the ground is what produces the best screening.

Sorry to be the ant at the picnic, but I can't think of many things as irresponsible as armchair botanists throwing out suggestions on what trees to plant when they don't have experience with them, in a place they've never been. Thailand is not in the Dark Ages. There are forestry and horticulture specialists in universities, government departments and gardens. Surely local expertise is the place to start. Acting on inspired ignorance is the sort of thing that in the past has led to introduction of invasive species or at best plants that don't succeed.

Thailand has an excellent forestry department and many good books on native flora and horticulture and many are in English or have English versions. White Lotus Press in Bangkok sells many of these(they even sell to via the internet). There is at least one Thai garden forum website which you can use to get further suggestions, but it's in Thai. A few of the posters do understand English though.

I will in my defense say I've been to Thailand(not to Chiang Rai, but have been to Chiang Mai not too far away) and I have experience with the trees I suggested, although with the exception of Riang Riang, not in Thailand. I have witnessed them in nearby Southern China though(Yunnan, Guangxi, Hainan, Guangdong, Guizhou).

Here is a link that might be useful: Thai gardening forum