The Borneo lowland rain forests blanket most of Borneo below about 1,000 meters elevation, spanning Indonesian Kalimantan, the Malaysian states of Sabah and Sarawak, and Brunei. These are classic dipterocarp forests: towering members of the family Dipterocarpaceae form the emergent canopy, with characteristic genera including Shorea, Dipterocarpus, Dryobalanops, Hopea, and Vatica. The climate is stable, wet, and tropical, with rainfall spread through the year and high humidity, and little seasonal temperature variation. Biodiversity here ranks among the richest on Earth, with the island holding the world's greatest dipterocarp diversity (over 260 species, more than 150 of them endemic to Borneo) alongside hundreds of bird species and flagship mammals such as the Bornean orangutan and Sunda clouded leopard. For gardeners, the region is the native home of horticulturally familiar plants, including a wealth of orchids and the fruit-bearing genus Artocarpus (breadfruit and jackfruit).

The Borneo montane rain forests cloak the central mountainous spine of Borneo above roughly 1,000 metres, spanning the Malaysian states of Sabah and Sarawak, the Indonesian region of Kalimantan, and Brunei. Rising above the lowland dipterocarp forests, the slopes shift into oak, beech, and chestnut forests of the Fagaceae and Lauraceae, with conifers growing more abundant at higher elevations and Ericaceae-dominated forest, including many Rhododendron species, near the peaks. This is a classic cloud forest: cooler and moister than the lowlands, it receives heavy rainfall and draws additional moisture directly from low clouds, grading from tropical conditions up to alpine zones on summits such as the 4,095-metre Mount Kinabalu. The ecoregion is a stronghold of endemic life, with the Bornean orangutan as its flagship species, twenty-three bird species found nowhere else, and a striking flora of carnivorous Nepenthes pitcher plants and orchids. Roughly three-quarters of its forest remains relatively intact, with about a quarter under formal protection. For gardeners, it is the wild home of montane Rhododendrons and tropical orchids long prized in cultivation.

The Borneo peat swamp forests stretch across the coastal lowlands of Borneo, spanning Indonesian Kalimantan, the Malaysian state of Sarawak, and the Belait District of Brunei, built up behind brackish mangroves and clustered around the inland lakes of the Mahakam and Kapuas rivers. These forests grow on waterlogged, acidic peat that can exceed twenty meters in depth, where anaerobic conditions slow decomposition; characteristic canopy trees include the valuable hardwood ramin (Gonystylus bancanus), Shorea albida, Dactylocladus stenostachys, and Dacrydium beccarii, alongside more than thirty palm species. The climate is tropical and monsoonal, with year-round waterlogging and seasonal flooding. The proboscis monkey is the ecoregion's flagship species, which also shelters the Bornean orangutan and the prized Asian arowana (Scleropages formosus), yet it is considered critical or endangered, with only about 14 percent protected and much of the peat drained, logged, or burned. For gardeners, the swamps' native understory includes ornamental screw pines of the genus Pandanus, grown elsewhere for their bold, strappy foliage.

The Brahmaputra Valley semi-evergreen forests blanket the alluvial floodplain of the upper Brahmaputra River, spanning northeastern India (chiefly Assam, with Arunachal Pradesh and Nagaland), southern Bhutan, and adjacent Bangladesh. As the name implies, the natural cover is semi-evergreen forest, mixing an evergreen canopy of Syzygium and Cinnamomum with deciduous trees such as Terminalia, Bombax ceiba, and Tetrameles, an understory including Mesua ferrea, and stands of bamboo. The climate is monsoon-driven, with heavy southwest-monsoon rains from June to September that flood the plain and renew its fertile silt, giving way to a cooler, drier winter. Though much of the original forest has been cleared and only a small fraction is protected, the surviving reserves are globally important: Kaziranga National Park holds the world's largest population of greater one-horned rhinoceros, alongside one of India's largest Asian elephant populations, and the golden langur is among the ecoregion's flagship species. For gardeners, several signature trees here, including the red silk-cotton Bombax ceiba and the fragrant-flowered Mesua ferrea, are long-grown as tropical ornamentals.

The Cardamom Mountains rain forests cover the wet evergreen highlands of the Cardamom and Elephant mountains in southwestern Cambodia and southeastern Thailand, extending to Vietnam's Phu Quoc island along the Gulf of Thailand. Forests here grade from lowland and hill evergreen stands into montane forest, where the beech family (Fagaceae) genera Lithocarpus and Castanopsis dominate alongside Lauraceae trees such as Cinnamomum and Litsea, with myrtle-family Syzygium and abundant epiphytic orchids. Rising steeply to over 1,500 meters, the range intercepts moisture-laden monsoon winds and receives very high annual rainfall, exceeding 5,000 millimeters in some valleys. Part of the globally important Indo-Burma biodiversity hotspot, it is one of Southeast Asia's most species-rich and endemic-rich forests, sheltering Asian elephants, tigers, clouded leopards, endemic chestnut-headed and Siamese partridges, and the critically endangered Siamese crocodile, with much of the range now within Southern Cardamom National Park. For gardeners, native genera such as the aromatic Cinnamomum and the ornamental Syzygium illustrate the region's warm, perpetually humid, frost-free climate.

The Chao Phraya freshwater swamp forests occupy the lowlands of the Chao Phraya River watershed in central Thailand, a broad floodplain extending roughly 400 km north to south. Historically these wetlands carried tall stands of Dipterocarpus alatus on the higher ground, with screw pine in swampier spots and a matrix of reeds, grasses, and sedges, grading into mangroves toward the estuary. The climate is monsoonal, bringing around 1,400 mm of annual rainfall, with maximum temperatures near 33°C and minimums around 24°C. Almost all of the original swamp forest has been cleared for rice agriculture and cities, including Bangkok, and large mammals such as tigers, Asian elephants, and Javan rhinoceroses have vanished, while Schomburgk's deer is believed extinct. The Lyle's flying fox now serves as a flagship species, and the plain still holds the world's largest populations of the Asian openbill stork.