The origin of the Chinese flowering cabbage ('Choi Sum' in Cantonese)has been a bit of a mystery. It was long popular in China and Japan before it became known to the western world. Most authorities think it originated in Southeast Asia. It certainly grows well in the tropics where it is widely cultivated by ethnic Chinese farmers, but I have never seen it wild in the Malay Peninsula, and the seeds are usually imported (from China or Thailand?). The plant also does not 'escape' and grow by itself without human help. These observations made me doubt the theory of its Southeast Asian origin.
However I recently noticed a small form of this vegetable in native markets in Kuching. It is less than half the size of the form grown by Chinese farmers. I made enquiries and found that it is known as 'sawi dayak'. It is grown by broadcasting the seeds on newly burnt hill slopes on which the inland communities grow hill rice. The very small seeds are sold cheaply in small packets, each containing hundreds if not thousands of seeds, so the plants must be prolific seeders and perfectly adapted to the local environment. Hence I now believe that sawi dayak is the parental wild type of Choi Sum and its origin is indeed Southeast Asia, specifically Sarawak in Borneo.
The Chinese form of this vegetable needs daily watering and tending. The sawi dayak is watered by rain only. It is just the vegetable for the 'let-it-grow-naturally' organic farmer.