Over ninety percent of the world’s rice is grown in Asia, and nearly all of this is produced by flooding rice paddies. In other words, rice fields are filled with water. Sometimes this happens naturally because of heavy rain. Often, this flooding is controlled by farmers. This method began in China over ten thousand years ago. Asian culture has been shaped by the need to cooperate and make sure that rice fields get sufficient water.
There are certain benefits to filling rice fields with water. For example, temperatures are more stable in flooded fields, because water holds on to the warmth of the sun better than dry earth. Another big benefit is that weeds are not able get oxygen under water.
Plants, just like animals, need oxygen to breathe. Generally, plants get this oxygen from the air. Of course, plants also need some water, but most plants die in flooded fields, because their roots are not able to get oxygen to breathe.
Rice is an unusual crop, because it remains healthy in flooded soil, although even rice may suffer damage if water is too deep for too long. Certain plants that float on water, such as lotus plants, develop special cells that allow gases to be passed from the surface to the roots. Rice also has these special cells for breathing.
As weeds cannot breathe under water, they drown. This means that rice does not have to compete with weeds if fields are flooded.