Redwood Trees are trees that naturally spawn in Redwood Forests. Like real deadwoods, they are very tall, very wide and can be harvested for a large amount of wood.
Redwood saplings require a lot of empty space around them to grow into trees - 5 blocks in each cardinal direction. The smallest area required is one sapling in the middle of a 3x3 square.
Version 1.1-6.0.2 - Their growth is dependent on climate/biome. They will not grow in cold and snowy biomes, possibly also not in very hot ones, the latter has not been tested yet. You need a plane, free area of 7x7 blocks with one sapling in the centre. Be careful not to stand in the 7x7 area when using bonemeal to grow them in survival, as the growing wood can easily kill you. Also the sapling itself must be placed below y=60.
Version 1.2.0+ - As of version 1.2.0, only one Redwood sapling is required to grow a tree.
Before version 1.2.0 - To spawn a redwood, the nine saplings must be planted in a 3 by 3 square. They must be planted on grass blocks, not dirt, and must have one extra block on each side, illustrated by the yellow blocks. The red blocks being where the saplings are planted.