Grass blocks can grow under all other transparent blocks like glass, fences, or torches. Though water and lava are both light-filtering blocks (meaning they decrease skylight by 1 level but do not affect block light), other light-filtering blocks such as ice and slime blocks do not block grass from spreading to the dirt block. When grass is random ticked, it spreads by checking 4 blocks within range and converts any dirt blocks selected into grass.

The block directly above the dirt block must not be lava, water, or a waterlogged variant of a block.

That means blocks that partially let light through such as stairs and slabs must not have their light-blocking side(s)‌ facing the dirt block.

The block directly above the dirt block must allow light to pass through it and hit the dirt block, even though the actual light level does not matter.

The source grass block must have a light level of 9 or brighter directly above it.‌.The dirt block receiving grass must be within a 3×5×3 range of the source grass block where the source block is in the center of the second topmost layer of that range.Grass spreading without player intervention depends heavily on the time of day.‌ For a dirt block to accept grass from a nearby grass block, the following requirements must be met: Grass can spread to nearby dirt blocks, but not coarse dirt or rooted dirt. Grass blocks generate naturally in most biomes in the Overworld, and as part of villages and ancient cities.

For more information, see Breaking § Speed.

↑ Times are for unenchanted tools as wielded by players with no status effects, measured in seconds.

Minecraft 1.8 sky overlay