I had planted a sapling and bamboo in an area and had covered them with string/barrier blocks to stop them growing. 

I became aware, looking to a neighbouring chunk, that the sapling and bamboo had grown in the neighbouring chunk, while their original forms were preserved in the chunk of origin. 

This was done in a flat world. 

To reproduce, simple plant a sapling or some bamboo and cover it so that it cannot grow, and over time (sped up by increasing randomTickSpeed (although related not that I was unable to get this command to work - tested with grass and dirt,) the plant will grow in another chunk, or replicate the sapling if it has no room to grow.