Can't place Shrub in Flower Pot

So it is possible to place a Dead Bush into a Flower Pot, but not a Shrub.

To get the shrub I did the following command on a flower pot..

blockdata ~ ~1 ~

{Item:tallgrass}

So I middle click the flower pot and I get 'Shrub', so I try and place it back in a different flower pot and it does not work. If I update the flower pot by putting a block next to it, it shows the dead bush.. I am not sure myself on this.