Sounds ridiculous, but that's what I get from placing a sign on the wall above the crop stick D:

Before placing the sign: (note the text in Waila: This plant is fertile and can grow)

After placing the sign (note the text in Waila: This plant can not grow here)

IIRC, due to the compatibility with 2 block tall plants in 1.4, the block above the crop sticks needs to be empty.

Yes, the block above the crop has to be air.