I've spent hours (who am I kidding - *days*) working on a massive pip-planted farm. It seems that pips can appear to take a seed out of a fridge (they get the animation for having something in their mouth), but they then never go on to actually plant a seed.
By comparison, when they pick up the same kind of seed (in my case sleet wheat) from the ground, they go on to then actually plant it.
I can't say for certain this is the case, but I have so far watched pips plant hundreds of seeds. I have seen lots of times where a pip appears to pick up a seed from a fridge, but not once have they then gone on to plant it. They stand still for a few seconds, then the chubby cheeks animation ends and they get on with their lives.
Have sleet wheat in a fidge.
Have a tile where something could be planted.
Add one or more pips.