Now the Pneumaticraft seeds are discontinued in later updates of Pneumaticraft, but Agricraft greenhouses still plant them by some sort of random seed algorithm so not even sure who to report this to. Everyone?

Try mentioning it on something related to either of those mods?

There's a seed blacklist in the agricraft folder in you .config directory. Add the following to it and they won't show up anymore.

PneumatiCraft:plasticPlant:0
PneumatiCraft:plasticPlant:1
PneumatiCraft:plasticPlant:2
PneumatiCraft:plasticPlant:3
PneumatiCraft:plasticPlant:4
PneumatiCraft:plasticPlant:5
PneumatiCraft:plasticPlant:6
PneumatiCraft:plasticPlant:7
PneumatiCraft:plasticPlant:8
PneumatiCraft:plasticPlant:9
PneumatiCraft:plasticPlant:10
PneumatiCraft:plasticPlant:11
PneumatiCraft:plasticPlant:12
PneumatiCraft:plasticPlant:13
PneumatiCraft:plasticPlant:14
PneumatiCraft:plasticPlant:15