Not sure if this is a compatibility issue as the mulch accepts flowers from other mods and the BoP flowers can be placed onto such things as modded flower pots.

Biomes O Plenty bug, they need to return EnumPlantType.Plains from getPlantType for their block and call canSubstainPlant in their placement checks

Thanks for the reply. I opened an issue with Biomes O' Plenty.

Looks like Biomes O' Plenty closed the issue on their tracker. As there is nothing I can do without hacks on my side, I will just have to call their plants unsupported.