Submitted this to the Forestry github and was asked to reference it here, not a clue how to actually do that so here's the link to the issue:

ForestryMC/ForestryMC#1260 (comment)

Steps to reproduce:

1. Biomes O Plenty and Forestry installed
2. Load chunks

Additional Information:

Crashlog: http://pastebin.com/fLsSttHg

Affected Versions (Exact numbers, do not use "latest"):

• Biomes O' Plenty: BiomesOPlenty-1.9.4-4.1.0.2023-universal
• Minecraft: 1.9.4
• Forge: forge-1.9.4-12.17.0.1935

I am also seeing this issue, just a variant on it which is:
java.lang.IllegalArgumentException: Cannot get property PropertyEnum{name=variant, clazz=class biomesoplenty.common.block.BlockBOPGrass$BOPGrassType, values=[spectral_moss, smoldering, loamy, sandy, silty, origin, overgrown_netherrack, daisy]} as it does not exist in BlockStateContainer{block=biomesoplenty:plant_0, properties=[variant]}

Thanks @drewbit you referenced it correctly. You can see a link back here on the Forestry issue.

#763

See my comment in #763