Some mod authors rename their assembly, which screws the lookup function. Here an example solution for PalmeraTree. Please share if you find solutions for other mod plants.

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The author of PalmeraTree has renamed the assembly, which makes it impossile to find it.

Goto your config file and look for this line:
"PalmeraTree.PalmeraTreeConfig, PalmeraTree-merged, Version=2019.9.5.0, Culture=neutral, PublicKeyToken=null"

Replace it with this line:
"PalmeraTree.PalmeraTreeConfig, PalmeraTree, Version=2019.9.5.0, Culture=neutral, PublicKeyToken=null"

You can set "AutomaticallyAddModPlants" to false to prevent it from importing the wrong assembly again. If you forget to do so, then it will still work but you get a warning (which you can ignore).

Alternatively you could rename PalmeraTree.dll into PalmeraTree-merged.dll, but you have to repeat it when PalmeraTree is updated.