After exporting a mesh from the SpeedTree Modeler, you can use the included Houdini OTL to load it into Houdini and recreate the materials as closely as possible.
To install it, in Houdini, install the digital asset found at <apps install>/scripts/Houdini/SpeedTreeImport.otl.
Note: If you are on the bleeding edge of Houdini and for some reason the OTL provided does not load, the python script that does the actual loading is also provided in the same directory (script.py). However, you will need to recreate the materials needed, or change the script to create some other type of material (see below).
Once the OTL is installed, you will now have a new SpeedTree Importer obj node. When you instantiate one of these, it will ask to load a SpeedTree Material (.stmat) file. You don't need to worry about the format of the actual mesh exported from the SpeedTree Modeler, as that information is stored in the stmat file.
Once chosen, it will ask you the type of materials to make for it, Principled or Mantra. You can also add your own material types by editing the script (see below).
If you wish to change how the materials are created, or add completely new material types, the importer's python script has been designed to be easily extensible in this manner.
All you need to do is add or change one of the classes that inherits from SpeedTreeMaterialCreator. You will need to have a “description” member and a CreateMaterial() function that does the actual creation of the material and returns a shop node, based on the information in the SpeedTreeMaterial class that is passed in. Hooking the material up in the correct place on the mesh is all handled for you elsewhere.
For examples, please see the SpeedTreeMaterialCreatorPrincipled and SpeedTreeMaterialCreatorMantra classes in the python script inside the SpeedTreeImport.otl file.