Getting a SpeedTree model into SideFX's Houdini is a three step process. First, select or create a model in the SpeedTree Modeler. Second, export the model using 'File->Export mesh…'. Third, import the generated '.fbx' file in Houdini using the provided scripts. See below for a detailed explanation of these steps.
The first step in the process is to use the SpeedTree Modeler to create the model you wish to use in Houdini. This can be as simple as opening a library model or as complex as creating a tree from scratch. The model should include material assignments, correct uv coordinates, and everything else you need to see the model exactly as it should appear in Houdini. The goal is to replicate the model exactly as it appears in the Modeler in Houdini. This includes features such as detail mapping, branch intersection blending, and wind.
Once the model is complete, select 'File→Export mesh…'. In the ensuing file selector, select 'Autodesk FBX' from the combo box labelled 'Save as type' and pick a filename for the export. The model will be processed and the following dialog will be presented. Select the options appropriate for your situation and press 'Ok' to write the .fbx file.
SpeedTree provides scripts for working with SpeedTree data in Houdini. Click here for complete details. Using these scripts rather than a basic .fbx import provides a mechanism for implementing features such as branch intersection blending in Houdini.