Force Editor to release memory?

I have an editor tool which is importing external heightmap files and creating terrains from them. Part of the tool is importing tile sets, which could be of extremely large sizes. Unity obviously has memory limitations, which I thought I got around by adding the ability to convert the generated terrains to prefabs, and then removing the terrains used to generate those prefabs from the scene.

I use GameObject.DestroyImmediate to destroy the terrains once they are converted to a prefab...

Force Editor to release memory?