There are basically two options to add foliage and trees to any scene in architectural visualization and the first one is by creating 3d tress, which can add a huge amount of data to your project and the other one is by simply use textures. From both options the use of textures is by far the best one, because you will have in this case only a face with a texture applied and no extra load to a possible complex render.
But, to be able to use that in a scene you have to first find some textures to use. An artist called jlnh shared a set of 25 tree textures on BlendSwap that could be useful for architectural visualization. Visit this link to take a look on the tree collection.
