Organic semiconductors based on small conjugated molecules show a high potential for applications in organic light emitting diodes, organic solar cells, and organic thin film transistors. Here, growth morphologies of crystalline para-sexiphenyl (6P) thin films on TiO2(110)-(1x1), mica(001), Au(111), and SiO2 have been investigated by Atomic-Force Microscopy (AFM). For TiO2, a delicate balance between sticking anisotropy and diffusion anisotropy is the reason for the formation of parallel needles at room temperature as well as anisotropic stripes of upright standing molecules in the first monolayer at higher temperatures. On the mica surface, deposition of a layer of carbon or ion bombardment prior to 6P growth changes the orientation of the molecules from lying to upright standing and isotropic islands are formed instead of anisotropic chains of crystallites. The observed mound formation in the growth of upright standing molecules can be explained by the Zeno-Effect that requires a high Ehrlich-Schwöbel Barrier (ESB). For the first time in organic thin film growth, this barrier has been experimentally determined (0.67eV) and verified for 6P films on SiO2. In addition, we could also demonstrate that substrate induced changes in the molecular orientation in the first few monolayers lead to a layer dependent ESB, a novel phenomenon characteristic for growth of anisotropic molecules.