Photon correlation spectroscopy in the polarized (VV) and depolarized (VH) geometries has been utilized in order to investigate the aggregation dynamics in toluene solutions of a hairy-rod polymer, poly(p-phenylene) with flexible dodecyl side chains. Static and dynamic results from the dilute regime suggest that polymer molecules form small aggregates, typically trimers, even at very low concentrations. In the semidilute regime, results at different scattering wavevectors (q) in both the VV and VH geometry reveal two new relaxation processes (slow and ultra slow), in addition to the faster cooperative diffusion and reorientation of the trimers, respectively. They are attributed to the formation of large anisotropic clusters (of typical size 570 nm), with inherent crystallization, as revealed by X-ray scattering. After about 2 weeks the clusters settle under the action of gravity and form an opaque sediment. Assessment of the aggregation modes has been carried out by sediment redispersion. The first stages of this process, which exhibit multiple scattering, reveal a steeper than exponential relaxation, associated with a sedimentation-induced velocity gradient in a transient pseudonetwork formed by the dispersed clusters. In the single scattering limit, reached at later stages, the two relaxation processes are related to reorientation (slow) and number density fluctuation (ultraslow) of the anisotropic clusters. On the basis of simple thermodynamic arguments, we argue that rodlike macromolecules exhibit a larger activation energy barrier compared to their flexible counterparts, in agreement with the experimental evidence.