Previous high-angular-resolution 225GHz (~1.3mm) continuum observations of the transitional disk DM Tau have resolved an outer ring at 20-120au radii that is weakly azimuthally asymmetric. We aim to examine dust growth and filtration in the outer ring of DM Tau. We performed the ~0.06" (~8.7au) resolution Karl G. Jansky Very Large Array (JVLA) 40-48GHz (~7mm; Q band) continuum observations and the complementary observations at lower frequencies. In addition, we analyzed the archival JVLA observations that were taken since 2010. Intriguingly, the Q band image resolved the azimuthally highly asymmetric, knotty dust emission sources close to the inner edge of the outer ring. Fitting the 8-700GHz spectral energy distribution (SED) with two dust components indicates that the maximum grain size (a_max_) in these knotty dust emission sources is likely >~300um while it is <~50um in the rest of the ring. These results may be explained by trapping of inward migrating grown dust close to the ring inner edge.The exact mechanism for developing the azimuthal asymmetry has not yet been identified, which may be due to planet-disk interaction that might also be responsible for the creation of the dust cavity and pressure bump, or the fluid instabilities and vortex formation due to shear motions. Finally, we remark that the asymmetries in DM Tau are difficult to diagnose from the >~225GHz observations, owing to a high optical depth at the ring. In other words, the apparent symmetric or asymmetric morphology of the transitional disks may be related to the optical depths of those disks at the observing frequency