We study and actively control the coherent properties of surface plasmon polaritons (SPPs) optically excited on a nanohole array. Amplitude and phase of the optical excitation are externally controlled via a digital spatial light modulator (SLM) and SPP interference fringe patterns are designed and observed with high contrast. Our interferometric observations reveal SPPs dressed with the Bloch modes of the periodic nanostructure. The momentum associated with these dressed plasmons (DP) is highly dependent on the grating period and fully matches our theoretical predictions. We show that the momentum of DP waves can, in principle, exceed the SPP momentum. Actively controlling DP waves via programmable phase patterns offers the potential for high field confinement applicable in lithography, surface enhanced Raman scattering, and plasmonic structured illumination microscopy.