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Here, we show that GaN nanowires (diameter <30 nm) can be used as strain relaxing substrates for the heteroepitaxial growth of stable In(x)Ga(1-x)N alloys of controlled composition and thickness. Thinner nanowires with their smaller interfacial area reduce the heteroepitaxial stress. Also, the limited adatom diffusion length scales on the thinner nanowires aid in reducing the kinetic segregation effects. In addition to being single crystal templates for heteroepitaxial growth, these thick single crystal overlayers on nanowire substrates can provide suitable architectures for photoelectrochemical applications. The stability and crystallinity of the In(x)Ga(1-x)N layers are preserved by the nanowires acting as compliant substrates. Photoelectrochemical water splitting requires In(x)Ga(1-x)N alloys with a 2.2-1.6 eV band gap (i.e. 0.45 < x < 0.65) and 150-200 nm film thickness for efficient light absorption and carrier generation. At such compositions, the In(x)Ga(1-x)N alloys are inherently unstable, the thickness-dependent stress builds up during the commonly employed heteroepitaxial growth methods, and adds to the instability causing phase segregation and property degradation. A dependence of the growth morphology on the GaN nanowire growth orientation was observed and a growth mechanism is presented for the observed orientation dependent growth on a-plane and c-plane GaN nanowires. Photoactivity of GaN and In(x)Ga(1-x)N films on GaN nanowires is also investigated which shows a distinct difference attributable to GaN and In(x)Ga(1-x)N, demonstrating the advantages of using nanowires as strain relaxing substrates.