We report on strain and composition effects associated with growth of self-assembled BaZrO 3 (BZO) nanorods in REBa 2 Cu 3 O 7− δ (REBCO) superconductors (RE= rare earth= Y and Gd), which have a profound effect on flux pinning and in-field critical current performance. The ab plane mismatch between BZO and REBCO is never fully coherently accommodated. Instead, the nanorods always assume a size at least one unit cell smaller than the corresponding'hole'in the REBCO matrix, thus providing deep minima of in-plane mismatch strain. Next, we show that the nominal BZO nanorods are in fact solid solution Ba 2+(Zr 4+ 1− z RE 3+ z) O 3− δ perovskite, thus strongly affecting the stoichiometry and relative amounts of REBCO and BZO. We demonstrate that by varying only the Ba content in the nominal composition of 15 mol.% BZO+ REBCO, the unit cell density of BZO can be tuned from 5% to 23%, and the linear density of RE 2 O 3 (REO) precipitates from 18 to 1 μm− 1. The results explain the wide range of pinning performances observed for the same nominal amount of Zr addition and provide insight into the mechanisms behind the complex phenomenon of growth of nanorods by self-assembly in REBCO superconductors.