Supplementing plants with silica (Si) enhances growth and improves multiple stress tolerance across a large variety of plant species. Despite its importance for agriculture, this phenomenon has no satisfactory explanation. Hydrogen peroxide (H2O2), a common stress messenger in planta, may play a central role in Si-related plant biology. In this work, we test the hypothesis that the known in vitro chemical interaction between H2O2 and silica plays a role also in planta. We thus compared growth rates and response to hypoxia and salt stress in Si-accumulators (sorghum, wheat) and non-accumulators (sorghum mutant defective in Si uptake, tomato) in relation to variation in Si and H2O2. Si-dependent root elongation was observed in seedlings of both Si accumulators and non-accumulators. Similarly, roots elongated more rapidly in seedlings grown in media that were supplemented with 0.01-0.1 mM H2O2. When H2O2 was quenched with 20 μM KI, the Si-dependent elongation disappeared. In vitro, freshly polymerized silica gel slowed H2O2 decay under oxygen and in the presence of lignin. Our results indicate that Si affects the redox balance at the apoplast. We propose that cell wall-bound silica sequesters H2O2 and protects it from degradation, thereby acting as a buffer of apoplastic redox homeostasis. This simple physicochemical interaction may provide a unifying explanation for the agronomic benefits of Si fertilization.