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Boron (B) fertilizers are essential for global crop production. Sustainable B use demands a higher efficiency of B fertilizers through controlled B release and an enhanced B recycling from secondary sources, e.g. B-impregnated cellulose fiber insulation (CFI) waste. In this study, thermochemical treatments based on combustion and pyrolysis were investigated for processing CFI waste into a slow-release B fertilizer. Hot-water extractions of obtained materials confirmed that slow-release properties were achieved, and material characterization with XRD, FTIR and 11B-NMR showed that these were largely the result of formation of sparingly soluble Ca–B phases. A seedling toxicity test with fodder rape demonstrated the benefit of this slow-release B in comparison with conventional soluble B to circumvent toxicity, suggesting potential for B application via seed coatings. Finally, a fertilization trial with fodder rape and poppy seed showed that most slow-release B compounds were equally effective as soluble B at recommended B doses, while slow-release B can still benefit from reduced B leaching losses. [Display omitted] • Thermochemical processing of B-rich cellulose fiber insulation (CFI) was tested. • Obtained combustion and pyrolysis products had slow-release B characteristics. • Slow-release B was largely the result of formation of sparingly-soluble Ca–B phases. • B toxicity to plants could be avoided using treated CFI instead of soluble B. • In a plant fertilization trial, treated CFI was equally effective as soluble B. [ABSTRACT FROM AUTHOR]