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Tall growing perennial grasses such as Phalaris arundinacea and Festulolium can be used as an alternative source for bioenergy production in northern latitudes as they can be grown in less cultivated areas and can be potentially used as a dual purpose crop (bioenergy and forage). The aim of studies was to investigate the effectiveness of using bioenergy waste products -- fermentation residues (digestate) and wood ash as fertilisers for perennial grasses. The field experiment was conducted in the central part of Latvia (56°42' N and 25°08' E) from 2013 to 2015. For all fertiliser treatments (wood ash, digestate once per season; digestate twice per season and mineral fertilisers) the same amount of plant nutrients (N, P, K) was applied annually: N (100), P2O5 (80), K2O (160); and the missing quantities of elements in ash and digestate were compensated by mineral fertilisers. Dry matter yield (DMY) in two harvest regimes (single cut and two cut) and chemical composition (ash content; total C and N) of grass biomass partitioning among tillers, leaves and panicles were estimated. Biomass yield in the three years of use varied considerably depending on the fertiliser, harvest regime and species, ranging up to 10.0 Mg ha-1 for RCG and 7.73 Mg ha-1 for festulolium. All fertilisers provided a significant increase of DMY, however, better results for both species were obtained using wood ash and mineral fertilisers. The harvest regime and species affected directly the quality of biomass, single cut of RCG contained significantly less ash and more carbon. There were significant differences between sward fractions -- culms in comparison with leaves contained less ash and nitrogen, and more carbon, what are desirable features for solid fuel. [ABSTRACT FROM AUTHOR]