•A washing pre-treatment was applied to livestock derived substrates.•The washing pre-treatment did not affected negatively substrate physical properties.•The growing media with washed composts showed positive effects on seedlings. The search for alternative materials for peat substitution in soilless systems has led to an increasing use of compost as ingredient of growing media. These materials may have similar characteristics to peat for their potential use as substitutes of this non-renewable material. However, the high salinity is one of the most usual limiting factors in compost for its use in growing media. This work evaluates the effect of the use of a washing pre-treatment to improve the quality of livestock waste-derived composts as peat substitutes in substrates for pepper (Capsicum annuum L.) seedling production. For this, different growing media were prepared using as ingredients five composts from livestock manures and anaerobic digestates (M1, M2, M3, D1 and D2), with or without a washing pre-treatment. Each respective material was mixed with peat in the proportions 25 %, 50 % and 75 % (volume:volume), using as control treatment pure (100 %) peat. The main physico-chemical, chemical and physical characteristics of the composts before and after washing and those of the growing media were studied, as well as germination and biomass production of the seedlings. The use of a washing pre-treatment was effective in the reduction of compost salinity, without implying a negative effect on the physical properties and nutrient content of the growing media, which were adequate for being used in seedling production, as reflected in the beneficial effects on the germination and growth of pepper seedlings.