Compressed growth substrate blocks which swell when moistened are prepared from predried peat (moisture content less than 35%) by compressing the same to a thickness of at least 1/3 of its thickness after drying, a higher degree of compression being used on the border areas of the peat blocks than on other areas. Thereby the peat fibres in the more compressed border areas orient themselves partly in the direction of pressure and are bonded together by the natural resinous substances in the peat under the influence of the higher compression forces, thus imparting good cohesive characteristics to the block also when moistened. Advantageously also a small indentation for a seed is simultaneously pressed into the growth substrate block. A pressing device for use in the above method comprises a press-plate and a moulding tool for pressing thereagainst the moulding tool being provided with substantially blunt-edged ridges projecting therefrom in the direction of compression for imparting the higher degree of compression on the border areas of the growth substrate block.
