Abstract: In the production of growth blocks of sphagnum there are considerable operational problems with the required compacting of the loose sphagnum, as both compaction worms and pistons may easily get stuck. Instead, the invention provides for the use of a suction technique, whereby the loose sphagnum (2) is sucked into a perforated pipe (34) in a suction chamber (28), whereby a marked compaction of the material is achievable without any use of mechanical devices. The compacted block material may be surrounded by a decomposable paper casing, which is laid in successively as a lining hose of air permeable paper. The compacted material can be delivered as a compact string (46) by successive projection movements of the suction chamber (28) for being cut into blocks (56), and the suction chamber can temporarily opened for free return movements into its initial position.