The work discusses the pressure required for forcing diluted suspensions of plant particles in a pipe. Their volume increases under the effect of the interface with the liquid. An aqueous suspension of hay particles with a size of up to 106 μm and an initial volume fraction of 10% was investigated for this purpose. Standard measuring instruments were applied for testing the rheological properties of the prepared substance, i.e. a Höppler viscometer and a rheometer with coaxial cylinders. The measurement results were utilized throughout several procedures for calculating flow resistance. It was found that the hasty application of commonly known and practically used simplifying assumptions or inadequate interpretation of the rheological properties of the substance lead to significant design errors of the flow system. As a result of imbibition, the volume fraction of the solid phase varies, and thus it becomes difficult to predict the rheological properties of the suspension. • Swelling of plant particles in water. • Unknown actual volume fraction of solids phase. • Difficulties in predicting the rheological properties of plant suspension. • Ambiguous assessment of the demand for pumping power.