The solid waste streams specific to soilless horticulture (substrate slabs, propagation cubes, and plastic films to cover the soil and to wrap the substrate slabs) were determined quantitatively and qualitatively, while methods to reduce these waste streams without yield loss were evaluated in a case study applied to the Flanders region of Belgium and based on an explorative inquiry among horticulturists. Rockwool used for substrate slabs and propagation cubes was found to be by far the most important waste stream. The use of long-lived, polyurethane (PUR) slabs could reduce the total slab waste stream by ≈90%. Moreover, if substrate blocks are used instead of slabs, this reduction could even increase to 95%. The introduction of new cultivation techniques could further reduce the required volume of substrate slabs. Rockwool propagation cubes could be successfully replaced with peat pots that can be composted after 1 year of use. The reuse of plastic films to cover the soil or to wrap the substrate slabs cannot be considered because of the danger of plant diseases. Due to the susceptibility of these films to contamination, they cannot yet be recycled on a large scale. The use of thinner films and the cultivation on profiled concrete floors were found to allow drastic reductions (of up to 80%) of the quantity of plastics used.