Background Stone wool is commonly used as a plant substrate in soilless cultivation and discarded after one growing season. Stone wool waste is difficult to recycle, and thus it is typically landfilled. Alkali-activation of stone wool (i.e., milling and mixing with an alkaline solution) has been shown to be a feasible way to upcycle this waste fraction into, for example, construction products. In this study, the aim was to develop recycled plant substrate from stone wool waste from greenhouses via alkali activation. Methods Waste stone wool from greenhouses was characterized by X-ray fluorescence (XRF) and mixed with sodium silicate solution either directly or after ball milling. The alkali-activation process was combined with the addition of H2O2, pre-made foam, or granulation to obtain suitable porous material for the plant substrate application. Preliminary greenhouse cultivation experiments of pea (Pisum sativum) were conducted with alkali-activated stone wool mixed with peat (a weight ratio of 1:1) and fertility analysis of the mixture were conducted. Results The results indicated that the most feasible production method was to use ball-milled stone wool and to combine alkali activation with granulation. The obtained granules could reach 2.7 MPa as compressive strength while the other methods resulted in very fragile material. The preliminary greenhouse cultivation experiments revealed that there were significant levels of nutrients (Ca, P, K, and S) and alkalinity leached from the granules which hindered the growth of pea. The high P and S amounts were also confirmed by the XRF results of stone wool. Conclusions It can be concluded that the developed granules did not function well as a plant substrate for pea but could enable the re-utilization of the nutrients contained in the greenhouse stone wool waste. Moreover, their application to acidic sulfate soils could be feasible as it would utilize the alkalinity of granules.