Castor bean (Ricinus communis) is an important non-edible oilseed crop and source of castor oil which has multiple applications ranging from cosmetics to biofuels industry. However, the extraction of castor oil generates large amounts of de-oiled castor cake containing ricin, a highly toxic glycoprotein that requires treatment prior to its valorization. In this study, biochar was produced by slow pyrolysis at 550 degrees C from castor stalks and de-oiled castor cake, both by-products of castor oil production, in view of sustainable valorisation of a potential large scale cultivation of castor beans in Greece. The produced biochars were highly alkaline and had significant differences in terms of surface area, morphology, nutrients content and ratios of atomic C:N and H:C. The biochars were added to potting mix at different rates (0, 1 and 5% mass fraction of the dry mix) for the cultivation of tomato and castor seedlings, and their growth was monitored over a period of two months without fertilization. Based on the results, castor biochars improved castor seed germination, achieving 90% success rate earlier when compared to control. On the contrary, biochar did not affect significantly the germination of tomato seedlings. However, in both species, the biochar treatments promoted lateral root initiation but increased the developmental rate only in castor. In addition, biochars affected soil cation exchange capacity (CEC), PO4, total N and P, K, Na, Ca, and Al levels. (C) 2017 Elsevier Ltd. All rights reserved.