Hydrothermal humification (HTH) is a novel way to treat biomass in order to produce soil conditioner or organic fertilizer. A plant seed germination technique was used to assess the effect of HTH material derived from garden waste on seed germination, root elongation and germination index (GI) of cucumber at different stages of incubation. The incubation experiment was carried out with the mixture of HTH material and soil at a ratio of 1:3, 1:5 and 1:10 (HTH material:soil, wet weight) in soil. The results showed there are direct positive effects of the HTH material on seed germination and root growth. GI all were over 80%. High addition rates had non significantly negative effects on plant growth.