The present invention relates to a method of producing a coherent growth substrate product formed of man-made vitreous fibres (MMVF), comprising the steps of (i) providing MMVF; (ii) providing an uncured binder composition; (iii) providing an additive; (iv) forming a mixture of the MMVF, the uncured binder composition and the additive; (v) curing the uncured binder composition in the mixture to form the coherent growth substrate product; wherein the uncured binder composition comprises at least one hydrocolloid; and wherein the additive is a micro-organism, fungus, biologically-active additive or combination thereof.