Grodan SBS (Single Block System) are individual plugs with or without a cell tray. Refills are sold without a tray. Excellent for the propagation of seeds and cuttings. Two sizes of plugs are available: Small 25mm x 25mm x 40mm and Large 36mm x 36mm x 40mm. It is recommended to pre soak all rockwool before use in water at pH 5.5 – 6 and root stimulator. After germination or rooting, a plant start feed is recommended before the use of Grow formulas. Horticultural grade rockwool is designed specifically for plant production. It differs from insulation and building grade rockwool and glasswool. Its made with varying proportions of both wettable and non-wettable rockwool with the fibres laid in different directions according to its use.