A-Ok Starter Plugs for starting seeds and rooting of cuttings prior to plugging into larger Gro-Blocks. Strip of 15 MM 40/40, 1.5 in. x 1.5 in. x 1.5 in. polywrapped blocks. Grodan rockwool is an excellent quality stonewool substrate with a very high water holding capacity. Rockwool is absolutely free of disease, salts, and weed seeds. It has no Cation Exchange Capacity (CEC) so the grower can take advantage of a nutritional program specifically tailored to the crop being grown.