Grodan Gro-Wool Medium Water Absorbent Granulate Rockwool, 3.5 cu ftGranulated rockwool can be used alone or as an amendment to organic based mixes. For example, the addition of water absorbant granulate to peat mixes at 25 50% volume increases water holding capacity and aeration, improving plant growth and shelf life. The water repellent granulates are superior substitutes for aerators such as perlite or Styrofoam. The granulate is prilled for easy mixing. Granulated rockwool as potting mix: When you use granulated