Made in Denmark since 1969, stonewool is widely recognized as the ideal growing substrate. For any growing application, stonewool offers speed, ease of use and cleanliness. Grodan stonewool is known for its quality, consistency and performance. This multi-density slab retains air/water extraordinarily well. The final stage for growing in rockwool. After pretreating your rockwool, slice a whole in the bottom of the bag for drainage. Place your rockwool block on top of the slab, removing the plastic directly under the block, giving your plants all the room needed for development. Available in many sizes; one is sure to meet your needs. The Expert slab has its fibers running in the vertical orientation to allow faster draining.