Rockwool comes in a standard set of formats. Plugs and small cubes start seeds and cuttings; blocks, often about 10 × 10 cm (4 × 4 in) or larger, hold the young plant; and slabs, long plastic-wrapped bags of stone wool typically around 90-100 cm long, are what the blocks are set on to finish. Roots grow down from the block into the slab.
In practice
Slabs are soaked in nutrient solution before planting, then drainage slits are cut in the wrap. Dripper stakes feed each block. The grower tracks slab weight or water-content sensors to control dryback, and slab EC to steer growth. Slabs differ in density and fibre orientation, which changes how they hold and release water; pick one designed for the crop and keep the type consistent run to run.
Used slabs can be recycled through specialist schemes in some areas.
Don't confuse with
- Rockwool cube or plug: smaller starting formats.
- Coco slab (grow bag): similar shape and use, but coco coir.
Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Drip irrigation delivers water or nutrient solution through tubing and emitters directly to each plant's medium, in measured amounts on a timer or controller.Dryback is the drop in a substrate's water content between irrigations, usually expressed as a percentage of its content at full saturation.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.A fabric pot is a breathable, sewn felt or non-woven container that lets air and water through its walls, drying the medium faster and air-pruning roots.Crop steering adjusts irrigation timing, volume and dryback, along with climate, to push plants towards vegetative growth or generative (flowering) growth.Drain-to-waste fertigation feeds each plant fresh nutrient solution and lets the runoff leave the system rather than returning it to the reservoir.Field capacity is the amount of water a medium holds after it has been saturated and allowed to drain freely; in pots it is called container capacity.
