Rockwool is created by mixing igneous rock and limestone, heating it and spinning it into fibrous mats or cubes. A well-aerated rooting medium, rockwool maintains about 15-percent air space even when saturated. Rockwool remains a common medium for germinating seeds in a hydroponic environment. It supports plant roots, but contains no organic materials, so all of the nutrients the plants need must be provided by a nutrient solution.
Mix a starter solution of 1.9 g of hydroponic nutrient solution and 1.9 g nitrate limestone added to a gallon of water. You will use this solution for watering the seeds until they germinate.
Cover your growing surface, which should ideally be sloped slightly to promote good drainage, with plastic sheeting. Place rockwool cubes on the plastic sheeting. Put one to three seeds in the hole of each cube.
Sprinkle the cubes containing seeds with just enough granular rockwool to cover the seeds and prevent them from getting washed out.
Wet the rockwool cubes immediately with the starter solution prepared in Step 1. Continue to water the seeds with the starter solution one to four times a day, or as needed to keep the rockwool moist.
Maintain the temperature above 68 degrees F in the germinating environment.
Start applying growing solution as the seeds begin to emerge. For the growing solution, mix 9.85 g of hydroponic solution and 3.42 g of nitrate limestone per gallon of water.
Thin seedlings, if applicable, to one per cube once the first true leaves emerge on the plants. Keep the most vigorous-looking plant in each cube and remove the others for composting.
Place rockwool cubes with seedlings in their final hydroponic garden location two to three weeks after germination. Space cubes according to how much room the mature plants will need, which depends on the type of plants you are growing.