How to Plant Seeds in Rockwool

A mixture of rock and sand, Rockwool forms in a process that resembles cotton-candy making. The process spins the rock and sand into fibres, pressing the fibres into cube shapes afterward. Rockwool is absorbent and airy. Cleaner than ordinary potting soil, Rockwool has its conveniences as a hydroponics (soilless gardening) medium. If you are using Rockwool for the first time, you will need to decrease the pH of the cubes before seed planting.

Fill two large bowls with water that has a pH between 5.5 and 6.5. Measure the pH of the water using a pH testing kit; this kit can be purchased at a garden centre. Most tap water has a pH of around 7.0 (neutral).

Decrease the pH of the water by adding lemon juice, one capful at a time, until the pH is between 5.5 and 6.5. Lemon juice is natural and will not harm the seeds. Soaking the Rockwool cubes in water will significantly reduce their pH, making it close to or equal to that of the water.

Slip on a dust mask, goggles and gardening gloves before handling the Rockwool product. Breathing in fibres from the Rockwool can cause respiratory problems. Avoid contact with your skin and eyes.

Place the Rockwool cubes into one of the bowls of water. Allow the cubes to soak for at least 30 minutes. Remove the Rockwool cubes from the first bowl after the 30-minute soaking period.

Add a scoop of plant food to the second bowl of water. Place the Rockwool cubes in the second bowl for 30 minutes. Soaking for a second time will work as a flush and transfer additional nutrients from the plant food to the cubes.

Set the cubes in rows inside of a glass baking tray. Drop one seed into each of the premade holes, found in the centre of each Rockwood cube. If your cubes have no holes, use the tip of a pencil eraser to poke a half-inch hole in the centre of each cube.

Place the tray on a plant warming pad, set between 21.1 and 26.7 degrees Celsius--the ideal temperature for the germination of most seeds. Place the tray in a location that receives bright, indirect sunlight during and after the germination process. Keep the Rockwool cubes moist at all times by adding water to the tray when the cubes begin to lose some of their moisture.


Rockwood grown seedlings will require transplanting when their roots begin to outgrow the cubes. When this happens, simply prepare larger cubes in the same manner; cut a hole in the centre and place the seedling plus the original cube inside the larger one.


Handle the Rockwool cubes carefully. Do not squeeze the cubes, which can rob them of their perfectly proportioned moisture-to-air ratios. Do not water the Rockwool cubes directly. Always pour the water into the tray.

Things You'll Need

  • Large bowls
  • Water
  • Water pH testing kit
  • Lemon juice
  • Dust mask
  • Goggles
  • Gardening gloves
  • Rockwool cubes
  • Plant food
  • Glass baking tray
  • Plant warming pad
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About the Author

Jonae Fredericks started writing in 2007. She also has a background as a licensed cosmetologist and certified skin-care specialist. Jonae Fredericks is a certified paraeducator, presently working in the public education system.