How to Use Rockwool to Germinate Seeds

Written by meg butler
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How to Use Rockwool to Germinate Seeds
Germinate your seeds in rockwool to give them a clean start. (Jupiterimages/Brand X Pictures/Getty Images)

Rockwool is a manmade mineral fibre. Originally used strictly for insulation, rockwool's water- and oxygen-retentive properties are now also used for soilless gardening. There are many benefits to starting seed without soil. One is to avoid damping off disease, a soil-borne fungus that often preys on emerging seedlings. Horticultural grade rockwool cubes are the perfect size for germinating seeds. They are simple to use and available at most home and garden centres and nurseries.

Skill level:

Things you need

  • Utility knife
  • Toothpick
  • Planting tray or irrigation system

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  1. 1

    Separate connected rockwool cubes into individual cubes or sections -- groups of cubes -- for each variety of seed. Not all rockwool cubes are connected. If your rockwool cubes are not connected, skip this step.

  2. 2

    Place the bottom of the rockwool cube in a tub of clean water. Allow the rockwool cube to soak up water for 30 minutes or as long as specified by the manufacturer. Squeeze the rockwool cube gently until it stops dripping.

  3. 3

    Push the seeds into the centre of the rockwool so that they are completely submerged. The rockwool may have a preformed slot in its centre. If it does not, cut one a little deeper than the length of your seed before inserting the seed. Cut the sharp end off of a toothpick and use it to push the seeds down.

  4. 4

    Place the planted rockwool cubes in a seedling tray or hydroponic irrigation system.

  5. 5

    Keep the rockwool moist with regular watering until the seed germinates.

  6. 6

    Separate sections into individual cubes once their seedlings produce two true leaves and roots begin to grow out of the bottom of the cube.

  7. 7

    Pot the seedling up into soil or into a larger section of rockwool. Before planting in soil, remove the plastic surrounding the rockwool (if present) and pull some of the rockwool away from the roots. Work carefully. The plant's roots are very delicate at this point. If you feel that you may damage the roots, leave the rockwool in place when planting the seedling. Wait to remove it when you pot the plant up again.

Tips and warnings

  • Wear a dust mask when cutting and handling dry rockwool. It sheds fibres that may irritate your lungs.
  • Don't wait too long to pot up your seedlings. Once they develop leaves, they will need a more nutritious medium to grow in.

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