What is Rockwool?

Rockwool is a product that is fairly popular among marijuana growers. It is made by melting rock and spinning it into a dense mass of intertwined fibers.  Sometimes a binding agent is added such as a food grade starch.  The process is similar to the spinning of cotton candy.  Rockwool isn’t inert though, so if you choose to grow with it, you will have to adjust your pH to compensate.

Why use Rockwool?

Rockwool is fantastic growing medium, it can hold ten times its weight in water, but it is impossible to over water the plants.  The spun structure of the Rockwool enables the water to be delivered on demand to the plant, yet drain so it won’t drown the plant.  Rockwool is typically used in hydroponic applications,  though it can be adapted to others.

How do you prepare Rockwool?

Materials needed to prepare Rockwool

  • Rockwool Slab
  • Water
  • Bowl
  • Plant Food (For vegetation stage) (=higher nitrogen content)
  • pH/EC tester

What are the steps to prepare Rockwool?

  1. Pour about a half gallon of water in a the bowl
  2. Test the pH balance of the water, it needs to be around 5.5, no less though. If its too high, use “pH Down” or lemon juice to adjust the pH balance, be sure to retest.
  3. Cut a 2″x 2″x 2″ piece from your slab and soak it for 30 minutes.
  4. While those soak, make another batch of water with a pH no greater than 5.5, this time mix a little bit of plant food in with it.
  5. After the cubes are done soaking, take them out of the water.
  6. Flush the cubes with the plant food mixture, do not re use this water once it has passed through the cube, throw it away.

Note:  Do not squeeze the rockwool, its spun structure can be damaged, rendering it less effective.

How do you plant in Rockwool?

Materials needed to plant in Rockwool

  • Knife
  • Prepped Rockwool
  • Prepped Seeds or Clones
  • Big Zip-Lock Bag

Steps to plant in Rockwool

  1. Cut a slit in the Rockwool for the seed/clone, about an inch deep.
  2. Put the seed/clone in. (don’t forget your cloning gel if it’s applicable)
  3. Place your starter cubes right side up in a zip lock bag.
  4. Close the bag, except for a 1/2 in.
  5. Adjust ventilation in bag as seedlings emerge.  Don’t let them go dry, mist them if you need to keep them from completely drying out.  If you mist them, use the same food mix.

Do NOT keep the starter cubes soaking wet. By 2-4 weeks the seedlings should be ready for transplant.  (There should be at least four leaves before you transplant.)

How do you transplant into Rockwool?

Materials needed to transplant into Rockwool

  • Rockwool- It comes in different sizes, you will have to choose which is best for your application.
  • Knife
  • Water
  • Plant Food  (For vegetation stage) (=higher nitrogen content)
  • pH/EC tester

Steps to transplant into Rockwool

  1. Prepare whatever medium you plan to use for your transplant, in this case it is Rockwool, again.  Follow the same steps for preparation as the starter cubes, soak, flush, etc.  It is very important that you use the same pH as before and the same food mixture as before, this is to reduce the shock that the seedlings endure during this process.
  2. After you have prepared your slab, you simply cut a snug little hole out of it for the cube and put it in.  If you are transferring from another medium like soil then just adapt, make a hole big enough to put the would be transplant in snugly with some of its current medium for support.  You can begin to remove the original medium after the transplant has begun to root, being careful not to damage the plant, this is optional, it won’t hurt the plant if a little of the original medium is still present.  Algae will sometimes grow on the rockwool, don’t worry, this will not affect the plants in any way.  Some sizes of Rockwool come covered in plastic, you can leave this on to reduce algae growth.
  3. After you complete the transplant, do not water for the first 2-4 days, this gives time for it to root.  Do not allow it to go dry though, mist it if you need to keep it from drying out completely.
  4. Once the 2-4 days has passed, you water the plants is in accordance with your method of growing.
  5. Once a week you should water through the top of your medium to flush out stagnant minerals and salt, aka leeching. (Use the same mix that you feed them regularly, so you don’t shock them.)


  • Always use pH adjusted water with plant food.
  • Never let the Rockwool go bone dry.
  • No light = No water-the plants are resting.
  • Make sure that the Rockwool can drain freely - avoiding situations where the Rockwool is sitting in water

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