How to Build a Rockwool Hydroponic Garden


Rockwool is a man-made mineral fiber made by melting basaltic rocks, then spinning the molten rock into fibers. It's very similar to the process used to make cotton candy. What does this have to do with growing a hydroponic garden? Rockwool makes a great soil substitute. It holds the plants and gives them the support they need, allows the nutrient solution to drain back into the grow tray so the plants aren't over-watered, and holds oxygen so the plants don't drown.

Set Up a Hydroponic System

Step 1

Place the bucket in an area where it can sit undisturbed for 24 hours. Fill the bucket two-thirds with water and let it stand uncovered for an hour. This will allow any chlorine present in the water to disburse.

Step 2

Use the pH meter to test the pH of the water. It should be around 7.0. Add one to three drops of phosphoric acid to the water. Mix well and re-test the pH. If it is higher than 5.0, add another 1 to 2 drops of phosphoric acid and mix well. Continue to test and adjust the pH until it is between 4.5 and 5.0.

Step 3

Submerge the rockwool cubes in the acidic water. Cover the bucket and let it sit undisturbed for 24 hours.

Step 4

Follow the manufacturer's instructions to set up your hydroponic garden. Make sure to test the system with plain water. Do not add nutrient solution to the nutrient reservoir until you are ready to transplant the seedlings into the rockwool cubes.

Plant Your Seedlings

Step 1

After the rockwool cubes have soaked for 24 hours, arrange them on the grow tray of your hydroponic garden.

Step 2

Place one seedling in the hole in each rockwool cube. Use your fingers to fluff some of the rockwool material around the seedling to hold it securely.

Step 3

Fill the nutrient reservoir with nutrient solution diluted with water. Check the manufacturer's instructions to determine the correct ratio of nutrient to water.

Step 4

Use a pH meter to test the pH of the diluted nutrient solution in the reservoir. It should be between 6.0 and 6.5 for most plants. If the nutrient solution is to acidic (the pH is too low), add one to three drops of potassium hydroxide. If the nutrient solution is too basic (the pH is too high), add one to three drops of phosphoric acid. Mix well and re-test. Continue to adjust until the pH is between 6.0 and 6.5.

Tips and Warnings

  • Make sure the bucket you use to soak the rockwool has a tight-fitting lid and is placed in a secure location, especially if you have young children or pets who could fall in and drown.

Things You'll Need

  • 5-gallon bucket with a secure lid
  • Water source
  • Hyroponic system
  • 3- to 4-inch rockwool growing cubes, available at garden supply stores
  • Meter for pH or liquid test kit
  • Phosphoric acid
  • Nutrient solution
  • Potassium hydroxide


  • University of Florida: Soilless Media Choice
  • University of Arizona: Hydroponics

Who Can Help

  • Rockwool as Growing Substrate
Keywords: rockwool cubes, rockwool hydroponics, rockwool

About this Author

Tricia Ballad has written professionally since 2004. She has authored three books, as well as numerous articles on parenting and website content involving green living. Her work has appeared in Natural Family Online and Budget Artists. She holds a Bachelor of Arts in English with a creative writing specialization from Bradley University.

Article provided by eHow Home & Garden | How to Build a Rockwool Hydroponic Garden