Rockwool has a naturally high pH of 8, which can hinder seed germination. To grow the seeds in the Rockwool, the cubes need to have their pH and EC concentration adjusted to create a more suitable environment for the seeds. 

NOTE: When handling Rockwool, keep the cubes away from the mouth or eyes as it can irritate the lungs and face. Gloves are suggested if the handler has sensitive skin. 

How to Prepare the Rockwool

1) Prepare a container with a gallon of lukewarm water around 80 F. 

• It is best to use distilled water due to its purity, but tap water works just as well.

2) Measure and add pH Down until the water is around a pH of 5.5.  

3) Submerge the Rockwool in the acidic solution for around 15 minutes. 

• Keep an eye on the pH to make sure it stays around 5.5.

4) Remove the Rockwool from the solution and correctly dispose of the pH solution. 

• When removing, DO NOT squeeze the cubes. Rockwool has a porous structure, allowing it to hold water and air for seeds. Squeezing the cubes will ruin the structural integrity of the Rockwool, removing the water retention ability. 

5) Refill the container with a gallon of lukewarm water around 80 F. 

6) Add Nitrogen, Phosphorus, and Potassium (or N-P-K) to the water according to the plant's needed ratio until the EC concentration for the plant is reached. 

• For lettuce, the N-P-K ratio is 4-1-6 for an EC of 0.8 mS/cm. 

7) Submerge the Rockwool again and let it sit for 15 minutes. 

8) Remove the Rockwool from the solution and correctly dispose of the nutrient solution. 

• Do not squeeze the cubes. 

9) Place the Rockwool in the grow tray.

• The Rockwool is now prepared for the seeds. 

NOTE: Try to keep your environment sterilized when preparing your Rockwool as bacteria can grow in the Rockwool. 

NOTE: It may be a good idea to place your Rockwool in the area where you will be growing the plants in your hydroponic system, ensuring the cube fits. If it does not, you can trim the Rockwool to fit the area. 

Setting Up the Seeds and Humidity Dome 

1) Place two seeds into each hole in the cube.

• Pinch close the holes so to keep the moisture on the top of the seeds.

2) Place tap water in the tray until the water reaches the bottom of the Rockwool.

• NOTE: It's difficult to overwater/underwater due to the Rockwool's ability to hold water.

3) Cover the growth tray with the humidity dome to lock in the moisture. 

• Leave the vents closed until the seeds sprout. 

• Keep the environment around 70 to 80 F. 

• Check the cubes daily for water content. If the tops of the cubes are dry, add more water sparingly. 

NOTE: The seeds do not need light until the seeds sprout.

When the Seeds Sprout

1) Remove the dome or open the vents and place the seeds under grow lights.

2) If more than one seed grew inside the Rockwool, cut away the shorter/weaker sprout so that only one plant is in each cube.

• Do not pull the plants out, as it can cause root damage.

Tips For Maintaining Seedlings

• Keep a consistent watering schedule and provide nutrients when needed. 

• A good sign of the seedlings needing water is when the top inch of Rockwool is dry. 

• When the first leaves grow in, increase the nutrient mix to match the EC amount the crop needs. 

  • For lettuce, the EC value is around 1.0 to 1.6 mS/cm. 

• Check the pH regularly for the specific plant needs. 

  • Lettuce needs around 5.5 to 6.5 pH.  

NOTE: The pH and EC of the seedlings do need to be monitored outside the main hydroponics system but not to the same extent. Using a pH strip and secondary EC sensor or using the same pH and EC sensors from the main system for a small check-in will be fine. 

• Keep the temperature within the 70 to 80 F range.

NOTE: The temperature range and humidity of your growth tent should be enough for the seedlings, as the atmospheric conditions are already set for plant growth. If you do need to raise the relative humidity or temperature, keeping the humidity dome on the seedling should suffice.

• Watch for bacteria or mold growth in the cubes.

• When the seedlings are 2 to 3 inches tall or the roots are through the bottom of the Rockwool, it is time to transplant.

  • The timing of transplanting is different for every plant, with lettuce being around 1 to 2 weeks.

  • Transplanting involves placing the Rockwool cubes into the designated growth area of the hydroponics system.

    • For Nutrient Film Technique systems, this requires placing the Rockwool in the holes of the water pipe.

    • If the Rockwool is too big to fit in the hole, the sides of the cube can be trimmed. 

      • Be sure to not hit any roots already present. 

Harvesting Crops

• There are two main ways to harvest lettuce from a hydroponics system:

1) Whole Plant Harvest

• Completely remove the lettuce + Rockwool out of the system, placing them in a bowl. 

• Or cut at the base of the lettuce where it meets the Rockwool, leaving behind the roots.

• This harvest method is good for one-time use of crops or if you are going to leave the system for long periods.

2) Cut and Come Again

• • • • Trim the outer leaves of the plant and leave behind the inner leaves.

      • The outer leaves become your harvest while the inner leaves allow the lettuce to continue growing.

      • You can do this method a few times, lengthening the harvest to around a month before the crop becomes stretchy and bitter.

        • Then, complete removal is necessary.

      • This harvest method is good for constant use of the hydroponic system.

On Rockwool Disposal

• Rockwool does not break down over time, so when finished: 

  • Reuse the cube if not damaged (Can be reused around 5 to 6 times in hydroponic systems).

    • Before reuse, heat treat them by pouring boiling water on them, killing off any bacteria or fungus.

    • Prior plant roots can be removed from the Rockwool when dried out, but if pulling the roots out starts to damage the structure of the cube, leave the roots in the Rockwool or dispose of it. 

  • Break up the pieces and work them into a garden or potting soil.