How To Use Rockwool Cubes

Last Updated on June 3, 2023 by Kimberly Crawford

Rockwool cubes have become increasingly popular among gardening enthusiasts and hydroponic growers due to their versatility and effectiveness. 

If you’ve ever encountered difficulties with maintaining proper moisture levels, ensuring adequate aeration, or struggling with nutrient deficiencies in your plants, Rockwool cubes can be a game-changer in helping you achieve healthy plant growth.

By harnessing the power of Rockwool cubes, you can take control of your plant’s growth and unlock its full potential. In this blog post, we will discover how to use Rockwool effectively for your gardening or hydroponics endeavors. 

So, let’s dive into the world of Rockwool cubes and embark on a journey of transformation and success in your gardening endeavors.

Understanding Rockwool Cubes


Before delving into the practical aspects, let’s take a moment to understand what Rockwool cubes are and their defining characteristics. 

A rockwool cube is a type of growing medium made from spun rock, also known as stonewool. Rockwool cubes are made from a synthetic mineral fiber called basalt and Chalk, which is heated to a high degree (3000℉) and spun into fine threads like cotton candy. 

These threads are then compressed to create dense, absorbent cubes that provide an ideal medium for plant growth. Rockwool cubes are known for their exceptional water retention and aeration properties, making them popular in both soil-based gardening and hydroponics systems.

Rockwool cubes come in various sizes and shapes to cater to different plant needs. Typically, Rockwool cubes are one to two inches in size and come wrapped, which is recommended to keep on during the entire growing process. For larger needs, you can also find Rockwool cubes measuring three to four inches. 

Smaller cubes are ideal for starting seeds or rooting cuttings, while larger cubes are suitable for transplanting established plants.

How to Use Rockwool Cubes

Before you begin using Rockwool cubes, it’s important to prepare them properly. Follow these step-by-step instructions to ensure the cubes are ready for optimal plant growth:

Step 1: Preparing Rockwool Cubes for Use

Let’s explore each process in detail.

1. Determining the Appropriate pH Level for the Water:

Appropriate pH Level for the Water

Begin by checking the pH level of the water you’ll be using to soak your Rockwool cubes.

To determine the current pH level, you can use a pH testing kit. Ideally, the pH should be between 5.5 and 6.0 for most plants and should never be dropped below 5.0. If necessary, you can adjust the pH using pH-up or pH-down solutions to ensure optimal growing conditions.

2. Soaking Rockwool Cubes in Water:

Soaking Rockwool Cubes in Water

Begin by soaking the Rockwool cubes in water for up to 24 hours. This step helps to eliminate any residual minerals or loose fibers that could affect the pH balance and potentially harm your plants. Fill a container with clean water and gently place the cubes inside, allowing them to fully submerge.

3. Addressing Safety and Handling Concerns:

Rockwool is generally safe to handle, still, you can wear gloves and a dust mask during the initial stages, such as soaking and cutting the cubes. This precaution helps minimize any potential irritation caused by fine fibers. Once the cubes are soaked and ready for use, the risks diminish significantly.

Step 2: Germinating Seeds 

Germinating Seeds
  1. To start the germination process, carefully insert two seeds into the hole on the top of each Rockwool cube. Here you can use a toothpick or a similar object and gently press the seeds down to the bottom of the hole. Once the seeds are in place, pinch the hole closed to secure them.
  2. Once you have prepared the Rockwool cubes, it’s time to place them in a nursery tray. The nursery tray will serve as a stable and organized space for the germination process. 
  3. After arranging the cubes in the tray, cover the entire tray with a humidity dome.
  4. The temperature inside the dome should be between 70° – 80°  Fahrenheit.
  5. It’s important to keep the Rockwool cubes moist during the germination process. So, water them every one or two days. Alternatively, you can use a spray bottle to mist the Rockwool cubes when they start to dry out.
  6. Once the seeds have sprouted, and you see the seedlings emerging from the Rockwool cubes, it’s time to remove the humidity dome and place the tray under grow lights. The light will provide the necessary energy for the seedlings to grow.
  7. If you planted two seeds per Rockwool cube, it’s recommended to cut the tops off the weaker or shorter seedling, leaving only the strongest and tallest one in each cube. Avoid pulling the weaker seedling out, as it may disturb the healthier one.
  8. Allow the plantlets to grow in the Rockwool cubes until they reach a height of 2 to 3 inches. At this stage, they are ready to be transplanted into their permanent growing containers or system.

If you would like visual guidance on how to prepare and use Rockwool, you may find the following video helpful.

Troubleshooting Common Issues

troubleshoot problems rockwool

Even with proper care, challenges can arise when using Rockwool cubes. Here are some potential problems and their solutions:


If your plants show signs of wilting or yellowing leaves, it may indicate overwatering. Allow the Rockwool cubes to dry out slightly before watering again. To maintain the optimal moisture level in your Rockwool cubes, it is important to adjust your watering schedule accordingly. 

Nutrient Deficiencies:

Symptoms such as yellowing or discolored leaves can indicate nutrient deficiencies. So, keep an eye on the nutrient levels in your hydroponics system and adjust the nutrient solution accordingly. If necessary, consult a plant nutrient chart to identify and address specific deficiencies.


You now have a comprehensive understanding of how to use Rockwool cubes in your gardening and hydroponics projects. Armed with the knowledge of Rockwool cube composition, preparation, and usage, you are now equipped to explore this versatile growing medium. 

Remember to experiment, observe, and adapt your approach based on the unique needs of your plants. With Rockwool cubes, you can create a thriving garden or hydroponics system and enjoy the rewards of healthy, vibrant plants. So, happy growing up.