How to Use Rockwool in Hydroponics

How to Use Rockwool in Hydroponics: A Comprehensive 6-Step Guide

Whether growing hydroponics or not, rockwool is my favorite substrate for starting seedlings. They are extremely practical and easy to use, and they form the perfect place for seedlings to root and begin their journey to the vegetative stage.

In this guide I will show you how to use rockwool to get the most out of your seedlings, how to soak and prep rockwool, what it is and more.

How to Use Rockwool in Hydroponics

How To Use Rockwool Cubes for Hydroponics & Rockwool Grow Cubes Preparation

** Rockwool can irritate your skin so use protective gloves when handling it.

1. Choosing the Right Rockwool Cube Size

For cannabis cultivation, there are typically two sizes you’ll encounter:

  • Starter cubes — Are smaller and perfect for cloning and for placing your germinated seeds (follow this guide on how to germinate cannabis seeds). You can also germinate the seed directly in the rockwool cube.
  • Larger cubes or slabs – The larger cubes and slabs can be used as a growing substrate if you don’t want to transplant the plants to a hydroponic system or other growing medium.

2. Preparing the Rockwool

  • Soaking — Before inserting your germinated seeds or clones, you need to soak the rockwool in water with a pH level of 5.5 for 3-5 hours. This is because rockwool tends to have a higher pH (around 8), and soaking will help to neutralize it. A pro-tip is to add a little bit of cannabis nutrient solution and Amplify to the soak.
  • Draining — After soaking, allow excess water to drain out. It’s essential not to let the rockwool remain waterlogged, as this can suffocate the seeds or clones. DO NOT squeeze the rockwool cubes because it may damage their structure and decrease their effectiveness. If the cubes are too wet, shake them gently to remove excess water.

3. Planting

  • Seeds — Place one seed in the hole of a starter cube, pushing it down gently with the root downwards so it’s about a quarter-inch deep. Make sure not to touch the root because it is very sensitive. Cover the seed lightly with a piece of the rockwool fiber.
  • Clones — For clones, gently insert the cut stem into the hole of the starter cube. It should fit snugly, but don’t force it. Learn how to make cannabis clones here.

4. Watering and Nutrients

  • Early Stages — During germination or early clone growth, use plain pH-adjusted water. Overwatering is a common mistake so ensure that the rockwool remains moist but not waterlogged.
  • Post-Germination — Once your seedlings have sprouted and developed a couple of sets of true leaves, or your clones have begun to establish roots, gradually increase nutrient strength as the plants grow.

5. Transplanting

Once the roots of your cannabis plants start to emerge out from the rockwool cubes, it’s time to transplant them to larger rockwool cubes or directly into a hydroponic system. If using larger cubes:

  • Make a Hole — Create a hole in the center of the larger cube that’s roughly the size of the smaller starter cube.
  • Transplant — Place the whole starter cube, with the plant and its roots, into the hole of the larger cube. Make sure it fits snugly but don’t damage the protruding roots.

6. Monitoring and Maintenance

  • Check pH regularly — While rockwool is inert, the pH of the water/nutrient solution can fluctuate. Aim for a pH range of 5.5 to 6.5 for cannabis.
  • Avoid Overwatering — Rockwool holds water well so overwatering can be an issue. Ensure adequate drainage and allow the rockwool to nearly dry out between waterings.
  • Keep an eye on nutrients — Regularly monitor your nutrient solution’s EC (electrical conductivity) levels. This will give you an insight into the nutrient uptake of your plants.

Rockwool Cubes Water Retention & Air Flow

A unique feature of rockwool is its ability to hold large amounts of water while still providing excellent aeration. It retains water for the roots to use, but its fibrous structure also ensures that excess water drains away, preventing waterlogged conditions that could lead to root diseases.

Should Rockwool Cubes Be Soaked in Nutrient Solution?

Soaking your rockwool cubes in nutrient solution can give your plants a head start. The rule of thumb when considering how long to soak rockwool is to do so for at least a couple of hours. The nutrient solution you use should be pH balanced to ensure optimal nutrient uptake.

Adjusting pH Levels

Speaking of pH, rockwool naturally has a high pH, which can be detrimental to plants. It’s essential to adjust the pH of your soaking solution to about 5.5-6.0, creating an environment conducive to nutrient uptake.

Keep a pH up and a pH down product nearby so you can adjust the pH as needed.

Placing Rockwool Cubes in Hydroponic System

Place the cubes in your system so the base touches the nutrient solution. The rockwool will wick the solution up to the plant roots. Over time, as your plants drink up, ensure the nutrient solution is replenished.


What are the Uses of Rockwool Cubes?

In hydroponics, rockwool cubes serve as an alternative to soil, providing a stable and controllable environment for plant roots. They’re versatile enough for starting seeds, propagating cuttings, and growing plants to maturity. Whether you’re a newbie or an experienced cultivator, rockwool cubes hydroponics offers a plethora of benefits that’ll make your green thumb greener!

What Rockwool Cube Size Should I get?

Rockwool cubes come in a range of sizes, each tailored to specific stages of plant development. Smaller cubes, generally 1-1.5 inches, are perfect for germinating seeds or rooting cuttings. Larger blocks, often used in conjunction with smaller cubes, support larger, mature plants. Thus, you can choose the appropriate cube size based on your plant’s growth stage.

Is Rockwool Organic?

The question of whether rockwool is organic can be as complicated as trying to complete a Rubik’s cube blindfolded. While rockwool is made from natural materials, it undergoes a synthetic process to create the fibers. Consequently, most certifying bodies don’t consider it organic.

How Long Do Rockwool Cubes Last?

In the battle against time, rockwool cubes stand as valiant warriors. Their lifespan depends on the plant species and the environmental conditions but, with proper care, they can support plant growth for an entire season.

Can Rockwool Cubes Be Reused?

Yes, but it’s not typically recommended because of potential disease and pest issues. If you do choose to reuse them, it’s vital to clean and sterilize them thoroughly first.

Alternatives to Rockwool?

Though rockwool is highly popular in hydroponics, alternatives like coconut coir, perlite, vermiculite, and clay pebbles can also be used, depending on the specific needs of your hydroponic system.

Where To Buy Rockwool Cubes?

The good news is, rockwool cubes are widely available. You can purchase them online, at a local hydroponics store, or garden centers.

Can Rockwool Cubes Be Planted in Soil?

Indeed, they can! Transplanting rockwool cubes into soil can be a smooth transition, making rockwool cubes good for cloning plants that will eventually be moved to soil.

Do Seeds in Rockwool Need Light?

Seeds in rockwool do not need light to germinate. However, once they sprout, they need sufficient light for photosynthesis and growth.

When To Transplant Rockwool Seedlings Into Hydroponic System?

It’s time to transplant your rockwool seedlings into your hydroponic system when the roots begin to peek out from the rockwool starter cube. At this point, they’re ready to take on the world (or at least the nutrient-rich solution in your hydroponic system!).

What is Rockwool Made of?

Rockwool is primarily composed of basalt rock and chalk that’s heated to extreme temperatures and spun into fibrous cubes. It provides excellent moisture retention and airflow – vital factors for healthy plant growth.

Cannabis Grow Series

This article is part of the cannabis grow series where we want to teach you how to grow cannabis step-by-step in a simple and effective way. Explore the full series here!

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