As you ponder the different grow media to start your crop with, one option you’ll come across is rockwool for hydroponics. And as you’ll discover in our complete guide to using rockwool in hydroponics, there is a reason commercial growers and hobbyists alike prefer using it.

Rockwool offers a host of advantages over other media choices – including incredible water retention & aeration, versatile use across plant grow cycles, and effortless pH regulation.

But what is rockwool for hydroponics, exactly? And how do you use it? Are there any drawbacks this type of media comes with? As with any media, there is a lot more than meets the eye. Fortunately, you’ll be left with no questions by the time we’ve finished our discussion today. We’ll teach you everything you need to know about using rockwool in hydroponics.

Let’s get right into the topic at hand!

What is Rockwool for Hydroponics?

First things first – what is rockwool for hydroponics? More specifically, what is rockwool made from? Here’s a brief overview of this amazing inert growing media.

Rockwool is a man-made rock made from basalt rock and chalk that is heated until it melts. The molten rock is then spun into fine fibers and shaped into blocks, cubes, or even granules (more on the types of rockwool later on).

This artificial rock was originally invented for use as insulation in the construction industry. However, horticulture experts quickly realized rockwool’s potential as a soilless grow medium – especially in hydroponic applications.

Nowadays, rockwool is one of the most popular types of hydroponic grow media choices among both commercial and hobbyist growers alike. It’s commonly used in systems such as ebb & flow hydroponics, deep water culture hydroponics, aeroponics, and other amazing beginner hydroponic systems.

Is Rockwool for Hydroponics a Good Medium?

Now that you have a general understanding of what rockwool is and where it comes from, is it really the right choice for your crop? Below, we’ll evaluate all the advantages and disadvantages of rockwool. Let’s start with all the reasons commercial cultivars and home growers alike choose rockwool for hydroponics over the alternatives.

Advantages of Rockwool

As we briefly mentioned before, rockwool boasts a plethora of advantages that have earned it favor among all sorts of growers. Here are just a few reasons you should consider using rockwool as your grow media:

  • Water Retention & Aeration: One of rockwool’s defining characteristics is its impressive ability to retain water while still allowing for adequate aeration. This quality is essential for preventing both overwatering and underwatering – two of the most common mistakes growers make. As a result, this is one of the most forgiving media you can choose – helping you avoid the dreaded hydroponic root rot. And, it leads to impressive plant growth rates.
  • Effortless pH Regulation: Rockwool is naturally neutral, which makes it incredibly simple to maintain the ideal pH level for your plants. You won’t have to worry about constant testing and adjustments like you would with other types of media. This means less work for you – the grower – along with fewer problems popping up over the course of your plant’s life cycle.
  • Can be Used Across Plant Grow Cycles: Rockwool can be used for everything from seedlings and clones to fully-grown plants. This versatility means you won’t have to invest in multiple types of media – rockwool will do the job no matter what stage your crop is in. But what really makes rockwool unique is how effortless transplanting into new media/containers can be. You can start seeds in rockwool blocks. As they sprout and harden off, you can place this block directly into a net pot or container with ease!

Disadvantages of Rockwool

No type of grow media is perfect – rockwool included. Here are a few potential drawbacks that are worth mentioning:

  • High pH Levels: Although rockwool is classified as neutral, it actually has a higher-than-average pH level. This can spell trouble for your crop if you’re not careful. The good news is, rockwool’s pH level can be easily lowered with common acids such as vinegar or lemon juice.
  • Not biodegradable: This media is man-made – and does not decompose naturally. Disposing of it safely and ethically requires a bit more work than traditional soil, coco, or clay pebbles.
  • Can be Harmful if Inhaled: Rockwool fibers are very fine – so fine that they can be inhaled if you’re not careful. Fortunately, this problem can be remedied by wearing a dust mask when handling dry rockwool.

The Different Rockwool Medium Types Explained 

As we mentioned earlier, this grow media comes in a few different forms. These include cubes, slabs, or granules. Below, we’ll briefly touch on each rockwool type so you can make an informed decision about which one is right for your needs.

  • Cubes: Rockwool cubes are small, square blocks of rockwool that come in pre-cut sizes. They’re by far the most popular form of rockwool and can be used for everything from starting seeds to growing fully-grown plants. The fact that they’re already cut to size makes them incredibly easy to work with – especially for beginner growers.
  • Slabs: Rockwool slabs are large, flat sheets of rockwool that are often used in commercial applications. They’re not as common as cubes but can be useful for certain types of crops (like lettuce or herbs) that benefit from a large, shallow root system.
  • Granules: Rockwool granules are small pieces of rockwool that resemble gravel. They’re not as popular as rockwool cubes or slabs but can be used in a variety of ways – including homemade potting mix, hydroponic systems, and even insulation.
rockwool hydroponics

How to Use Rockwool in Hydroponics: Step-by-Step Guide

Now that we’ve gone over all the advantages and disadvantages of rockwool, it’s time to get down to business. Here’s a step-by-step guide on how to use rockwool in hydroponics, so you can get started on your very own rockwool grow operation!

Start With a Soak & Plant Your Seeds (or Transplant Cuttings)

Start by soaking your rockwool cubes or blocks in pH-balanced water for 24 hours. Doing this will help them fully expand and prepare for planting.

Once your rockwool is soaked, it’s time to plant your seeds. For best results, we recommend using a small rockwool cube or block. Simply make a small hole in the top of the rockwool, drop in your seed, and lightly cover with more rockwool. If you’re propagating your crop through hydroponic cloning, you can skip all this and just place your cuttings in the soaked rockwool blocks. Then, proceed to the next step.

Add Heat & Light

Place your seeded rockwool cubes on a heat mat set to 75-80 degrees Fahrenheit. Doing this will help ensure that your seeds germinate quickly and evenly. If you’d like to learn more about how to start hydroponic seeds, we have a complete guide on the subject!

Once your seeds have germinated (you should see little sprouts poking through the top of the rockwool), it’s time to move them under hydroponic grow lights. Place your baby plants on a shelf or table about 18-24 inches below the light source. Slowly move them up a few inches each day until they’re about 6-12 inches below the light.

Water your rockwool cubes or blocks daily, using pH-balanced water. As your plants grow, you may need to water them more or less frequently – just be sure to keep an eye on the rockwool and never let it dry out completely.

When to Feed Seedlings in Rockwool

Water is just one of the many components your rockwool-grown crop needs to thrive. You also need to consider when to feed seedlings in rockwool. Again – this will depend on the specific crop being grown and the age of your seedlings.

At first, you should fertilize your rockwool plants every 2 weeks with a high-quality hydroponic fertilizer. Be sure to follow the manufacturer’s instructions carefully, as too much fertilizer can damage your plants. If you’re growing your crop in an automated hydroponic system, you’ll only need to worry about topping off the reservoir every so often as EC drops. If you’d like to learn about the best hydroponic plant nutrients, we have an in-depth article on the topic.

And at a certain point, you’ll need to transplant your rockwool-grown seedlings into a larger block. We’ll talk about that next.

When to Transplant Rockwool Seedlings

Transplanting rockwool seedlings is a bit different than transplanting other types of plants. That’s because rockwool is made of inorganic materials – meaning it doesn’t break down like soil does. As a result, you can’t simply pull rockwool-grown plants out of their blocks and transplant them into bigger ones.

Instead, you’ll need to carefully cut the rockwool block in half with a sharp knife. Then, you can transplant each half into a new, larger block. Just be sure to do this quickly and carefully, as rockwool can dry out quickly.

FAQs About Using Rockwool in Hydroponics

That concludes our guide on how to use rockwool in hydroponics. Now, you know just about all there is using rockwool in hydroponics. But at this point, we want to address a few of the most frequently asked questions we get asked on rockwool for hydroponics.

What Can I Grow Using Rockwool Cubes?

You can grow just about any type of crop using rockwool cubes. That’s because rockwool is such a versatile medium – it can be used for everything from starting seeds to propagating clones to growing full-fledged plants. It even works great with sea of green hydroponics.

One of the most popular crops to grow using rockwool cubes is hydroponic tomatoes. Tomatoes love rockwool because it provides them with an ideal environment for root growth. And since rockwool is so easy to use, it’s a great option for beginner hydroponic gardeners who are just getting started with tomatoes.

Other popular crops that can be grown using rockwool include lettuce, peppers, cucumbers, and herbs. Just about any type of vegetable or herb can be grown using rockwool cubes!

Does rockwool raise pH?

No, rockwool itself does not raise pH. However, rockwool can become alkaline over time as it absorbs nutrients from the water. For this reason, it’s important to test the pH of your rockwool-grown plants regularly and adjust accordingly.

If you notice that the pH of your rockwool-grown plants is getting too high, you can lower it by adding some acidic fertilizer or by using reverse osmosis water. Conversely, if the pH is too low, you can raise it by using alkaline water or adding some dolomite lime to the rockwool.

Will Roots Grow Through Rockwool?

No, roots will not grow through rockwool. Rockwool is made of inorganic materials that roots cannot penetrate. However, rockwool does provide an ideal environment for root growth, as it holds moisture and nutrients well and is full of tiny pores that allow roots to breathe.

For this reason, rockwool is an excellent medium for growing crops in hydroponics. It’s often used to start seeds or propagate clones, as well as to grow full-fledged plants. Just be sure to transplant your rockwool-grown plants into a larger block when they get too big for their current one – otherwise, they’ll quickly outgrow their rockwool home!

Does Rockwool Contain Asbestos?

No, rockwool does not contain asbestos. Asbestos is a type of fiber that was once used in insulation materials. However, it has since been banned in many countries due to its health risks.

Rockwool is made of basalt rock and slag wool – two materials that do not contain asbestos. So you can rest assured that rockwool is safe to use in your hydroponic garden.

Is Rockwool Itchy Like Fiberglass?

No, rockwool is not itchy like fiberglass. Rockwool is made of natural materials that are not known to cause irritation. However, we still recommend wearing gloves and a dust mask when handling rockwool, as it can be dusty.

Do Seedlings Need Nutrients in Rockwool?

Seedlings do not need nutrients in rockwool until they have at least a few sets of “true leaves”. If you add nutrients to the water too early, they can actually damage the delicate roots of your seedlings. So it’s best to err on the side of caution and only add nutrients once your seedlings have been transplanted into their own rockwool blocks.

Can You Reuse Rockwool Cubes?

Yes, you can reuse rockwool cubes – but only if they’re in good condition. If your rockwool cubes are starting to break down or become waterlogged, they won’t be able to provide the same level of support to your plants. In this case, it’s best to start with fresh rockwool.

To reuse rockwool cubes, simply soak them in a solution of 10% bleach and 90% water for 30 minutes. This will kill any harmful bacteria that might be lurking in the rockwool. Then, rinse the rockwool cubes thoroughly and allow them to dry completely before using them again.

What Can I Use Instead of Rockwool for Hydroponics?

There are a few different types of media you can use instead of rockwool – each of these has its time and place. Our two favorites are hydroponic coco coir and hydroponic clay pebbles. We encourage you to take a look at these complete guides to learn more about each. Beyond these two options, you can also consider using perlite, vermiculite, or sand in your hydroponic garden.

Final Thoughts on How to Use Rockwool in Hydroponics

There you have it – our complete guide to using rockwool in hydroponics. We hope this article helps you gain confidence in starting your neck crop using rockwool in hydroponics. After all, this is a great choice for any crop, any experience level, and any system type. 

With an incredible balance of water retention and aeration, versatile use, and forgiving growing – what more could you ask for in a medium?