How To Use Rockwool In Hydroponics: Complete Guide

When I first looked at customizing my hydroponics system, I came across this product called Rockwool I’d never heard of before.

It seemed like an easy way to make your own seed pods and more, so I researched and experimented and found something that changed my process altogether!

Rockwool with hydroponics is a great way to help seeds sprout and grow into full plants. Rockwool is a spun-fiber combination of basalt and limestone. Its open fibers allow for great water retention, effective hydration and aeration of roots, and provide a great place for roots to grow.

Use Rockwool as a homemade seed pod and growing medium for your plants.

Check out the rest of the article for many more details and our guide for how to use Rockwool.

What Is Rockwool?

Rockwool is a growing material made artificially. 

It’s similar to what many people know as insulation in their homes. 

Of course, now we use other types of insulation, including spray foam, but Rockwool is similar to what we used to use. 

People discovered the insulation made a great medium for use in hydroponics with a slight change in the make-up of the material. 

Rockwool is also known as Stonewool, but it’s all the same type of product. 

It’s a combination of basalt (old volcanic rock) heat-treated and combined with limestone.

In the heating process, it all melts together in molten rock. Then, it’s spun into looser fibers similar to the making of cotton candy with sugar. 

After the fibers form, they add a binder material to the mix and form it into a mat shape, often a mat with cubes. 

Rockwool is an ideal growing medium because it transfers water very efficiently. 

On top of this, the fibers hold moisture for a long time. 

In hydroponics, all the nutrition needed for plant growth comes from a nutrient-rich water solution. 

Rockwool’s double-effectiveness around water makes it desirable for this process. 

The loose fibers also allow for great root growth while playing the water-retention role that soil normally does (without the mess!). 

A lot of DIY hydroponic reservoir ideas include Rockwool for more cost-savings. 

Click the link above to check out our list of the best ideas and other tips for making your nutrient tank. 

What Can I Grow Using Rockwool?

Rockwool comes in all sorts and shapes and sizes, from small seed sprouting cubes to massive blocks. 

This means you can use your Rockwool to help your hydroponic growing with any vegetable, green, or herb you want. 

In commercial farming, many hydroponic growers will use Rockwool to sprout seedlings and then transfer them to a larger and long-term system. 

For your home farming, it’s common to make your sprouting cubes and then transfer them into bigger blocks or systems made of even more Rockwool. 

Flowers are another common plant people grow in Rockwool. 

It’s great for germinating seeds, cloning plants, or even propagating plants. 

The high water levels help the plants stay healthy all the time, even without soil. 

How To Use Rockwool In Hydroponics

For those who want to use Rockwool in your hydroponic system, this section offers a few steps and tips for taking your plants from seed form until fully grown. 

If, at any point, you wanted to transfer the plants to soil or another medium than Rockwool or stonewool, just do it. 

Rockwool can be buried straight into the soil or any other medium with no adverse effects. 

Items You’ll Need

• Rockwool
• pH Tester
• A straw or toothpick
• Garden scissors
• Vermiculite
• Seeds
• Box cutter or another knife

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Soak Your Rockwool

The very first you MUST do is soak your Rockwool in water. 

The creation of Rockwool leaves it with a high pH level, and plants prefer a lower pH to grow, especially at first. 

It’s not uncommon to find a pH level of 8 coming from these cubes. 

Soak the cubes in water until you reach a pH range of 5.5-6. 

Don’t let it go below 5, or it’s too acidic for plants. 

Your specific Rockwool product will give you some advice on how to long to soak it for. 

Anywhere from 30 minutes to 24 hours is the range. 

It depends on the exact product and the water you use. 

Testing is critical, but it’s easy to complete too. 

Also, remember when soaking to never squeeze them dry. This will collapse the fibers. 

Shake the Rockwool until it stops dripping. 

Check the next section for even more details. 

Insert Two Seeds And Cover Them Up

Use a straw or toothpick to widen the hole in each cube. 

You may not have to do this, but you want the seed to go well into the hole.

The straw will widen so you don’t have to work as hard. 

Put two seeds in each hole, and then use the straw or toothpick to push it down into the Rockwool. 

After, pinch the top of the hole closed. If the hole is too big, use another piece of Rockwool to cover it up. 

Put Your Seed Pods In Their Nursery Or Sprouting Spots

As we’ve just done, starter plugs with fresh seeds put them on a tray and place a dome to trap humidity over the top. 

The ideal temp for sprouting seeds quickly is 70-80° degrees Fahrenheit (27° C). 

Soak the cubes with the water solution so a little drips out, and then close your humidity dome. 

Check on the cubes once per day, but don’t be surprised if they’re still wet from before. 

Rockwool holds on to a lot of moisture. 

At this point, water them only a little or mist them with a spray bottle. Always use the nutrient solution. 

Once Sprouted, Move Them To Growing Spots

Depending on the plant you’re growing, you’ll start to see little sprouts within a few days or longer. 

At this point, remove them from the nursing tray and place them under grow lights. 

Some people put them in their full systems, but most just keep them in a second growing light spot real close to the lights. 

Trim Back The Smaller Sprout

As the sprouts grow, you’ll notice one becomes more dominant than the others. 

When it’s clear one of the seeds has become dominant, trim it back with garden scissors into the Rockwool to ensure the better one gets all the nutrients. 

Warning! Don’t pull the smaller sprout out of the Rockwool. This could pull the whole plant out and kill it.

Add Vermiculite (Optional)

Adding vermiculite at this point may be a good option. This makes the water transfer and oxygenates the plants even better. 

Of course, Rockwool mostly takes care of this on its own, but for those who want to go overkill on helping your plants, it’s a good option. 

Either Move To Your Main System Or Harvest

If you put the sprouts right into your main system, you don’t need to take this extra step. 

Just keep your watering up and going until it’s ready to harvest, and then enjoy! 

If you used an in-between growing place, move them into your main system once your plants are 2-3″ inches tall. 

Remember: don’t remove them from the Rockwool. They can stay in the Rockwool forever. 

Rockwool has those fibers that allow for awesome root growth. 

It’s similar to how the pods work when you buy seed pods pre-made by a company such as AeroGarden. 

Speaking of, check out our picks of the best AeroGardens for lettuce and herbs.

How To Prepare Rockwool For A Hydroponic System

We talked a bit about preparing your Rockwool, but there are some other considerations you’ll need to keep in mind about the process and this product in general. 

Pre-Soak The Rockwool To Lower The pH

As we mentioned before, soak the high pH stonewool with low pH water to get it to the ideal 5.5-6 level. 

Wear Gloves And Maybe A Mask

Because this product is similar in structure to insulation and essentially made of heated stone strings, it may be irritating for some to touch. 

When doing most of the touching and handling, you should first use gloves to prevent such irritation. 

Rockwool isn’t as hazardous for the lungs, and you won’t be breathing it in either. 

But if you’re worried or use a lot of it in one space, a mask isn’t a bad idea.  

Break Them Apart To Fit Your System But NEVER Unwrap

Don’t be afraid to break the pieces apart into different shapes or pieces to fit into your exact system. 

During this process, take care to cut them apart with your knife and not pull. 

Pulling may cause them to unwrap, and once that happens, they’re useless. 

Allow For Drainage

One common mistake people make with Rockwool is to leave it sitting in the water. 

Rockwool holds a lot of water naturally on its own, so it doesn’t need to soak while you’re growing the plants. 

This is harmful to your plant. 

Too much soaking starts to transfer salt from the Rockwool into the plant’s roots, which could kill them. 

Always allow for drainage, and you won’t have this problem. 

Don’t Squeeze The Rockwool

Have you ever tried to squeeze cotton candy?

If you haven’t, let me tell you what happens; after all, my kids are weird sometimes and like to “experiment,” so I know what happens firsthand. 

The cotton candy collapses into a dense piece of sugar. And it’s a sticky mess. 

Rockwool is spun fiber, like cotton candy, though not made of sugar. 

It collapses when squeezed too. 

While it’s not sticky, a dense block of Rockwool loses all its value: 

• No space for roots to grow
• Fails to hold water
• Fails to transfer water

Heat Treat Them For Repeated Use

There is some debate around whether or not it’s safe to reuse Rockwool for new plants once you harvest your original. 

Experts will all agree that it’s possible to reuse them, though many people go about it wrong. 

This leads to the myth they’re not reusable. 

Roots left in the cube will begin to rot like they do in soil.

But unlike in the wild, there aren’t any bacteria or fungi to help break them down into nutrients.

They just sit in the cube, rotting. 

This is what kills new plants. 

To reuse your Rockwool cubes, make sure you remove all signs of roots with either a pair of tweezers or a heat treating. 

The root may end up wrapped around and through the fibers, so it’s a difficult process. 

Some owners have had success with heat treating them to burn out the rot. 

Put the Rockwool in the oven at a low temperature for 30 minutes or so, and then check on it. 

Rockwool is safe for the oven. In fact, some ovens use Rockwool as their insulation. 

We usually bury the Rockwool when we’re done, as the reusing process is more work than we want to take on (and Rockwool is quite affordable). 

We talk about this later on in the article.

Concerns About Rockwool

Health Concerns On Rockwool

As we touched on before, Rockwool is similar to insulation. 

There is a small risk of skin irritation and an even smaller risk of dust inhalation when using large amounts. 

Simply wear gloves when handling them a lot and a mask if the area isn’t well-ventilated or the Rockwool is dry to avoid all these issues. 

Preventing Algae And Mold

All mediums for growing come at a higher than normal risk for algae and mold, and Rockwool is included in this. 

The combination of proximity to light and high levels of nutrients in water is the cause. 

Many owners will cover the surface of the cubes with a lid of some kind to prevent direct exposure to the grow lights. 

Just make sure you have a hole in the lid for the plants to flourish through. We want the leaves to get light for proper growth. 

Avoiding Root Rot

Rockwool isn’t at much higher risk for root rot than any other hydroponic system, but it’s still important to keep in mind. 

If a plant dies, its roots start to rot. 

This may also happen if the hydroponic reservoir gets some bacteria into it. (Always keep an air pump in it to keep the water moving). 

Whatever the reason, check for root rot every day because once you see it, you need to remove the plant right away. 

Root rot spreads to other plants quickly and may kill your whole harvest if you’re not careful. 

Signs of root rot include: 

• Stunted leaves
• Discolored leaves
• Shoots that grow and then shrink back quickly
• Soft roots instead of firm
• Brown roots instead of white

Break Them Up And Bury Them When Done

Rockwool doesn’t break down in the wild, so if you throw it away, it’ll stay in the landfill for a long time. 

This leads many to think it’s bad for the environment. 

Rockwool and stonewool are made of rocks. They’re just there. 

Still, we suggest you get the most out of your Rockwool by either breaking them into small pieces and burying them in your gardens or compost. 

Returning them to the soil allows for any remaining roots to be broken down into nutrients. 

Putting them in your gardens or compost helps a lot. 

After all, Rockwool helps with retaining moisture which is great for both of those places. 

Just make sure you break the cubes up into small pieces and don’t bury them all in one place.