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How to Start Seeds in Rockwool – #1 Guide to Success

I have found that there are quite a few different methods when it comes to growing plants.

However, one method that is quite popular is using Rockwool in order to grow plants.

This method is popular due to the fact that it is quite effective and I can use this method to grow a wide variety of plants.

But how to start seeds in Rockwool?

Naturally, there are alternatives to this method, but there is a reason that the majority of growers tend to prefer using Rockwool.

Here is what I think you should know about how to start seeds in Rockwool so you can grow plants effectively.

How to Start Seeds in Rockwool

To start seeds in Rockwool first soak them in an acidic solution with a pH between 5.0-6.5) as Rockwool has a high pH between 7-8. To start seeds the ideal pH is between 5.5-6.0. Put the Rockwool cubes in a nursery tray and put a humidity cover over them. You should make sure that the temperature is maintained at between 70-80°F (21-27°C). Water the Rockwool cubes occasionally every few days. 

How to Start Seeds in Rockwool

How to Start Seeds in Rockwool

What Exactly Is Rockwool?

Rockwool is an extremely common growing medium for hydroponics growers.

Commercial hydroponics growers do tend to use Rockwool quite a bit, so it is not just used by everyday growers.

Rockwool was not utilized as a growing medium right away.

In fact, it was first used as insulation, not as a growing medium.

Two other terms for Rockwool are stone wool and mineral wool.

The origins of using Rockwool for gardening can be traced back to Denmark and Rockwool was first used there for gardening during the 1970s.

Rockwool is liked by growers because it retains both water and moisture well.

Rockwool also does not get in the way of root growth.

It is worth mentioning that I do have to prepare Rockwool before I use it for gardening.

Rockwool has a rather high pH between 7-8. The ideal pH for hydroponics is 5.5-6.0. Rockwool therefore should be soaked in an acidic solution (pH between 5.0-6.5) before the Rockwool cubes are used.

Rockwool retains moisture well

Rockwool retains moisture well

How Do I Prepare My Rockwool Cubes?

Soak the Rockwool cubes in acidic water. This reduces the PH of the Rockwool cubes.

The acidic water decreases the PH because it dissolves away the accumulated line on the Rockwool cubes.

Use distilled water for soaking the Rockwool cubes.

Once they have soaked and their PH is decreased to 5.5, they should be ready for me to use in starting seeds.

The next step is to submerge the Rockwool cubes in water. I should let them soak for a maximum of 24 hours.

Take them out of the water once they are done soaking.

Add them to the hydroponics system carefully and let the system operate without any plants until the system’s PH remains between 5.5 and 6.

If that is the case, it means I can use the cubes since they are now stable enough for seed germination.

I do have to test the PH to check the system’s PH during this process, of course.

One difference between Rockwool and other materials used in hydroponic growing is that Rockwool is not a PH-neutral material.

In fact, Rockwool naturally has a high PH.

Typically, the natural PH of Rockwool is roughly 8. This is a result of how Rockwool is manufactured.

Lime gets deposited on Rockwool fibers during Rockwool manufacturing.

Since lime naturally reduces acidity, it also increases the PH of Rockwool.

The pH of Rockwool is between 7-8. This is too high for seedlings. The cubes should therfore be soaked in an acidic solution

The pH of Rockwool is between 7-8. This is too high for seedlings. The cubes should therfore be soaked in an acidic solution

How to Start Seeds in Rockwool?

Preparing Rockwool cubes usually takes several days.

One thing that I can do to prepare Rockwool for gardening is to soak it in acidic water.

Doing so will decrease the PH of the Rockwool cubes.

The acidic water does so because it dissolves lime that forms on the Rockwool’s fibers during one part of the Rockwool manufacturing process.

Distilled water is the best choice for this method of preparing Rockwool for gardening. However, if I only have tap water available, then I can use that, as well.

I have to gradually adjust the PH of the Rockwool until the PH gets to 5.5. That is the PH I want for my Rockwool, in this case.

I have to ensure that the PH of the Rockwool does not decrease below 5.

If it does, the acidic water will begin damaging the fibers of the Rockwool because the Rockwool is too acidic.

After seeds were started in Rockwool they can be transferred to net pots and nutrients need to be added into the water according to the University of Illinois.

Rockwool is available as stonewool and mineral wool

Rockwool is available as stone wool and mineral wool

Rockwool Cubes Are A Good Choice for Starting Seeds

I can use a variety of different forms of Rockwool in order to start seeds. However, one type of Rockwool that works well for starting seeds (also known as germinating seeds) is Rockwool cubes.

One reason for this is that Rockwool cubes hold quite a bit of water considering their size.

As a result, this means that they give me some level of protection against power outages that cause timers or pumps to shut down.

Another benefit of Rockwool cubes holding so much water is that it is difficult to overwater my plants. Growers typically utilize Rockwool cubes for two main purposes.

Starting seeds is one of these purposes. However, a grower may also use Rockwool cubes to propagate new cuttings, as well.

For this reason, I would say that you might hear Rockwool cubes proposed as an idea for how to start seeds in Rockwool.

The ideal pH for seedlings is 5.5-6.0

The ideal pH for seedlings is 5.5-6.0

How Do I Start Seeds in Rockwool Cubes?

It is not always easy to properly germinate seeds. In fact, even for more experienced growers, the process can be quite stressful at times.

One reason for this is that I have to balance the water level carefully. I have to make sure that the seeds are wet enough that it promotes germination.

On the other hand, I have to ensure that the seeds don’t get so wet that they die due to being waterlogged. Rockwool cubes are a popular choice for germinating seeds because they retain moisture very well.

Rockwool cubes work well in making sure that seedlings or seeds are wet enough that they don’t dry out.

At the same time, Rockwool cubes help ensure that seeds or seedlings are not in a waterlogged environment that could kill them.

I should mention that moisture is the most important factor when it comes to seed germination.

Water goes into the seed via a micropyle, which is a very small opening, or it goes into the seed via the seed coat.

Water’s presence activates the seed’s enzymatic reactions that lead to seed germination.

I recommend checking out this paper for a detailed device into how enzymes are critical to seed germination.

Frequently Asked Questions About How to Start Seeds in Rockwool

Is Rockwool expensive?

Rockwool is a bit more expensive than other types of growing media, generally speaking. However, its benefits outweigh the costs for many growers.

Are there any downsides to Rockwool cubes?

The primary drawback of Rockwool is that there is usually a difference in the moisture level at the top of the Rockwool cube compared to the moisture level at the bottom of the cube. Because of this, it is possible to overwater seeds.


Conclusion About How to Start Seeds in Rockwool

Rockwool cubes are an excellent choice for starting seeds due to their excellent moisture retention.

It is not quite as affordable as some other growing media, though.

How to Start Seeds in Rockwool?

I have to prepare Rockwool cubes before I use them for gardening.

This is due to their high PH level.

More experienced growers will probably find Rockwool cubes fairly easy to use.

Author Bio

Daniel Iseli

Taking care of houseplants and gardening are my greatest passions. I am transforming my apartment into an urban jungle and am growing veggies in my indoor and outdoor garden year-round.