9 Differences Between Horticubes & Rockwool (+ When to Use)

Horticubes and rockwool are common growing mediums for hydroponic systems. I was initially unsure which of the two should be used in my system. So I bought a small pack of each to learn about their differences and decide which one is best for me. Here’s what I found.

Generally, horticubes and rockwool vary in terms of their material, appearance, weight, uses, pH, need for pre-soaking, handling and safety precautions, tendency to break down, and reusability. Horticubes are preferable until the transplanting stage, whereas rockwool can be used until the harvesting stage.

Which one is appropriate for your hydroponic system? This question will be answered after you finish reading this article, so let us proceed with the discussion!

1. Material

Horticubes are made from water-absorbent phenolic foam while rockwool cubes are composed of processed limestone and granite.

The materials used to manufacture horticubes and rockwool have a significant physical difference that we can observe at first glance.

Horticubes: What are They Made Of?

To give you a better idea of what horticubes look like, imagine a dishwashing sponge cut into cubes! Most horticubes are made of floral foam, a type of phenolic foam that can absorb water.

Floral foam is commonly used to propagate flower cuttings. They are preferred for this because they retain water well, extending the vase life of cut flowers like roses!

They do not have subtypes, unlike other growing mediums which have fine and coarse classifications.

Conversely, the story of rockwool is quite complicated…

Rockwool: How are They Made?

Rockwool is made by heating granite and limestone to extreme temperatures. These rocks are then turned into thin threads after they have melted. Only after that can they be formed into blocks that we use in our hydroponic gardens!

Like horticubes, rockwool are also not divided into further classifications.

2. Appearance

Horticubes have a sponge-like texture and are typically brown or white in color. Meanwhile, rockwool cubes are typically green in color and have the texture of thick cotton fibers.

You see, the texture and color of horticubes and rockwool are really different.

Rockwool has a texture like cotton candy! Can you see it? It has these fiber-like strands that are clumped together as a cube. On the other hand, horticubes look and feel just like your regular kitchen sponge!

3. Weight

Rockwool is heavier than horticubes. This is due to the difference in the material used to make each growing medium—rockwool is made of granite and limestone, which are heavier than foam.

As shown in the illustration above, 1 cube of rockwool weighs 1.3 grams, whereas a seemingly bigger horticube weighs only 0.5 grams. These values vary depending on size, but the main point is that rockwool is heavier than horticubes.

How is this information helpful for a gardener? You will discover that as you read forward!

4. Uses

Horticubes are ideal for seed starting and plant propagation via cuttings. Rockwool, on the other hand, has a wider range of applications, from being a seed starter substrate to rooting medium to growing full-grown plants.

In the previous section, we have established that rockwool is heavier than horticubes. This property of rockwool makes it a growing medium that can support the growth of plants up to their mature stages.

From my experience, rockwool is a great medium to grow tomatoes, lettuce, kale, and bok choy! They can also support the growth of herbs that can grow tall such as basil and mint.

This is contrary to horticubes which only work best until the transplanting stage!

5. pH

Horticubes have a neutral pH of around 7.0, whereas rockwool has a basic pH (7.0-8.0) because it is made from high pH minerals namely granite and limestone.

Here’s a golden piece of information.

The pH of a growing medium can also have an impact on the growth of your hydroponic plants. For example, if the pH of a growing medium is too acidic or basic, the roots may be unable to absorb oxygen and water, resulting in leaf curling and yellowing.

So for horticubes, this is not a concern because they have a neutral pH. Now let us dig deeper into this aspect of rockwool by moving to the next section.

6. Need for Pre-Soaking

Horticubes does not require pre-soaking because they absorb and retain water just like a sponge and they have a neutral pH. Rockwool, on the other hand, performs best when pre-soaked in a slightly acidic solution to lower its pH.

After discussing the effects of pH on your hydroponic plants, we will move on to how to act in light of the characteristics of a growing medium.

From the last section, you have learned that rockwool has a basic pH. This means it has a high pH. So for it to be normal, you need to expose it to an environment that can lower its pH. This can be done by low-pH solutions.

Here are steps you can follow to lower the pH of rockwool before using it:

1. Get a bottle of acidic water with a pH of 5.5 to 6.5.
2. You can add drops of lemon juice to make it more acidic.
3. Check the pH using a pH meter or pH strips.
4. When the water is already acidic (pH meter must read 5.0 to 6.6), get your rockwool and soak it in the solution for an hour.
5. After that period, it is now ready for use.

7. Handling & Safety Precautions

Rockwool must be handled with caution because cases of skin irritation have been associated with its use. Horticubes, by contrast, do not have this problem and are typically considered safe materials to use in gardens.

I can attest to this because I got a mild skin itching that lasted for an hour after I worked with rockwool. The fibers and dust from mineral wool are the components that can irritate the skin and nose.

So if you also have sensitive skin or a sensitive nose, it is recommended that you wear garden gloves and a face mask when working with rockwool.

It is better to take preventive measures than suffer, right?

8. Tendency to Break Down

In a hydroponic system, rockwool does not easily break down. More specifically, rockwool can last for 3–4 years. Contrary to that, horticubes have a higher tendency of breaking off and clogging pump filters.

Rockwool is less likely to break down in the hydroponic system because it is made up of small but durable threads from mineral rocks clumped together to form a heavy cube.

To help you visualize what I mean by this, consider a handkerchief. It is made from thousands of different fibers and will not break apart if no powerful force (such as fire or scissors) acts on it.

Now let us look at horticubes.

At the beginning of this article, I compared horticubes to a dishwashing sponge. Let me ask you, how often do you change your kitchen sponge?

Why? More probably, you are changing because it is already breaking off.

The same thing happens with horticubes as it is exposed to continuous hydroponic pressure. It is more vulnerable to breaking off because it is a lighter and less durable growing medium.

9. Reusability

Rockwool can be reused after sterilization because it does not degrade. Horticubes, on the other hand, are not recommended for reuse because they can easily break off during the sterilization process.

Rockwool will not degrade because it is made of non-renewable minerals. Thus, sterilizing it with hot water or hydrogen peroxide allows gardeners to reuse and resterilize it for up to six different plants and growing cycles.

This means you can use rockwool in growing, for instance, lettuce for 1 cycle, basil for another cycle, and so on… After that, the structure of the rockwool will likely be damaged by the roots of the different plants.

Horticubes, on the other hand, are not ideal for reusing because they are a light growing medium that will break apart when exposed to disinfecting agents such as hot water.

Reuse your Rockwool

When Should You Use Horticubes?

Using horticubes is ideal when germinating seeds of leafy and fruiting vegetables and herbs. It can also be used when propagating plants using stem cuttings.

Horticubes, as a lightweight growing medium, cannot support the stature of plants during their development and maturity stages. As a result, they are better suited for germinating seeds and sustaining their growth until the transplanting stage.

   Explore more options in our article about the best hydroponic growing mediums. 

When Should You Use Rockwool?

Rockwool is a more flexible option since it can support the growth of vegetables with large biomass such as eggplants, peppers, and tomatoes. It can also be used in germinating seeds and propagating through cuttings.

You can use rockwool throughout the growing cycle because it can support plant growth from germination to harvest time. In this regard, using rockwool can save you money because you won’t need to purchase another growing medium for transplanting.

    You can also check out rockwool alternatives.  

Horticubes vs Rockwool: Pros and Cons

Overall, rockwool is a more versatile growing medium, whereas horticubes are less expensive and easier to work with. However, disadvantages can also be observed such as the need for pre-soaking due to the high pH of rockwool; and breaking off in horticubes, which can cause system clogging.

If you are still undecided, I have summarized the different advantages and disadvantages of horticubes and rockwool in the table below.

Growing Medium	Pros	Cons
Horticubes	Neutral pH  High water retention  Versatile, can be used with different hydroponic systems  Cheap   Can be DIY using a common sponge  Pre-soaking is not required  Available in many sizes	Does not contain any nutrient  Usage is somehow limited to germination and rooting   Breaks off easily  Can cause clogging in pump filters
Rockwool	High water retention  Porous, can accommodate 18-25% air  Available in many sizes  Reusable, can save you money in the long run  Does not break down easily when used in a hydroponic system Versatile, can handle leafy greens, fruiting greens, herbs, cuttings, and seed germination	Basic pH  Needs pre-soaking  Non-biodegradable, which can adversely affect the environment  Roots are not entirely free since rockwool has a compact design  Can be irritating to the skin for some people

FAQs

What are the similarities of horticubes and rockwool?

Horticubes and rockwool are both inert and sterile. They also hold moisture well in their porous structures, so there is less chance of the plant roots drying out. Furthermore, they efficiently maintain oxygen and lower the risk of overwatering.

How should I choose a growing medium to use in hydroponics?

When selecting a hydroponic medium, consider the medium’s water holding capacity, porosity, pH, and particle size. This is because the hydroponic growing medium must be suitable for the specific plant and hydroponic system. It is also necessary to consider how tall the plant will grow to plan for support and anchorage.

Summary of Horticubes vs Rockwool

Horticubes and rockwool are different from each other in terms of material, appearance, weight, uses, pH, need for pre-soaking, handling and safety precautions, tendency to break down, and reusability.

In terms of usage, horticubes are a more appropriate growing medium from seed starting to transplanting. In contrast, rockwool can support plant growth and development from seed germination up to the harvesting stage.

Horticubes are less expensive and easier to work with than rockwool. However, rockwool is reusable and can last longer. Limitations for both growing mediums also exist namely pre-soaking due to rockwool’s high pH, and breaking off in horticubes which can cause clogging in the pump.

Sources

• “Propagation” by n/a in University of Arizona
• “Soilless Growing Mediums” by Thakulla, D., Dunn, B., and Hu, B. in Oklahoma State University