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17 Rockwool Alternatives for Hydroponic Systems

Rockwool has been a growing medium for hydroponic systems for around 40 years. Rockwool cubes are often used to germinate seeds as well as as an alternate to soil in hydroponic systems. It is favored for its chemical and biological inert characteristics, such as a high pH. However, it isn’t environmentally friendly and can be harmful if mishandled, leading some growers to seek alternatives.

In this post, we’ll discuss 17 Rockwool alternatives for hydroponic systems:

  1. Polyurethane grow slabs
  2. Perlite and vermiculite
  3. Expanded clay pellets
  4. Coco coir
  5. Rice hulls
  6. Gravel
  7. Sand
  8. Sawdust
  9. Oasis cubes
  10. Sphagnum peat moss
  11. Sure to Grow
  12. Starter plugs
  13. Growstones
  14. Pumice
  15. Brick shards
  16. Potting soil
  17. Gardening soil

Although Rockwool is one of the preferred growth mediums in the hydroponic plant world, there are some significant issues to consider before using it.

  • Despite its popularity, Rockwool comes with a steep learning curve. Yet, it’s often recommended to veteran and novice horticulturists alike. 
  • Rockwool dries quickly and can be expensive depending where you live. 
  • The most critical aspect to consider when shopping for the perfect growth medium is considering what’s best for your crop. 

Each of the growth media listed below has various benefits and drawbacks, but they all work incredibly well for specific types of vegetables and plants. 

Let’s dive into all the Rockwool alternatives for hydroponic systems to discover the pros and cons of each, and help you choose the perfect growing medium for your hydroponic needs.

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1. Polyurethane Grow Slabs

Polyurethane grow slabs are especially beneficial for cucumbers based on studies published by the International Society for Horticultural Science (ISHS). 

Interestingly, polyurethane is one of the newest growth media used in hydroponic systems. Although it may seem beneficial to be the latest exciting medium on the scene, it can lead to availability and reliability issues.

According to Greenhouse Product News Magazine, this rigid foam substrate lacks sufficient data on its performance because it hasn’t been utilized for as long as some of the other growth media listed below. 

However, because polyurethane was created specifically for hydroponic systems, it offers excellent aeration and about 80 percent air space. It also has a 15 percent water retention capacity.

Perlite and vermiculite spilling out of pots
Photo by Elena Petrenko

2. Perlite and Vermiculite

Perlite is a lightweight substance made of materials made from volcanic rock. Light vermiculite is created from heat expanded mica and has a flaky composition. 

Perlite and vermiculite are often combined as a growth medium because of their complementary properties. Individually, they pose some benefits—especially perlite—but they tend to negate each other’s harmful properties when these substrates are combined. 

On the plus side, this growth combination is affordable in most places and is therefore widely used.   

Alone, perlite has demonstrated a high crop yield, specifically when growing cucumbers. In a study published by M. Bohme, cucumbers did significantly better when grown with a perlite growth medium than expanded clay pellets. 

Although the benefits of perlite are promising, there are some drawbacks to using this in your hydroponic system.

While perlite and vermiculite provide excellent aeration, they have poor water retention. 

Because this medium tends to dry out quickly, it also creates dust that can damage your health. If you choose to use a perlite and vermiculite mixture in your hydroponic system, be sure to wear a mask when performing maintenance. 

3. Expanded Clay Pellets

Third on this list of alternatives to Rockwool is expanded clay pellets. It’s expensive but sustainable and highly aerated, but does the cost make it overrated? 

This inorganic medium is one of the few reusable growth media which also contributes to its environmental friendliness and its cost. 

Clay pellets are cheap compared to single-use grow media when considering their long-term potential due to their reusability.  

Another factor to consider is the type of hydroponic system that you have and what will best supplement and support it. Expanded clay pellets, for example, are best for the ebb and flow hydroponic systems. 

Expanded clay pellets are available in many sizes and shapes, providing superb aeration, but this feature can also make them heavy.

As stated previously, other media, such as perlite, have demonstrated superior crop yields compared to clay pellets. 

Also, some studies suggest that expanded clay pellets also have a higher likelihood of producing some issues in crops, such as tomato blossom-end rot.  

Beyond disease, clay pellets have also been known not to retain much water. Those that have used expanded clay pellets in their hydroponic systems have found that they require frequent watering. 

Because of the inorganic nature of this medium, like any other inorganic medium, there are environmental concerns about the lack of biodegradability.  

However, inorganic media have also shown a better crop yield in most vegetables. 

4. Coco Coir

Coco coir is an option that many growers prefer. It’s an organic medium made and processed from dried coconut husks.   

Unlike inorganic materials, organic substrates like coco coir cause decomposition concerns. Coco coir has gardening roots from the 19th century but was abandoned because of its rapid decomposition. 

Coco coir has been known to decompose quickly during crop production, which may change the composition of the growth medium. However, it’s not known whether this affects the crop yield or growth of the plants. 

Coco coir is expensive and doesn’t have any nutrients for your plants. Also, some research suggests that this growth medium is high in salt content and can block critical components needed for a healthy crop, including calcium, magnesium, and iron.  

5. Rice Hulls

Rice hulls are another organic medium that can be used as a Rockwool alternative. Their history is rooted in successfully replacing perlite and other mixes in commercial-sized crop productions.

Rice hulls are more effective if mixed with other media, and users often create specialized formulas for specific plants. They also have the same demonstrated success as polyurethane foam and perlite.

Unfortunately, rice hulls create a significant salt build-up and therefore need to be replaced often. Usually, most hydroponic systems that use rice hulls have to be replaced after each or every other crop. 

6. Gravel

Gravel is simple, easy, and one of the most user-friendly growth media available for your hydroponic system.

Gravel comes in lots of different sizes, which provides many options. However, you should consult an expert when selecting the right size for your hydroponic system. They’re also inexpensive and provide lots of oxygen for your plants.

Another plus is the availability of rock and its reusability. Longevity plus sustainability equals a potential perfect match for your hydroponic system!  

Unfortunately, though, gravel doesn’t retain water well. This can lead to overwatering for your plants and excessive draining. Not to mention that rock also alters the pH levels of the water that moves through your hydroponic system. 

Altered pH levels can also alter how you care for your crop, as high pH levels would chemically react with your plants.

Gravel is also heavy and hard to maintain in your hydroponic system. Two quarts, a reasonably small package of gravel, weigh almost 50 pounds (23 kgs)! Is your planting system prepared to handle that much weight?

Photo by Glen Jones

7. Sand

The next Rockwool alternative on the list is sand, which often lands in the same category as gravel.

Sand and gravel share a lot of similarities. For one, sand is heavy. Therefore, your planting system must be able to hold a large amount of weight if you use sand, especially if it’s above ground level.

On the plus side, sand is readily available in many places, sometimes for free or at least cheap.

Sand is also a great additive to other media and, when combined, has been shown to produce good results. 

Yet, despite its benefits, sand isn’t a widely used growth medium in hydroponic systems. It’s often thought of as archaic, and it has some drawbacks.

One of the issues with sand is that it packs tightly together. When sand packs together, it limits the oxygen flow and doesn’t provide good aeration for the crops. 

8. Sawdust

Sawdust has shown some success as a growth medium in Australia when used on tomatoes, and a study on hydroponically-grown strawberries. Sawdust is a great medium to use when your budget is tight because it’s very cheap, readily available, and its results are worth noting.  

However, sawdust retains moisture, making it easy to overwater crops, but when it is combined with other grow media, its properties can be enhanced. 

Also, the success of sawdust seems to be tied to the purity of the wood. Contamination, moisture, and decomposition are all factors to be considered.

9. Oasis Cubes

Oasis cubes are foam-based media that are highly porous. These Rockwool alternatives are rigid water-absorbing foam substrates that offer a tremendous rooting medium for starting seeds. 

Another positive to oasis cubes is that they can hold 40 times their weight in water, don’t pose a danger to your well-being, and are considered optimal for growing herbs, lettuce, and other similar veggies.

Oasis cubes have an excellent reputation for rapid germination and provide an ideal home for seeds. Beyond seedlings, horticulturalists seem to find them somewhat limited in growing plants beyond the above-listed vegetables or the early stages of a plant’s life. 

Other than their semi-limited use, another con to oasis cubes is the expensive initial investment. However, they’re somewhat reusable, so the up-front cost is less over time compared to other Rockwool alternatives. 

10. Sphagnum Peat Moss

Sphagnum peat moss’s natural and organic properties make it a readily available growth medium for your hydroponic system.

The structure of this substrate makes it optimal for providing excellent aeration. In addition, Sphagnum peat moss is a sponge-like material that makes it highly absorbent. 

Unfortunately, this growth medium decomposes over time in a way that sheds small particles that can plug your pump or drip emitters.

11. Sure to Grow

Next up on the list is Sure to Grow cubes, sometimes sold as a pad, which have received mixed reviews. Again, gardeners seem split on the overall results of this product.

Product reviews on show that the Sure Grow Sure to Grow Cubes have a high success rate in germination but mixed results when working with cuttings and seedlings.

Growing with this product may be difficult for first-time or novice hydroponic gardeners and seems less favorable than Rockwool for most hydroponic systems.

However, Sure to Grow is relatively inexpensive, depending on the retailer and the amount of product you’re purchasing.  

12. Starter Plugs

Starter plugs are made of organic materials, usually derived from coco coir or peat and bound with an adhesive. 

Some positives to starter plugs are the ease of use which makes them beginner-friendly. Also, they’re usually pre-packaged and shipped in already dampened and ready-to-use trays. The convenience of these starter plugs can be a significant advantage.

Starter plugs have also been known to have faster germination than other media types, but they lack the reusability that many other alternatives on this list have.  

13. Growstones

Growstones are an inert material made of recycled glass wholly reduced to a smooth, small-medium. It comes with a high recommendation from the American Society for Horticultural Science, which stated in a 2011 article that growstones are an ideal alternative to Rockwool and perlite and rice hulls.

Growstones are commercially available and relatively easy to work with. However, like Rockwool, the inert nature of this material means that it doesn’t supply plants with additional supportive or interfering elements. This is both good and bad as it means that no chemicals interact with the nutrient system, but it also means you may have to monitor your crop for any additional support it needs. 

The findings in the ScienceDaily article suggest that the highly aerated porous nature of growstones added to the superior results in this substrate. The recycled glass component of this material also makes it a popular choice because it won’t damage your plant with contaminants or toxins. 

Growstones are an optimal choice, as they have about three times more water saturation than expanded clay pellets, twice as much aeration than perlite, and four times the oxygenation abilities versus coco coir. 

So, you might be wondering if growstones have any downsides.

Well, the tiny recycled glass components that make up this medium are gritty to work with. The little shards can be powdery, and they tend to float. So if you’re washing your growstones, they can become a bit of a fine, gritty mess. 

It’s also concerning when tiny bits of growstones break off because the smaller stones can leak through holes in your pots and possibly into the drain, leading to issues with the pump.   

14. Pumice

Pumice is another inert, inorganic medium that’s cheap and readily available. Primarily utilized in New Zealand, it’s another growing medium made from sponge-like volcanic rock. 

The design makes it another excellent Rockwool alternative if you’re looking for something with great aeration and moisture retention. 

Pumice also has a wide-ranging reputation for improving the growth of most plants and vegetables, actively fighting against compaction and runoff, as well as providing excellent aeration.

However, pumice has a ton of sodium because it’s made from volcanic rock. The sodium needs to be extracted before use in your hydroponic system, which can be an added inconvenience to use. 

Also, the salt creates water retention issues for this medium. Unfortunately, the sodium inherently in pumice leads to reduced water holding capacity, negatively impacting your crop.   

Overall, there are several benefits to using pumice in your hydroponic system, like longevity, pH balance, and nutrients. Check out this article on pumice, and its effects on hydroponics from a soilless grow organization, Ponics Stone.

15. Brick Shards

Brick shards, gravel, and sand all have pretty much identical properties. But, most notably, they have a lot of disadvantages.

Like gravel, brick shards can alter the pH levels of your hydroponics system by chemically altering the water filtering through. However, they’re relatively inexpensive.

If you’re new to the hydroponics system, and you have a supply of brick shards, gravel, or sand available for free or cheap, it’s worth a shot and could help you learn more about your system through trial and error. 

16. Potting Soil

Potting soil is a staple item in most greenhouses and horticultural environments. 

The great thing about potting soil is that it provides optimal aeration due to its varied composition. Also, it has good water retention, and it drains well.

Potting soil can also be beginner-friendly as it already includes and stores nutrients for your plants, which means one less thing for you to worry about because you don’t have to give your crops much fertilizer. 

Please note that we recognize that the oint of hydroponics is to use a meidum other than soil when growing your plants, but in a pinch if you are looking for an alternative to rockwool, it is a possible solution for you.

If you go this route, keep in mind that potting soil already has nutrients, so it reduces how much control you have on your plant’s nutrient intake.

17. Gardening Soil

Last but not least, gardening soil is another option. And no, this isn’t the dirt you have lying around in your backyard or garden area. That soil is potentially filled with bugs and contaminants that you don’t want in your hydroponic system. 

Gardening soil, another staple in greenhouses, is a medium rich with organic matter. This growth medium offers perfect water retention but has a lot of drainage issues.

Another downside to gardening soil is the variation in brands. Each brand seems to have a unique reputation for crop yield, horticultural benefits, and issues.

Again, like potting soil, this should be used as a last resort and not the most ideal for hydroponics.

Closing Thoughts

As this list demonstrates, there are many options when identifying the best alternatives to rockwool to support for your hydroponic system. Whether you’re a veteran horticulturist or a new gardener experimenting, there are limitless options for caring for your garden. 

Remember, though, to weigh your options carefully to avoid a potentially costly mistake. Not every medium is a perfect fit for every system or every plant. Therefore, it’s essential to carefully consider your unique situation before deciding on a growth agent.