If you want a hydroponic garden to be successful, in most cases, the nutrient solution will not be enough to give your plants the constant supply of nutrients they need; you will also need a growing medium.
A growing medium is a solid material, in the form of pebbles, sponge, fibers or shreds and bark that is used to withhold the nutrient solution (water and nutrients) and then release it slowly to the roots of the plants. But which ones are the tried and tested growing mediums, and how can you choose the best one for your hydroponic garden?
Using a growing medium has done to hydroponics what ‘Strawberry Fields Forever’ by the Beatles has done to pop music: it has revolutionized it.
But choosing the right growing medium is not easy, and you will need to know all the details of the different ones available and how they behave to choose the best one for your gardens and crops.
Main qualities of a growing medium
We can’t use any solid as a hydroponic growing medium. It does need to have some basic qualities in order to be functional:
Choosing the best growing medium for your hydroponic garden is a bit like choosing the wood to make a violin; yes, the strings are all important and so is the shape, but unless you choose the right material, the sound will never have that perfect ring…
Is a growing medium strictly necessary?
Technically speaking, you can have a hydroponic garden without a growing medium; however, most gardeners will strongly advise that you use one.
What is more, using one will make such a huge difference on the management of your garden and of your resources (water, nutrients etc.) that the small expense for a growing medium is easily justified, and will actually save you lots of money in the long run.
What are the advantages of using a growing medium?
If you are still not convinced that using a growing medium is a smart and wise choice, just look at the difference it can make to your hydroponic garden:
As you can see, if you use a growing medium, you can have much better control on the growth and health of your plants, on how you use nutrients and water, on the atmospheric conditions of the microclimate of your garden, and on the independence of your garden in case of faults and breakage.
And all this while actually reducing the amount of work and energy you put in. In 1979, The Clash had a single out called ‘Bank Robbers’ and the B side was ‘Rudie Can’t Fail’…
Well, with hydroponic growing mediums, all gardeners can be called “Rudie”.
Are there any drawbacks to using a growing medium?
“Nothing is perfect in God’s perfect plan,” Neil Young used to sing back in 2011… Still, when it comes to using a growing medium, the disadvantages are really small:
So, if nothing is perfect in God’s plan, it is also true this may remind us of our role as gardeners, help along with the work of Nature, and, at least in this case, she is really asking very little from us…
Do all hydroponic systems use a growing medium?
Not all hydroponic systems need one, and not all methods can use one. Some, however, depend so much on it that doing without is almost unheard of, even nonsensical.
This should give you a broad perspective on using (or not) a growing medium according to the hydroponic method you choose.
This is a good start, but now, after this general “overture” , it is time to dive deep into the “underwater world” of growing mediums, a bit like Wagner does in Tristan and Isolde…
Ready for the some “drama” now? Not real drama, actually, just lots of facts, details and tips…
How Many Growing Mediums Are There?
Hydroponic gardeners have been experimenting with different growing mediums for decades, but it was only when rock and roll started from vinyl records that researchers understood importance of using the correct medium; in the 1950s, in fact, studies in hydroponics found out that using a poor, inadequate growing medium had a negative impact on the plants you grow.
Over the years, three main groups, or types, of growing mediums have come out as being the best for hydroponic gardens:
Not all materials that fall into these categories are suitable; however, these are three groups of materials that comprise all the growing mediums.
1: Pebbles and stones
From the idiom “a rolling stone gathers no moss” comes the name of one of the most famous bands in history, but this old saying also tells us something about why pebbles like expanded clay, lava pebbles, vermiculite etc. are good growing mediums… Let me tell you why…
2: Foam materials
Growing mediums like rockwool, oasis cubes, floral foam and even polyurethane foam insulation can be used as growing mediums.
I will be straightforward with you: they are not my favorite choice and they do not the most popular among hydroponic gardeners. But first, let’s look at their advantages.
However, there are some off-putting factors you will need to take into consideration:
3: Natural organic fibers
Hydroponics is a branch of organic gardening, thus, the enthusiasm for eco-friendly solutions should come as no surprise.
If you are coming to hydroponics with this perspective, then natural organic fibers may be a very appealing option for you.
Pine bark, pine shavings, coconut coir, coconut chips and even rice hulls have now become common so, you will be drawing on a well established tradition.
Still, they have many pros but also some cons.
However, they do have some small disadvantages as well:
A final note, more on fading than a high C, to natural organic fibers is that not all of them are fully inert; some, like pine bark, need to be aged if you want them to be as inert as possible. Once they are dry, though, they will not release nutrients into your solution or absorb them from it.
11 Different Hydroponic Grow Mediums And Their Pros And Cons
You can even improvise your own make shift growing medium; this has been done by amateur (and not just) hydroponic gardeners with a “free jazz” inclination or inspiration.
Yet, while breaking conventional rules and tried and tested methods can be good for experimentation, if you want to be on the safe side, you’d better stick to a list of growing mediums that have proved to be reliable…
On top of this, the list of mediums is fairly long, including common building material, cheap mulch and even plain sand… Shall we now look at them in turn?
1: Expanded clay (a.k.a. grow rock or even hydrocorn)
Expanded clay is the Pachelbel’s ‘Canon in D’ of all growing mediums; possibly the most common, most easily recognized and most eclectic of all the ones we use.
If you heat clay, which is a fully natural material, at 2,190oF (or 1,200oC) it expands, forming small air bubbles inside, in a honeycomb structure.
It is widely used building material, cheap, light and very durable. Hydroponic gardeners soon found out that this is a very good growing medium, because it can hold the nutrient solution in its porous structure and then release it slowly. But there is more…
Even if expanded clay pebbles are very common, they are not perfect:
Still, because they are very cheap and durable, they can be used in combination with other mediums with longer water retention, like pumice or vermiculite.
2: Pumice and lava rocks
Still within the “pebble” category, another cheap and readily available option is pumice and other porous lava rocks.
Pumice is a volcanic rock formed when a volcano erupts lava with lots of water and gas in it. The water evaporates quickly and this forms lots of bubbles and pockets inside of it.
It does have some good advantages, and it is also used in soil gardening.
Pumice is very common with tomatoes and herbs grown with the nutrient film technique.
It needs washing and sterilizing before you use it, as it breaks easily producing dust. The most common range of pebbles is between 1 and 7 millimeters in size.
While some gardeners hail it as the “perfect growing medium” for hydroponics, it is sometimes too light for some hydroponic systems; in fact, it floats on water, so, while it is excellent for drip systems, wick systems, nutrient film technique, it does pose some problems with ebb and flow and deep water culture systems.
Pumice (and similar lava rocks) too can be used in combination with other growing mediums, like coconut coir.
This mineral with a beautiful name has the internal structure of an accordion; it has, in fact, many internal layers with thin plates which create pockets that fill with water and air.
This, however, only after you heat it at 1,652oF (or 900oC). In fact, this growing medium expands with heat, a behavior that sets it aside from other mediums.
This mineral has many pros as a growing medium:
However, vermiculite is not a perfect growing medium either. It does have some major drawbacks:
This is why vermiculite is often used in combination with another medium, one that allows good aeration, very often coconut fibers or, even more frequently, perlite, which we will see next.
Another material with a beautiful name, derived possibly from its white “complexion”. It is a glass found in volcanoes, which, when erupted holds on to water inside.
When it is heated at 1,600oF (which is 870oC), it “pops”, a bit like popcorn and it expands to up to thirteen times its original size, becoming light and filled with small pockets of air.
We could see perlite as the complementary growing medium of vermiculite; in fact, while it does not hold water for long, it is excellent for aeration.
This is why the two are often found together, in different rations, though, usually 50:50 is the most common.
Starting with the advantages…
Still, you will never find a professional hydroponic gardener using perlite on its own, and this is because of its many drawbacks:
Yet another building material that has found its way into the list of hydroponic growing mediums, rockwool has that feeling of early synthesizers used in music alongside classical instruments in the sixties and seventies…
Not only does it look like something from a movie or even television series from that era, but it is also an early industrial product coming from the transformation of organic material.
In fact, it imitates the fibers of plants, while being produced industrially.
It is, in fact, an insulation material which hydroponic gardeners have adapted into a growing medium.
If you are prepared to put up with a growing medium that looks cheap and industrial, then rockwool may be a practicable choice for you. In fact, it does have some advantages:
However, it also does have some major disadvantages, and its fortune as a growing medium seems to be declining steadily:
6: Polyurethane foam insulation
Little did Pink Floyd know that their experimentation with synthesizers would lead to full blown techno music, but it did…
Similarly, with the development of the oil industry, came fully synthetic insulating materials, like polyurethane, a synthetic foam with a spongy and semi-rigid texture.
It is not very common with hydroponic gardeners, but it may be used as a makeshift growing medium in case of necessity.In fact, it does have some of the key qualities of a growing medium:
7: Floral foam
Image Source- https://www.youtube.com/watch?v=n1Mdikw3GNo
If polyurethane is “in your face” synthetic, floral foam is a bit like EDM music; it may “look” more natural, but it is still synthetic. You may picture it covered in cut flowers in a beautiful composition, because this is how florists have made it popular. But you can also use it as a growing medium.
However, there are some main disadvantages:
On the whole, while some people use it, it is maybe better only with limited and small applications. For example, if you have some and you desperately need it for seeding, you may as well recycle it, but I would not recommend buying it on purpose nor using it extensively.
Image Source- https://www.maximumyield.com
Welcome back to the natural world… After a trip into artificial growing mediums, from now on, we will breathe only fresh air, a bit like moving from computer generated to instrumental music. Sand is a readily available growing medium; it is basically rock in very tiny pieces, so, it has some good qualities as growing medium.
Still, even sand is not perfect:
Hydroponic gardeners who use sand also like to mix it with another growing medium, usually perlite and vermiculite or even coconut coir; this is to provide better aeration, with a ratio of 70:30 or 80:20 between sand and other medium.
Finally, if you wish to use sand, choose the one with the biggest possible grains; this way, the pockets between them will be larger.
9: Aged pine bark
Pines and conifers are fast growing trees, used to make furniture, whole homes and many musical instruments, like guitars and even violins.
But what makes them appealing to hydroponic gardeners is their bark; thick and with natural chunks that are easy to separate, they have been used for epiphytes like orchids as a growing medium as well as mulch for decades.
However, before you use pine bark as a growing medium, you need to render it inert; fresh pine bark will absorb nitrogen from your nutrient solution.
Professional gardeners can remedy this by adding nitrogen directly to the bark they use, but if you want an easy life, your best option is to get aged bark. In fact, it does have some great qualities:
However, even pine bark does have some problems, as gardeners know quite well.
Pine bark is often also used in combination with other natural mediums, like perlite and vermiculite.
10: Rice hulls
Rice hulls break down easily, but they also have a very strong texture; this means that while they will absorb your nutrient solution, they will also have a basic strong structure. If you are thinking about them, here are the advantages:
On the other hand…
Rice hulls are rarely used as the sole growing medium. Instead, they are often used especially with pine bark, usually 30:70 or 40:60 rice and pine bark.
11: Coconut coir and coconut chips
The Stradivarius of growing mediums is coconut; you can use both the coir (the fibers outside the husk) or husk chips. Both are excellent in many ways:
“But,” you may ask, “is there a difference between the two and are there any disadvantages?”
Apart from the aesthetic differences (chips look better than coir), coconut chips also form bigger pockets.
You can, of course use both together, maybe with the coir lower down and the chips on top.
Finally, a little detail that hydroponic gardeners know well… Removing coconut coir from the roots when you want to change crop can be a bit “fidgety”…
What Grow Medium Is Right For You?
The range of hydroponic growing mediums is like full orchestra; there are so many “instruments” that choosing the best one for your “piece”, garden or crop, can take some time… But in the end, you will need to choose which “voice” you wish your garden to have…
Most hydroponic gardeners prefer fully organic medium with low environmental impact, and here, coconut coir and fibers are by far your best choice.
Others may prefer natural pebbles, like expanded clay and vermiculite or even sand. Few will choose a synthetic material, not just because they are not organic, but they also have quite a few drawbacks…
Then again, you can mix growing mediums to get the best “harmonic composition” for the plants you want to grow.
Still, whichever you choose, my final tip is to invest in the long term; a pH neutral, durable and recyclable growing medium will spare you expenses in the future and a lot of work as well.
Look at it like choosing a violin; a low quality one may be good to learn the basics, but a good one will pass down from generation to generation and always regale us with its beautiful notes.
After many years as an academic in London, Adriano Bulla became a writer, publishing books like A History of Gardening, Organic Gardening and Elements of Garden Design; he then decided to become a gardener, following his childhood dream, and has been following his dream writing and gardening professionally in Southern Europe, where he has specialized in new and innovative organic gardening fields and techniques, like permaculture, regenerative agriculture, food forests and hydroponics.