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For newcomers to indoor gardening and hydroponics the voluminous array of products on offer can be something of a headache: from choosing a system, lighting, ventilation, what to grow, where to grow, to considering nutrient solutions, and so on. And then of course there’s growing mediums, or growth media if you prefer. There seems to be no end of options and permutations. It’s enough to make one throw one’s hands in the air and give up before even starting. But don’t despair – this is where you must let the paternal arm of Hydromag wrap around your shoulders.

  • In this article we're covering :
  • Coco Coir
  • Perlite
  • LECA (Clay Balls)
  • Rockwool
  • Peat Moss
  • Vermiculite
  • Diatomite
  • Sure To Grow

Firstly, some of you just starting out might be wondering what “Growth Media” actually is (fortunately nothing to do with medical journals dedicated to discussing tumours). It’s essentially soil substitute (or substrate as it’s also known). As your plants’ nutrition will be derived from solution, not soil, the growth media we’re talking about is, in effect, just there to hold the plants in place (think Neo in The Matrix, after taking the red pill, waking up to find himself suspended in a strange liquid – or growth medium!).

Almost anything can simply hold your plants in place, from polystyrene to sand. But before you rush off and improvise with old pairs of socks and toenail clippings, know that your medium should also be inert, lightweight, water-retentive and porous; to allow oxygen and solution access to the plants’ roots with minimal interference. And in this article we’ll be taking a closer look at some of the more popular growth media products out there in the market, with a particular focus on Coco Coir, Perlite, Rockwool and LECA (or clay balls). Why these four? Because they’re easily obtainable, simple to use and will service practically all your growing needs. But to pique your interest we’ve bunged in some facts about other commonly-used media.

Coco Coir

Coco Coir, growth media, growing medium, soil substitute


“Coco Coir has become increasingly popular in recent years. It’s inexpensive, highly absorbent and can be used many times.”

If you thought Coco Coir sounds like some drunken, ageing diva in Los Angeles, clinging to her fast-disappearing youth in an undignified manner … well you’d be right. That is exactly what Coco Coir sounds like. But it’s not what Coco Coir is.

Coco Coir has become increasingly popular in recent years. It’s inexpensive, highly absorbent and can be used many times. Its pH value fluctuates slightly which can aid the absorption of nutrients. It also has an excellent cation exchange capacity (i.e. its ability to hold and release the various elements and compounds in your nutrient solution). And if that wasn’t enough here are a couple of other juicy bonuses: it’s got roughly double the porosity of rockwool; and a water-holding capacity that’s greater than peat moss. Interestingly its effectiveness has been shown, rather like a good cheese or Hydromag’s writers, to actually improve with age. After the first use or two it tends to degrade in a favourable way.

But nothing in life is perfect. For one, Coir has a habit of retaining oxygen – which can lead to increased microbial activity; bacteria and fungi are rather partial to a bit of coconut. This is a positive when the bacteria and fungi are friendly, but something of a drag when the unfriendly variety takes root. The other thing of note is that nitrogen can also get “caught” in coconut media, so extra nitrogen may be required for your growing solution. And while Coir is reusable, it does break down and has been known to cause drainage issues.

Coco Coir Overview:

  • Environmentally Sustainable
  • Excellent cation exchange capacity
  • Water-holding capacity greater that peat moss
  • Nitrogen can get "caught' in coconut media, extra nitrogen may be required
  • pH value fluctuates slightly aiding absorption of nutrients
  • Double porosity of rockwool
  • Retains oxygen
  • When breaking down can cause drainage issues


Perlite, growth media, growing medium, soil substitute



Made from volcanic glass, perlite is cooked at such high temperatures it puffs up like a depressed, ice-cream-gorging menopausal woman going through a break-up. Or more sensitively put – like popcorn. This process is called Fusionic Metamorphosis. The glass heating that is, not the ice-cream gorging (“What’s happened to Susan, she doesn’t seem to have taken the split with Fred very well?” “She’s just going through some Fusionic Metamorphosis”).

Perlite is bone dry due to its hydrophobic qualities (it’s water-resistant), but the thousands of tiny bubbles in its structure allow a degree of water retention. It’s cheap, light, and can be used repeatedly. For the drinkers amongst you, perlite is also often used by breweries to filter beer. So if you ever start your own microbrewery, you might want to keep those sacks of perlite handy.

On the flip side, the lightness of perlite causes it to “work” its way to the top of your soil substitute mix over time. If you’re offended by the sight of these little white pebbles popping up at the top of your mix, we recommend not using it if you plan on keeping plants in the same container for more than six months. And if you use an “ebb and flow” system with your plants you’ll notice pieces tend to float away during the flooding cycle. For this reason it is good practice to mix perlite with a heavier media such as the aforementioned Coco Coir. It also has a very low cation exchange capacity, something else the Coir can counterbalance.

Perlite Overview:

  • Cheap
  • Can be used repeatedly
  • Light
  • Thousands of tiny bubbles in its structure allow a degree of water retention
  • Hydrophobic qualities mean it is water-resistant
  • Lightness causes it to "work" its way to the top of the soil

LECA (Clay Balls)

Leca, clay balls, growth media, growing medium, soil substitute



“The drainage and aeration qualities of Clay Pebbles are superb and they can be sterilised and used time and again.”

Lightweight Expanded Clay Aggregate. Otherwise known as Clay Balls (Morph fans look away now). These are, well, balls of clay – cooked until they puff up like a thyroidal fat kid who’s just had his face stung by a wasp. Or more sensitively put – like popcorn. The result is a mountain of extremely light, highly porous, spherical objects that look like very stale Maltesers. They might even taste like very stale Maltesers but for God’s sake don’t try to find out. The drainage and aeration qualities are superb and they can be sterilised and used time and again. They’re also chemically inert so won’t affect pH levels. They don’t retain moisture as well as Coir or perlite though and are best used in conjunction with another product such as Coco Coir. Use as a base layer in an ebb and flow system, or in top-drip pots to cut out algae growth.

LECA Overview:

  • Extremely Light
  • Superb drainage and aeration qualities
  • Chemically inert so won't affect pH levels
  • Highly Porous
  • Can be sterilised and used time and again
  • Doesn't retain moisture as well as coir or perlite


Rock wool, growing medium, growth media, soil substitute


“The best quality rockwool is made from pure basaltic rock (diabase) and is inert. ”

This is a man-made mineral-based fibre, tweaked to be conducive to hydroponic plant growth. A bit like fibreglass, this can be bought in two different forms – repellent or absorbent (bit like Kevin Pietersen; absorbing cricketer, repellent person). You’ll need the repellent type. Rockwool is used in hydroponics on an industrial scale and is the choice of many large-scale, professional outfits – mainly because it’s relatively cheap, holds its structure, retains water well, and has little cation exchange capacity (allowing expert gardeners to control the release of certain nutrients). The downside is it’s not the simplest of substrates to work with, as quite a lot of initial effort is required to ensure it’s ready for use. It is imperative to test your rockwool first.

First piece of advice: don’t opt for the cheapest version on the market. The best quality rockwool is made from pure basaltic rock (diabase) and is inert. The crap version is often a byproduct of industrial smelting – basically slag – and will be awash with reactive metals that your plants won’t appreciate. Another thing to watch out for: like perlite, rockwool is hydrophobic due to the natural presence of mineral oils. But good quality rockwool will have these oils replaced with a wetting agent – it should wet easily without becoming waterlogged – yet another test you’ll have to conduct (as well as needing to adjust its pH value).

Another drawback is that its fibreglass-like quality can be something of an irritant when dry. Mind those fingers. Although rockwool requires extra initial effort, the major bonus of being forced to test it from the get-go means you know you’ve got your set-up just right before you’ve even started. A bit like doing all your homework on a Friday, you can now safely watch EastEnders and Hollyoaks omnibuses all through Sunday without that gnawing sense of dread! (I appreciate most of the readership has probably forgotten this feeling).

Rockwool Overview:

  • Rockwool Overview:
  • Comes in two different forms - repellent or absorbent
  • Retains water well
  • Hydrophobic but good quality rockwool will have a wetting agent
  • Holds it's structure
  • Has little cation exchange capacity

Others: Peat Moss

Peat Moss has nothing to do with the ill-advised relationship between the Babyshambles frontman and model Kate Moss, and everything to do with a spongy green substance found in peat bogs. Despite the plethora of modern, non-organic substrates out there, peat moss is still in common usage in hydroponics. All organic substrates, which of course peat moss is, have some cation exchange capacity. This will give the grower a buffer against nutrient changes in the root zone and aids nutrition management. Other advantages include its excellent water retentiveness and its relatively low cost. However this needs to be balanced by the fact peat moss has a tendency to become quite unstable – you’ll need to keep on top of pH levels. On purchase you will need to make a pH adjustment and introduce a wetting agent. Other problems include the fact it can decay, clog irrigation systems and lead to the growth of fungal spores if you’re unlucky. Growers also report that it takes up calcium and magnesium meaning your plants might miss out. Final note: there’s a raging debate amongst peat producers and conservationists as to just how renewable peat moss is as a resource. If you’re considering using it and you care for the environment we suggest you do your own homework and make your own choices.

Peat Moss Overview:

  • Low Cost
  • Needs pH adjustment
  • Non-Renewable
  • Some cation exchange capacity
  • Needs a wetting agent
  • Tendency to become quite unstable

Others: Vermiculite

Similar to perlite, vermiculite is mineral based. And like both perlite and LECA, it’s heated at such extreme temperatures that it puffs up like the face of a jowly middle-aged man with a tooth abscess who’s somehow just survived ten rounds with a Klitschko… Or more sensitively put – well, you’re starting to get the picture. Unlike perlite however, vermiculite retains a Hell of a lot of water so needs to be used sparingly. It provides excellent anchorage to young roots but needs to be mixed well with other media.

Vermiculite Overview:

  • Mineral Based
  • Provides excellent anchorage to young roots
  • Retains a lot of water
  • Needs to be mixed well with other media

Others: Diatomite

Tiny fossilised rocks or powder that can be added to your mix but can’t be used as a standalone medium. Diatomite comes from salt and fresh water sources. DO NOT get the salty version as the salinity levels are through the roof. Look at this seemingly harmless powder under a microscope and you’ll see what looks like thousands of tiny razors – and that’s how they act if our insect chums should stumble into it. The sharp edges blitz the bugs’ protective coating until all the moisture gets sucked out of them and they die a death from a thousand cuts. Diatomite is a useful addition to your growing mix.

“Look at this seemingly harmless powder under a microscope and you’ll see what looks like thousands of tiny razors”

Diatomite Overview:

  • Tiny fossilised rocks / powder
  • Sharp edges destroy bugs' protective coating so they die
  • Use the fresh water version
  • Can't be used as a standalone medium

Others: Sure to Grow

This is a relatively new product created from recycled plastic and looks and feels like cotton wool. It’s pH neutral (no pre-soaking required), lightweight, versatile, durable and has excellent water retention and aeration properties.

What’s different about it? Sounds like all the other products on the market… Well it’s also “non-wicking”, which in real terms means it doesn’t suck up nutrients using capillary action like rockwool or Coir – instead it simply holds the nutrients in a state of suspension. The upshot is plants don’t have to work so hard to suck up your solution, and can expend some of that reserve energy on growth instead! Available in blocks, pads, loose fill and other forms – all of which hold their shape so don’t leave unsightly bits and pieces floating down your irrigation channels. This product is gaining a lot of fans. Just be aware that this substance doesn’t have the rigidity of some of its rivals so larger plants might lack the necessary support (you’ll need to invest in some plant stakes or grow nets).

“Sure To Grow is a relatively new product created from recycled plastic and looks and feels like cotton wool.”

Sure to Grow Overview:

  • Created from recycled plastic
  • Lightweight
  • Excellent water retention and aeration properties
  • Less rigid so larger plants might lack the necessary support
  • pH Neutral
  • Versatile
  • Non-Wicking - Holds the nutrients in a state of suspension

All Else

Anything that can hold a plant above its nutrient solution is effectively a substrate or growth medium. Polystyrene, marbles, gravel, sand – all sorts. It is advised however to stick to recommended products such as those illustrated here in this article. None of them cost the earth but if you really do want to be a cheapskate, by all means experiment. But don’t come crying to us when all your hard labour results in shitty crops.

One final growing medium, best left to the experts, is AIR! Aeroponics, as it’s known, uses no growth media at all. But we’ll leave this for our sister publication Aeromag (kidding). Aeroponics involves the plant roots being suspended and receiving solution through a fine mist, pumped at timed intervals. More on this in a future article.

Growing Systems and Growth Media

Okay, you’ve seen what’s out there. Now it’s time to apply the right media with the right system. There are six main systems – “Wick”, “Water Culture” “Ebb & Flow” (a.k.a. “Flood & Drain”), “Drip”, “N.F.T” and “Aeroponic”. This is Beginners’ Corner as we’ve already been at pains to point out, so we’ll scrap the last two systems as they’re best left to the experts. That and they don’t really use growth media. We’ll address them in detail when you’ve earned your stripes, soldier.


The simplest of all systems – only suitable for small plants. The grow tray sits atop a reservoir of solution. A wick (as in a candle), sucks up the solution into the tray. It’s a passive system because there are no moving parts. There’s no flow of water so you don’t have to worry about your media getting bashed about, but as the wicks’ absorption rate is slow you’ll want to use media with good retentive

A mix of Coir and perlite are just fine for this system. If you’re feeling adventurous whack in some vermiculite.

Water Culture

The type you see in classrooms. Cups of plants in Styrofoam, floating atop a tank of solution – again, only suitable for small plants. Very little growth medium is required as the roots should dangle directly into the solution. Water retentiveness is therefore not that vital.

A small amount of perlite to hold the plants in place will do the job.

Ebb & Flow

A pump on a timer will flood your grow tray at intervals throughout the day. When the pump switches off the solution will then drain back into your reservoir. Because of the regular flow of solution you will find the growth media potentially moving or coming apart, so you’ll want to mix in a heavier medium like Coir.

A 50/50 mix of Coir and perlite are perfect for this end. If for some reason your pump or timer fails and you’re not there to jump in and fix it, your poor plants’ roots will dry quickly if you don’t use a medium with lots of retentiveness. Again, this is where Coir comes in handy (vermiculite or rockwool are also good), but use an inch or two of clay balls (LECA) at the base of your mix to prevent the Coir being washed away.


This system also operates on a pump and timer basis; except the solution from your reservoir is released through a drip line to the base of your plants. There are two versions of this system – Recovery and Non-Recovery. Quite simply, the former will return excess solution to your reservoir; the latter won’t.

A 50/50 mix of perlite and Coir will again suffice, and some growers will recommend an inch of clay balls at the top of your mix to cut down on algae.

Fancy putting your chosen substrate to use at home? First you’ll need to plan your indoor garden or grow room.


This article was originally published in Issue 002 of HYDROMAG (November – December 2012).

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