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A commonly uttered nonsense has it that media-based grow beds must be at least 300mm (12 inches) deep and, I have to say, that this is one of my favourite aquaponics myths.

How did this idea come about?

Probably the person most responsible for the current interest in gravel grow beds was aquaponics pioneer Tom Speraneo.

While he wasn’t the first to use media-based grow beds (hydroponicists had been using them for decades) and he wasn’t even the first to use them in aquaponics (UVI had tried and rejected them several years earlier), he made them work  for his purposes and he popularised their use.

The early promotion of aquaponics in Australia had a fundamentalist flavour about it so 300mm gravel grow beds became something of a mantra.

Much more recently, local aquaponics kit suppliers built grow bed products that were premised on the notion that they should be 300mm deep and this has further consolidated (indeed intensified) the dogma around grow bed depth.   One such supplier states:

“It has been found that grow beds must be 300mm deep as grow beds less than 300mm deep do not support bacteria growth properly resulting in a lack of nutrient processing, poor plant growth and water returning to the fish tank un filtered.

Utter nonsense!

Let me be clear…..I’m not saying that 300mm grow beds don’t work.   My own experience (and that of hundreds of others) evidences that they do work.

What I am saying is that there is nothing magic about the figure of 300mm (12 inches) – they can be deeper or shallower.

Some of the more common arguments in support of 300mm+ grow beds include:

  • Greater temperature stability
  • More effective nitrification
  • Greater plant support

It’s logical that the greater the mass of a grow bed, the more stable the temperature is likely to be.

My response to this perceived benefit, however, is that there are much more cost effective ways to achieve temperature stability.

The claim of more effective nitrification in 300mm+ grow beds ignores the fact that so long as (to ensure the ongoing health of the nitrifying bacteria) the media remains moist and at a consistent temperature between watering cycles, nitrification will occur……regardless of the media depth.

The notion that deeper grow beds offer greater plant root support has seems reasonable enough on the surface.  Mature sweetcorn plants, for example, require plenty of root support if they are to resist being blown over in a stiff breeze.   Upon closer examination, however, plant support is probably more a question of media type than grow bed depth.

Gravel is certainly the preferred media for growing sweetcorn….and 150mm of gravel will support sweetcorn just as effectively as 300mm.  Expanded clay is much less effective for use with sweetcorn – not even 300mm is enough to keep it upright when the wind blows.

Even if there was validity in the plant support argument, operating 300mm+ grow beds simply so that you can grow sweetcorn fails the cost effectiveness test.

The 300mm+ crowd point to the numbers of 300mm grow beds that are in existence as evidence of their superiority.  Given the single-mindedness and vigour with which they are promoted by the suppliers (who offer no other option), this is the silliest argument of all.

For practical purposes, I have found that properly managed grow beds can be as little as 75mm – and will work just as well as deeper ones.

Moreover, there are some benefits to be gained from using shallow grow beds and they include:

  • Cleaning shallow grow beds is a much easier task than it is with 300mm+ ones.  Regardless of what you are told, all media-based grow beds will require cleaning sooner of later.  Some of those touted as being examples of how they don’t need cleaning are already long overdue for renovation.
  • Shallower grow beds are cheaper to make than deep ones…..and they cost less to buy.
  • If you are using expanded clay media, filling a 150mm grow bed will cost half as much as filling a 300mm one.
  • Alternatively, you will get twice the growing surface for the same overall volume of media.
  • A 150mm grow bed will weigh a bit over half as much as a 300mm one.  The support framework for a shallow grow bed can be made of less robust materials because it doesn’t need to support the same weight.

To summarise…….the arguments to support the contention that grow beds must be 300mm+ are not particularly compelling…….particularly in light of the advantages of using shallower ones.