Growing Media - Part 2

By Joseph Farrugia

(click here to read part 1)

Basic materials for Growing Media.

A growing medium is any type of material used for growing plants. Technically it is used for the root system of the plant to anchor itself and to extract nutrients. A substrate is a free flowing solid growing medium. So a rigid medium or water in a container, are both disqualified as substrates.

Sphagnum Peat

By far the most popular and versatile growing medium all over the world is Sphagnum peat.

The source of this peat is the Sphagnum plant. This plant grows mainly in the northern hemisphere around and above 52º Latitude. Places like northern Germany, Ireland, Poland and the Baltic States in Europe, and Canada, Russia and parts of the US elsewhere are ideal for this plant to flourish. In the southern hemisphere, this plant is found in limited supplies in New Zealand, Tasmania, South Africa and southern Argentina.

Thousands of years ago, after the ice sheets of the last Ice Age retreated back north, vast barren silthy lands were newly exposed. Water and mud gathered in places where there were glacial moraine had formed depressions on the surface of the earth. Deep depressions led to the formation of lakes while shallower ones led to the development of wetlands. The Sphagnum moss was quick to take root and flourish in this favourable environment and soon vast tracts of the northern lands were turned into Sphagnum bogs..

After each successive growing season, the dead Sphagnum plants collapsed into the marsh waters. In this anaerobic (oxygen starved) environment, the dead plant material was unable to decompose as decay bacteria need oxygen to survive and work. Year after year, dead plants accumulated at the bottom of the bog. Since dead Sphagnum is acidic, the level of acidity (or pH) of the bog or fen rose slowly. With the rise in acidity, other plants and trees which were semi-acid loving (pH 6-6.5) and acid hating (pH 6.5 to 8.0) were slowly eradicated. In time, only Sphagnum and other ericaceous plants which could flourish in pH 3.0 acidity survived. For thousands of years, these semi-mono-culture bogs, rose to heights of sometimes more than 10 meters with layer upon layer of peat into which a new water table evolved. Raised bogs were formed where the upper level of the peat exceeded the original level of the water and the surrounding lands. Blanket bogs which cover wide areas of hillsides were aided by human action when Neolithic man cleared trees for agriculture and farming

Now the beauty of the Sphagnum plant is that the cells that make up the plant, because of the presence of phenolic compounds and because of the way the cell walls are built, can hold incredible amounts of water inside them, after the contents of these cells have leached into their surroundings. The dead cells, with their cell walls intact, can hold as much as 20 times their weight in water. Due to their acidic environment, they are usually free from weed seeds and other pathogens. It is thus obvious, that such a material, once harvested and treated will make an idea base for a growing medium.
Peat bogs – Poland
(Photo from


Peat Milling – Riga, Latvia

Peat bogs are drained and levelled before harvesting could begin. In places like Ireland, the top layer of the bog to a depth of 50cm, containing the live seeds is put aside and returned to the site when the usable peat is exhausted, thus facilitating the rehabilitation of the area. Bogs are drained by digging a grid of 1 meter deep trenches in the area of the bog earmarked for harvesting. When the field is well drained the peat is harvested in one of two ways, milled or sod. Milled peat involves scraping 2 to 3 cm of the surface of the bog with a special milling machine and piling it into heaps for further drying in the field. This method results in a peat with a greater amount of fine particles than sod peat.

Sod Peat – Riga, Latvia.

Sod peat is harvested by levering blocks roughly the size of a stone block used by Maltese masons and stacking them in rows and leaving them on the bog to freeze over the winter. The dynamic action of freezing and thawing opens the densely compressed peat mass. The following summer the sods are picked up and placed in stock piles. Nowadays sods are cut by a special mechanized sod cutter. This work is very dependent upon weather conditions, so peat output is very erratic and not very uniform. If the supplier is small and does not own fields in different areas or countries from where he can deliver a constant supply, the buyer in Malta might get sub-standard peat which has been lying in storage or which is acquired by the supplier from third parties. So, again please do buy from reliable sources.

There are two types of peat, black and blond. Black peat comes from bogs that contain large quantities of “Carex” and other plants, together with Sphagnum, although this is not a rule as places like Ireland have large areas covered with black sphagnum peat bogs. In Europe, these peat bogs are found mainly in northern Germany and Ireland. Black peat is semi-decomposed and generally older than blond peat and releases small amounts of salt. Being such, the cell walls are less intact than blond peat and of poorer quality plant material, which reduces their ability to hold water especially so for peat that is harvested from the bottom part of the bog. Blond peat on the other hand, is found in the Baltic states, Poland, Sweden, Norway and parts of Finland together with the steppes of Russia. This type of peat is less decomposed, is much younger, and has a lower salt content. Peat decomposition is measure in the Von Post scale which ranges from H1 for un-decomposed peat to H10 for fully decomposed peat. Typically blond peat ranges between H1 to H3 and black peat between H3 and H6. Other types of peat like reed, sedge, hemic and septic peats are usually between H4 and H10 but most of these latter peats are useless or nearly so for agricultural uses. It should also be noted that for hundreds of years peat has also be used as a fuel. In fact in some places, power stations as still fuelled by peat which is sub-standard for agricultural use.

In short, the useful life of black peat does not exceed 4 to 5 months while that of blond peat can be up to a year. So if you want to plant something for quick sale from your nursery, and provide that plant with intrinsic nutrition from the growing medium, and prolong or restrain drainage, use black peat as a base. Inversely, if you are not re-potting your plant before one year, and would like a wet but free draining medium, then you would want to use a blond peat as your primary ingredient. Remember that decomposed or decomposing matter hinders the free draining of water and the intake of oxygen. One has also to keep in mind that good peats are graded, it is not advisable to buy un-graded peat as this will invariably consist of leftovers from other peat products. Peat grades are measured in “fractions”, peat for seedlings is 0-5 where the numbers represent the size of the particles in mm. Potting peat will be 0-5 or 0-10 or a mixture of both. There are other fractions like 0-20 for large pots exceeding 20 litres capacity (opinions may differ on this, as some producers recommend fraction >10 for 10cm pots) and even 10-20 or 10-40 for ground mixing as a soil amendment.

Blond Peat milling and piling Kaunas Region – Lithuania.
(Photo – Author)

Another factor affecting the viability of peat is CEC or Cation Exchange Capacity, but this will be discussed in a later article.

That basically covers all you need to know about peat except for one last detail. As we have seen above, peat starts to slowly decompose immediately after leaving the anaerobic environment of the bog, so always buy fresh peat from a reputable supplier, (fresh peat smells sweet, while old peat smells sour) and never reuse used peat from re-potted plants.

Finished baled product prepared on pallets for export. – Riga, Latvia.
(Photo – Author)

Coir (or Coco-peat)

Coir is a by-product of the coconut industry. The shell of the coconut is crushed and while the fibres are used for carpets and other weaves, the pit is collected, washed and pressed into coir blocks. Coir has a very stable structure and will usually take about 4 years or more to de-compose, it also has an ideal pH of 6.0 to 6.5 which means that it can be readily used without the need of liming (pH and liming will also be discussed later). The major setbacks of coir is that it has only 1 grade and that it tends to be salty if the quality of the product is not up to standard. Coir, coming as it does from the far east, is very tricky and unreliable due to the possible content of salt and pathogens, and the ones that are available in Malta are nearly always of cheap quality and very difficult to judge. Personally, I keep them at arm’s length.

Compost (or humus)

As the name implies, this medium comes from composted plant matter. It is a by-product, with raw material collected from huge processors of vegetables and other field grown matter. Compost is rich in nutrients and organic matter as it still possesses nearly all the essential elements required for plant nutrition since it is intrinsically plant material itself. Composting is the controlled breaking down of organic matter by the action of bacteria while preventing it from rotting. As one can imagine, compost is heavily decomposed and will

not last long if used as the sole ingredient of a potting mix. Initially it is free draining but very soon it will start clogging, blocking the drainage and generally turning sour. It would maybe be more appropriate to treat it as an additive rather than as a base.

Rock-wool (Stone-wool)

Rock-wool is formed by super-heating a type of rock to about 1600C and turning it into a mass of fibres with typical diameters of 6 to 10 micrometers. The process is not dissimilar to that of a candy-floss machine involving a centrifuge.

This wool is formed into blocks and used as is in green houses. It could be said, and rightly so, that this is a hydroponics medium rather than a substrate. Hydroponics is a system of growing where the roots of the plant are continuously immersed in a mineral nutrient solution. The solution is usually flowing, temperature controlled, oxygenated and of course pre-fertilized.

Expanded Clay Pallets

These pallets are made from clay which is heated and popped into pallets. These pallets range from a diameter of 5mm to that of 12mm, they are either spherical or irregular (crushed).

When used dry they have to be mixed with a finer medium to fill in the large gaps between the pallets, but when used in hydroponics it is used alone. Clay pallets have a very high CEC and retain a lot of water while being very light. They are ideal for use in roof gardens where weight could present a problem.


Although revered as the king of substrates in Bonsai circles, in climatic conditions such as ours, where the sun relentlessly beats down on us all through the summer, and sometimes even the winter, it is not highly recommended especially for the beginner. I have personally tried using it and while it was fine during the rainy season, it became very difficult to maintain during hot weather. This is a pity, because it has all the goodies one expects a substrate to have. It is light, retains water, has a high CEC, neutral pH, free draining and what’s more it is clean and looks great around the bonsai tree. But alas, unlike in Japan, rain here is the exception and not the rule so you need extra care to keep your trees thriving in this medium. When Bonsai Master Salvatore Liporace visited us recently he strongly recommended this medium for advanced bonsai and so a number of our members are experimenting with this medium in our climate's conditions we are eager to see if we can find the secret to sucess.

Click to Read Part 3