To fully understand the answer to this question one must first understand a fungus – plant relationship called “mycorrhiza association” , mycorrhiza is a nutritional relationship between certain fungi and the roots of plants including orchids. Orchids that are saprophytic depend almost entirely on this relationship for their nutrition while others have no real need for this relationship at all.

Mycelium’s are fungal strands – endotrophic mycelium’s grow within the plant whereas ectotrophic mycelium covers the roots or tubers externally but do not enter it. There are two different types of endotrophic mycelium’s relevant to orchid plants:
(i) Tolypophagy where the fungal coils of the mycelium enters the roots and are killed and digested by the plant.
(ii) Ptyophagy where only the tips of the fungus enter the digestive cells of the orchid roots and the fungal mycelia’s contents are squirted into the digestive cells of the orchid plant.

Whether the fungus receives carbohydrates or other useful nutrients in return from the orchid for supplying these inorganic nutrients to the orchid seems hard to discover. It is known however, that the fungal mycelium consists of protein, carbohydrates and oil, so when this is digested by the orchid plant more than just inorganic nutrients are obtained.

So why treat bark?
It is known that not all forest trees support orchid growth. Some trees can support populations of orchids while an adjacent tree can be devoid of any orchid plants. It is however, known that some barks exude phenolic substances such as tannic, elegiac and gallic acids and these are suspected of being toxic to orchid seed and the growth of the orchid plant. It is more likely however, that these phenolic substances are inhibitory to the growth of fungi particularly the mycelium fungi. The reason therefore for treating bark prior to its use is:-

(i) To remove any of these phenolic substances that maybe present.
(ii) To feed the micro-organisms present and commence a healthy growth of mycelium fungus to live with the orchid from the time of repotting.
(iii) To leave within the bark structure traces of nutrients such as iron, phosphorus and magnesium in a soluble form that the plant can take up as it grows.
(iv) To correct the pH to the ideal range for orchid culture.

These mycelium fungi are “Good fungi” and have no connection with the pathogen that causes the form of bacterial pneumonia known as “legionnaires Disease”.

Therefore the answers sought are:
1. Treated bark is bark that has been inorganically fed with a suitable blend of fertilising chemicals to promote a healthy growth of fungi prior to its use, thereby leaving it in what is believed to be the best suitable condition for orchid root growth. This treatment promotes the growth of these fungi without accelerating or affecting the normal decomposition of the bark itself.

2. Composted bark is bark that has had its decomposition commenced or accelerated by chemical means to achieve the same result. This means that the breakdown of the bark medium has been commenced and the useful life of that medium is greatly reduced. Orchids grown in composted bark must therefore be repotted more frequently.

3. Treated or composted bark can be purchased from specialised orchid nurseries or the hobbyist can save a few dollars and do the bark treating themself. Large quantities done by the nurseries take approximately four weeks to complete the process but the hobbyist can do the treating in one to two weeks.

To treat bark at home it is a matter of buying a bag of bark and a tub of preferred treatment, (the tub treats one 50-litre bag of bark). Put the bark into a suitable water container, (many people use their Otto bin or a 75-litre garbage bin) tip the chemicals in and cover the bark with water.

Allow this to stand for one to two weeks then thoroughly rinse in fresh water. The treated bark is then ready to use.

Many People believe that treating the bark makes a difference whereas others says it is of no advantage. The best would be for an orchid grower to obtain some treated bark and try it for themselves and make their own decision as to whether they feel it makes a significant difference under their own growing conditions.


from an article in the “Eastwood & District Orchid Circle News”
Sydney, Australia