Orchid Potting Mediums

The three most important characteristics that all orchid potting mediums should possess are:

1. An ability to hold water.
2. An ability to hold nutrients.
3. An ability to allow the roots of an orchid a surface to cling to.

The Mediums

Fir Bark is the most popular orchid potting medium.  It is inexpensive, fairly light and easy to handle.  It has a rough surface and does not compact allowing air and water to be obtained by the plant's roots.

Fir bark is available in three grades:

1. Fine -- Used for mericlones (orchid seedlings) or mature plants with fine roots.
2. Medium -- Used for epiphytic orchids.
3. Coarse -- Used for vandas and large phalaenopsis orchids.

The drawbacks to fir bark are:

1. It decays and as it decays it begins to compact.  As it compacts it becomes more efficient at holding water than air and at this point, unless the orchid is repotted, its roots begin to rot.  Orchids potted in fir bark and grown in Florida should be repotted every year.
2. Fir bark "absorbs" nitrogen.  Actually, the micro-organisms responsible for fir bark's decay consume all the available nitrogen from the medium in the process of decaying the fir bark.  Since there is little nitrogen left for the plant, the use of high nitrogen fertilizers are required for orchids planted in fir bark.

Lava Rock is a very good growing medium for orchids.  It does not rot, is well aerated and retains water.  The only problem with lava rock is that it tends to accumulate mineral salts.  Do not use lava rock if your water contains large amounts of dissolved minerals.

Man-made Mediums such as expanded clay or expanded shale, like lava rock, are a very good growing medium for orchids.  They do not rot, are well aerated and retain water.   Their bad points are that they are expensive, difficult to obtain and because of their weight, costly to ship.

Perlite is a processed volcanic material most often used as an additive to other potting mediums.   Perlite is low-cost, holds water well and is decay resistant, thus it is a popular additive to fir bark.

Sphagnum Moss is a soft springy khaki-colored plant that grows on the surface of bogs.  It is capable of retaining up to 10 times its weight in water.  Sphagnum moss also contains an antiseptic which inhibits the growth of some fungi.

Sphagnum moss is available in the form of long or short fibers, alive or dried.   Live sphagnum moss is the best for orchids.  If not over watered or over fertilized, it will continue to grow after being placed in the orchid pot.  It is most often used as an additive to other potting mediums, though it is sometimes used alone.

Crushed Cork is another mixture additive.  It should not be used alone as it tends to break down quickly.

Peat Moss is what results when sphagnum moss dies and sinks into a bog.  It breaks down slowly to form peat.  Peat moss has an even greater water holding capacity than does sphagnum moss, but degrades quickly.  As an admixture to potting mixes peat helps to retain water, but should not be used as the sole potting medium as it is too dense to pot orchids in alone. 

Osmunda Fiber is from the roots of ferns in the genus Osmunda.  This medium has become quite expensive, thus is is not used as much as it used to be in orchid culture.

Tree Fern is the second most popular potting medium for orchids.  The fiber from tree ferns is resistant to decay and aerates well.  Tree fern fiber is the preferred medium in Florida as it stands up well to high humidity and warm temperatures.

Tree fern fiber tends to be rather expensive and does not retain water well.  For this reason many orchid culturists mix tree fern fiber with fir bark to make their dollars go further as well as the interval between waterings.

Redwood Bark is similar to fir bark, but is more resistant to decay.  Because redwood bark is imported into Florida from California, it costs more than tree fern fiber so it tends to be used more as an addition to potting mixes.

Charcoal made from hardwoods is added to cork or redwood bark potting mixes to absorb the acids from these highly acidic mediums.  Charcoal, like lava, collects mineral salts so avoid it if you have hard water.