Ingredients to Use when Making a Potting Mix for Orchids

EPIPHYTIC ORCHIDS GENERALLY need a coarse, open, well-drained growing medium that is somewhat, yet not too, moisture retentive. Over the decades, numerous materials have been employed for the purpose, with varying degrees of success. While each grower has his preference, it is safe to say that a growing medium must suit your cultural conditions — the quality and amount of light, temperature, humidity and air movement in your growing area. You may also have to adjust your growing medium to suit your watering practices and the types of orchids you cultivate.

One medium is not correct for every grower. Generally, large plants with big roots need coarser mixes than those with finer roots. Also, young plants sometimes need a mix that is more moisture retentive than a mature plant of the same type would require.
Here are some of the more usual alternatives and remarks about them:

ALIFLOR  This is one of several expanded-clay products used by the concrete industry for manufacturing blocks. It is a quick-draining medium that is neutral in pH and provides no nutritional value to the plants. I have noticed that many Florida growers seem to favor such media, presumably because they protect their potted orchids from a soggy root system during the rainy season, plus they last forever.

BARK  Fir bark is the most commonly employed. It is available in small, medium and coarse grades. This is probably the most popular medium at present and is frequently used in mixes. When fresh, it tends to dry quickly; after it ages and begins to break down, it can stay damp too long. It will usually stay in good condition for a year or two.

CHARCOAL  This one is a popular ingredient in mixes for adding drainage and reputedly has the ability to help keep the medium fresh by absorbing toxins. I have even seen orchids happily growing in 100 percent charcoal chunks. Various sizes of chunks are available and should be selected to suit the scale of the plant and its root system. It will last for years.

COCONUT HUSK  These chunks are available in small, medium and coarse grades and provide an alternative to bark in mixes or can be used alone. Compared with bark, coconut husk is more moisture retentive and longer lasting. Its somewhat spongy texture when moist can allow it to be easily packed too tightly around the roots, especially when used alone.

COIR  This material comprises the fibers of the coconut husk and can be used by itself as an orchid medium. It is gently tucked into the pot and around the roots, somewhat as you would long-fiber sphagnum, and is particularly useful with plants that insist on good drainage. It is not widely available or especially popular, but I have used it occasionally with success.

CORK  Cork bark slabs are useful for mounting orchids. Wine corks can be recycled in coarse mixes and used alone in pots or wooden baskets for thick-rooted species such as those in the Vanda Alliance. Cork lasts for years and is, of course, light in weight.

LAVA ROCK  This porous, in-organic medium is an alternative to the Aliflor type of material and may be more readily available in some regions. Choose this if you want a fast-draining medium that is not too moisture retentive. The pieces should not be sharp and can often be found in a variety of sizes.

OSMUNDA FIBER  This is the root mass from a fern and was the medium of choice in the decades before bark came into popular use. It usually comes in chunks and is typically moistened and gently packed around the root zone. It is rather moisture retentive. I tried it once for the sake of experience and cannot say that the chosen orchid was happy about it.

PERLITE  This is a white, natural, long-lasting product created with extreme heat. It is light in weight and is popular in mixes for providing porosity and texture. I generally prefer the largest chunks I can find, which aren’t much larger than peas. 

ROCKWOOL  I have never tried this and do not know anyone who has. That is not to say it does not have its following. It is written that it can be used in pieces or as a mat. It sounds like it is best for situations where you need an indestructible medium with considerable capillary moisture available. I prefer organic and natural alternatives.

SPHAGNUM MOSS  The long-fiber New Zealand sphagnum is best. It holds many times its weight in water. A bit of practice may be required to learn to pack it properly when used alone. It can add moisture retentiveness to mixes. Some orchids seem to thrive in it and a few commercial growers seem to favor its exclusive use for getting young plants off to a good start.

STYROFOAM  Those weightless, nearly indestructible “peanuts” that support the packing industry often seem to find their way into the bottom of orchid pots these days as drainage material, where they appear to do a good job. They provide plenty of air space and do not hold water. I hate to see them turn up anywhere, but applaud the recycling effort.

TREE FERN  This is the dry, durable root of tree ferns that is used primarily in mixes, or as porous slabs that are used similarly to cork for mounting orchids. It is not widely available but certain orchids seem to love it. I like to add some of the broken fibers to my potting mix for cattleyas.

When working with these in-gredients, it is often wise to wear a mask to avoid inhaling dust, especially when working with perlite and rock wol, as they are hazardous to the respiratory tract.

CONTAINERS  Of course, you cannot divorce your considerations for the best growing medium from your preference for a particular type of container. Plastic and glazed ceramic containers are moisture retentive, while clay pots “breathe” and help draw moisture away from the growing medium. Any one mix would behave quite differently in each.

Even such variables as how tightly a grower packs the medium around the roots when repotting affects the amount of air space in the pot and its water-holding capacity. Almost all of these media should be rinsed thoroughly before use. The organic ones usually require soaking for anywhere from a few minutes to overnight. Knowing the characteristics of various types of growing media and evaluating the ingredients for mixes, coupled with careful observation of your plants, should help you develop a predictably successful medium for your own orchid collection.