COCO HUSK as a POTTING MEDIUM by Brian Milligan

Coco husk, the chopped, dry outer coating of the coconut, has become a popular potting medium over the last few years. The majority of commercial growers in Europe have also changed to coco husk, and many cymbidium growers in Australia are also claiming great success with its use. However, growers of other genera have had mixed success here – a well known Victorian paphiopedilum grower found that his slipper orchids did not grow as well in coco husk as in pine bark, while others have been unhappy with the performance of their phragmipediums in coco husk. This article relates some of my experiences with coco husk as a potting medium for a range of orchids.

Coco husk is marketed in compressed blocks of dried material. Before use it must be soaked in water, which causes the coco husk to swell enormously in volume. Considerable quantities of salt are released in the process, sufficient to kill many orchids if not removed. I soak the coco husk in water overnight, then decant the slurry through a sieve, and repeat the process twice more (soaking overnight each time), after which the water contains a negligible quantity of salts. A cymbidium grower has told me that this is over-kill, and that I am removing some useful potassium salts in the process but I believe that it’s better to remove all (or most) of the salts present, and then to replace any ‘good’ salts by application of liquid fertilisers.

I have found that the quality of coco husk varies from batch to batch. Most coco husk floats when it is soaked to remove salts, and I prefer to use this material. Some batches contain a considerable amount of material that sinks, and I discard this, as it retains too much water when used as a potting mix. Initially I blended the coco husk with 20% (by volume) of perlite, in line with an early recommendation. However, I found that it gave no better results than coco husk alone, and eventually gave up adding perlite, as it roughly doubled the cost of the potting mix.
A major advantage of coco husk over bark is that it is much easier on the hands at re-potting time, and much more easily removed from the roots than bark. Also, it does not deteriorate as rapidly as bark does, and therefore re-potting should not need to be carried out so frequently. Certainly, in the 18 months that I have been using coco husk as a potting medium, no appreciable deterioration appears to have taken place.

A possible disadvantage of coco husk is that it retains water much longer than does pine bark. In terms of water conservation this may be a good thing but only if plants grown in coco husk are kept separate from those grown in bark, and watered separately. Those growers who use sphagnum moss as a potting medium will already have encountered a similar problem. Probably coco husk falls between bark and moss in its ability to retain water. A useful tip for those who are unsure when their orchids need water is to keep several ‘control’ pots containing only coco husk (or bark, or moss) in their collection, and to water them at the same time as the orchids. Simply upend these pots to determine whether the material near the bottom of the pot is wet or dry, and therefore, whether your orchids need to be watered again or not.

After 18 months using coco husk/perlite mix (4:1 by volume) and coco husk alone as potting media, I am pleased with the results in some cases, less so in others. Best results have been obtained with cymbidiums and lycastes. All of the cymbidiums that I checked had good, healthy roots, including those back-bulbs that I removed during re-potting. The presence of perlite in the mix seemed to make no difference to the orchids’ growth. Those lycaste hybrids that I had potted in coco husk/perlite mix did as well or better than those potted in bark. A pair of LycasteShoalhaven back-bulbs, potted in coco husk had leaves 200 mm tall and a well developed root system within five months – remarkable results for such a short time. On the other hand, one pair of Lycaste Macama back-bulbs potted at the same time failed to strike, while the new growth from a second pair was only 20 mm tall. However, that’s about par for the course – in my experience a 50% strike rate is as well as can be expected for lycastes, provided that one uses pairs of pseudobulbs; it’s much lower if one uses single bulbs.

My greatest success has been achieved with the Madagascan species Angraecum magdalenae, which has struggled ever since I acquired it six years ago. Since potting it in coco husk/perlite a year ago it has developed two new basal growths, while the main plant has flowered for the first time, and the pot is now filled with new healthy roots. I suspect that I have not provided it with sufficient water in previous times. Several coelogyne species have also done well when potted in coco husk, especially those that spent the winter in my heated glasshouse. The New Guinea native Dendrobium lawesii is also doing well in the new mix, while others have obtained similar results with Dendrobium victoriae-reginae potted in coco husk.

Results with masdevallias were mixed. Masdevallia Parlatoriana, which is a vigorous grower, thrived and flowered prolifically but other, less vigorous, masdevallias seemed to struggle in coco husk/perlite, and one or two died. There seems to me to be no advantage in transferring the rest of my masdevallias from my traditional bark/sphagnum moss or moss/polystyrene foam potting mixes. Australian native dendrobiums have grown reasonably well in coco husk but the mix remained wet for too long after watering during winter. The same applies to soft-cane dendrobiums – their keikis have grown well but need to be watered very sparingly during winter.

Please bear in mind that the above results apply to orchids grown for only 12-18 months in the new mix. Whether those orchids that are growing well continue to do so in future remains to be seen. After all, most potting mixes give good results for the first few months before beginning to deteriorate, sometimes becoming acidic with deleterious effects to many orchids. My recommendation is to try a few orchids in the new mix for a year or two first, and not to undertake a major re-potting program until you are happy with the reults.