LAVA ROCK as a potting medium... PROS & CONS

a post by Craig Morell (Pinecrest Gardens) and
Tom Kuligowski (Angraecums.blogspot.com)

 



       Although this page in the blog is dedicated strictly to the culture of various Angraecoids, I though it advantageous to look into the possibility of using lava rock as a potting medium  Most of the Angraecoids I grow are mounted to various hard woods, cork or tree fern material.
       With some very surprising results observed from growing a few other genera of orchids with lava rock; I thought it time to try a couple of my larger hybrids of Angraecums in lava rock as well.  While attending the Redlands Orchid Festival this past spring, I heard of a vendor that was selling Angraecum Memoria George Kennedy.  (The sad part of the story is that I never did find them.)
       I had the opportunity to tour an orchid grower's greenhouses while in Durham, North Carolina to give my Angraecums presentation to the Triangle Orchid Society.  In one of the greenhouses, I spot not one but four Angcm. Memoria George Kennedys; all four were in perfect shape and in six inch clay pots with nothing other than 100% red lava rock.  John Stanton, the owner of Orchid Trail Nursery, told me that he had just repotted the four plants several weeks prior to my visit.  I told him  the story of my search at the Redlands show and the fact that I came up empty.  To make a long story short, about ten days after my visit to North Carolina, I received a box via FedEx containing all four plants.
       As well as the plants had been packed, three of the pots were completely shattered.  Looking on the brighter side of this issue, I did get to inspect the root systems very closely and was amazed as to how much the roots had developed and actually flourished.  Keep in mind though that the plants are going through their peak growing period during the warm weather season.
       My decision to experiment with lava rock is based on the results I have observed after mounting a Dendrobium rigidum to a large piece of red lava rock.  This particular species is known to be a slow grower; but I was told to mount it to red lava rock; that Den. rigidum would take advantage of the iron content as well as the other minerals within the rock.  The plant has tripled in size in just eight months.

 

       When growing orchids, we must always keep in mind the sometimes overwhelming variables with which we deal.  Variables include types of fertilizers, pH of the irrigation water, light quality and intensity, as well as other chemicals we use on our orchids.  Of course, the types of media we use as potting materials and use as orchid mounts can make quite a difference.  In this post, lava rock is the medium I will investigate further.

       Information gathered over the last few weeks has given me a better understanding as to what is happening and what can happen when using lava rock.  I have not used anything other than the red type.  A geologist has confirmed that there is an iron content within the rock.  Needing to know what other minerals may be available to plants when exposed to this material was also important to determine the other avenues in which the plants may possibly thrive.

       I find it ironic that the first three minerals listed that are found in red lava rock are nitrogen, phosphorus and potassium (the three main components represented by the three numbers you find on plant fertilizers' containers).  Other minerals are zinc, iron, copper, magnesium, boron, sulfate, calcium, manganese and sodium.  The above minerals are not in a form that can be absorbed by the plant.  A plant's root system releases organic acids such as citric acid and oxalic acid.  These acids turn the solid minerals into a form that can be absorbed and used by the plant.  To the average lay person or hobbyist, this lone paragraph would lead us to believe that the minerals contained in red lava rock would give our plants a jump start in their growing patterns.  This can be very true; but as I said earlier, there are "...overwhelming variables with which we deal".

       There are numerous considerations in choosing red lava rock as a potting medium for your orchids.  Depending on where you live, you may not have good access to the medium.  The lava rock should be washed thoroughly before using; it has a LOT of fine dust particles which can affect the orchid roots badly, as well as make a mess underneath the pot or basket after watering.  Irrigation water pH will make a large impact on whether the medium is successful or not.  High pH water (over 8) will eventually lead to calcium buildup on the lava rock, as well as make many of the minerals and metals contained in the lava rock unavailable to the plant.

       Many of the Cymbidium tribe of orchids grows beautifully in lava rock; but the combined weight of the rock and a large plant preclude it from being used as a medium in hanging baskets.  One of the major down-sides to lava rock is that it will hold a lot of water on the center of the pot; especially in pots over 8" in diameter.  Many growers will use an inverted plastic pot in the center of the larger pot to allow extra airspace in the center of the pot when using lava rock to further air and water drainage.

       Various growers report differences in lava rock from year to year; in some cases, orchid roots will turn away from the medium and in other batches of rock, roots will flourish in the rock.  Growers do report that certain genera seem to disfavor the lava rock.  Brassavola seem to shy away from it, as do some terrestrials.  Growers of Aroids (such as Philodendron, Anthurium, Monstera etc.) find lava rock to be an ideal medium and in Hawaii it is the favorite medium for stock plant production of many flowering plants.

       If nothing else other than giving a plant with a fairly thick (velum material) root system; the roots will have a quick drainage when watered and ample space to breath.  My suggestion is to talk to someone that has had experience with lave rock as a medium.  Keep in mid, what works for one may be detrimental to someone else.

       For those of you that are using lava rock now or have in the past; please leave your experiences and thoughts to the matter in the comments section just below this post.  Thank you all!