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! 



Eurychone rothschildiana

       Eurychone rothschildiana was formerly know as Angraecum rothschildiana (still considered the synonym).  The plant originates in Tropical West Africa and east to Uganda and Zaire.  Commonly found near small amounts of moving water and at elevations up to 3900 feet (1200m) in evergreen forests.

Eurychone rothschildiana (1st time with buds)

 

       The plant pictured above is an older seedling that is in bud for the first time.  Having a group of three seedlings, two were mounted to rough cork slabs (average size about 6 x 4 inches (15x10cm) in height and width and one was potted in a three inch (8cm) clay pot.  Those mounted to the cork slabs were laid on a loose and thin layer of moss and covered by small pcs. of coconut tree fiber to protect the outer most roots.  The sole plant that was potted is in a mostly non-organic mix with about a 10% amount of medium coconut pieces.  The entire mix is medium in size for fast drainage letting the root system partially dry after watering.

       Eurychone rothschildiana is a compact plant that will have a short monopodial stem.  Specimen plants will rarely have a stem reaching 4 inches (10cm).  There can be anywhere from 3 to 8 broad egg shaped leaves that are up to 8 inches (20cm) long and 2.8 inches (7cm) wide.  As with all Angraecoids, the leaves are distichously placed along the stem.  They have a leathery substance and range from a medium to dark green.  The roots are .1 to .2 inches (.3-.4cm) thick.

       Depending on the climate in which you are growing Echn. rothschildiana; special attention should be made to the temperatures that the plants are exposed to.  In its natural habitat, the warmest daytime temps average only 81 degrees F (27 degrees C) while in cooler months the average daytime temperature is 71 degrees F (22 degrees C).  This plant should do well in temperate climates within greenhouses that can keep this range along with a relative humidity range of 71% - 84% year round with the higher humidity levels being during the summer months.  Here in sub-tropical South Florida, while plants are grown in open aired greenhouses or arbors, it is important to keep the plants cooler during the extreme warm months of late spring into early fall (almost a six month period of temperatures 10 - 15 degrees F (8 degrees C) higher than that of the plants natural habitat.  This can be accomplished with a constant strong air movement.

 

Echn. rothschildiana

 

       The active growing season can last up to ten months.  Only coming to a rest period during a short winter dry period.  During the active growing season, plants should be watered very regularly.  Mounted plants during the hot summer months up to twice a day; which can help with the humidity levels.  Potted plants should be watered every two to three days as long as the drainage is fast.  You must also be careful not to let water sit in the crown for any length of time.  Giving the root system an opportunity to slightly dry.  During the two month rest period, the plants should be watered enough to keep them from drying completely.

       I use a standard 20-20-20 fertilizer every 7 days during the active growing season while backing off to every three weeks for the two month rest period.  A systemic fungicide such as Thyomil is used to treat the entire plant as well as the root system and is switched to Dithane 45 every third month to prevent a resistance build up.  There is always the possibility of minor issues arising so a quart spray bottle with Physan 20 is kept on hand for spot problems.  Keeping a constant air flow also helps prevent serious problems from developing with fungus.

       The amount of light that Echn. rothschildiana is given ranges from 1000fc - 1800fc.  Both of the mounted plants are in an area that is actually a bit lower.  They receive absolutely NO direct sunlight during any part of the day; in other words they are in what I consider a low Phalaenopsis type of light.

 

Eurychone rothschildiana

 

       Eurychone rothschildiana will bloom here in the northern hemisphere throughout most of the year.  It is most likely to bloom in July and least likely to in the cooler winter months.  Mature plants can bloom at any point though.  The inflorescence is pendant like and will range in length from 1.2 - 3.5 inches (3-9cm) and can carry 3 - 6 flowers.  Mature specimen plants have been know to have as many as a dozen blooms.  The blooms are fragrant and have white to light green sepals and petals with the lip being white edges turning to green and then a dark maroon or brown throat.  The flowers average just under 1.25 inches (3cm) wide by 1 inch (2.5cm) high.  The lip can be 1.1 inches (2.7cm) long by .8 - 1.0 inches (2-2.5cm) wide.  It is very concave and scalloped around its edges.  The spur is narrow in the middle and swells at the tip; its length is .8 - 1.0 inches (2-2.5cm) long.

       As mentioned earlier in this post, Echn. rothschildiana is a compact plant.  Potted or mounted, it will do well in a sub-tropical climate as well as the colder climates in the northern hemisphere.  Be well aware of the fact that it will freeze if placed to close to a window during harsh winters. 

       There is one oddity about this plant you should realize.  It is known to be short lived.  A healthy mature plant can die suddenly without warning.  I have not experienced this issue myself; but have had several discussions with Craig Morell, the horticulturalist at Pinecrest Gardens in Miami, Florida.  He has had several well cared for mature specimens die suddenly for no apparent reason.

       One last note: Eurychone rothschildiana has been crossed with Aerangis and Angraecums to create at least three hybrids to date; Euryangis Victoria Nile, Eurygraecum Lydia and Eurygraecum Walnut Valley.  The last two being created by Fred Hillerman.