Rock Wool + LECA

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Ray

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A long post, but I wanted to explain the evolution of a concept, in case anyone cares to do some experimenting

Three or four decades ago, I traveled 6-7 days a week, making it difficult to keep up with my plants, and that’s what led to the development of semi-hydroponic culture for orchids. Paphs were my first “Guinea pigs” and they did so well for me, it encouraged me to expand to other genera. All of my slippers have been growing that way since, as were most of the rest of my collection.

However, as I no longer have a nice, warm, humid greenhouse and have returned, after 40 years, to in-home growing, I am now experiencing temperature and humidity issues - in the winter, when the plants are indoors and the thermostat is moved lower to save energy, the evaporative cooling from the moist, open LECA pushes the root zone temperature too low for hot growers like phalaenopsis, so I took them out of S/H and moved them into straight sphagnum, which they seem to love, but I don’t. Since I grow outdoors for 7-8 months a year, the heat, humidity, and frequent watering means I have to replace it at least every 6 months. So...I thought I’d experiment with rock wool cubes as a longer-lasting sphagnum substitute.

Grodan Mini Cubes are made by melting basalt and spinning the fiberglass, compacting it to a fixed density, and slitting them into 1 cm cubes. As long as you don’t pack them tightly, they stay open and airy while holding a LOT of water, and the phals took nicely to it. That gave me an idea about a possible cure to the “S/H Dry Line” - the phenomenon in which the evaporation of the water from the LECA exceeds the wicking rate, leading the top to be totally dry, with an apparent “dry line” moving down into the pot.

If you cannot raise the humidity to slow the evaporation, it is possible to place a “barrier” on the top surface of the LECA to do so. Some use sphagnum, some aquarium gravel, others sheets of plastic - anything to slow the evaporation from the “rocks”. I figured the rock wool cubes would also work, but I wondered if having little “reservoirs” scattered throughout the LECA might solve the problem.

For the last year, I have experimented with 100% cubes or a 50/50 blend of cubes and LECA. As of now, all of my paphs and A few phrags are in that blend either as a semi-hydro medium or as one for traditional culture (free drainage with no reservoir) and I am very pleased with the results. Plants currently in straight cubes wil get moved to the blend and it is a great deal airier.

Some folks are experimenting with the mix ratio while others have potted in layers - 2” LECA, 1/2”-1” cubes, more LECA, more cubes, etc., and are reporting a great deal of success. Considering how paphs living in leaf litter on the forest floor tend to grow their roots laterally, it occurs to me that a “layered” medium might be a particular benefit, but I have not tried it at this point.

For folks wishing to experiment, I think lowering the percentage of cubes in the blend is a good way to go - maybe 25%-33%?

A practical tip: when mixing rock wool cubes with LECA, make sure the cubes are saturated first. They are of similar size to the LECA, so blend well, but if they are dry, they tend to “float” to the top when hand mixing before use.
 

Ray

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Every plant I own is in inorganic media, using one of these configurations:
  • S/H using 100% LECA
  • S/H using 50/50 LECA & rock wool (will probably reduce the rock wool content over time)
  • Traditional culture using 50/50 LECA & rock wool
  • Traditional culture using 100% rock wool - these will be transferred to the 50/50 blend.
 

littlefrog

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I've had good luck with 50% fir bark and 50% Flexi-mix (an inorganic, polyurethane foam kind of stuff I think - looks like chunky peat but isn't). I love the fleximix. But evidently I can't get it anymore, so now I have to find something else. I've done a few experiments with rockwool, seems to work pretty well.
 

terryros

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Ray, I am visualizing that a layer of rock wool cubes over LECA might be a bit of an inverse SH situation. Would the rock wool cubes be a small reservoir of liquid that would be gradually released to flow down through the LECA? More than just a barrier to liquid loss in the top part of the LECA?
 

Ray

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Terry, I think the opposite is true.

Ignoring the minor amount of evaporation through the drainage holes, all of the water losses are via evaporation from the top. When the cube layer is wet, the evaporation occurs from it, and the water in the LECA below is pretty much blocked from evaporating.

Yes, if you water only enough to saturate the rock wool and drip down a bit, that slows the wetting below, but that is a mistake in watering, not a problem with the setup.
 

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