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.
 

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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.
 
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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.
 

eds

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Ray,
Have you tried any of the grodan chunks, the larger pieces? (I have bought a bag of both sizes to experiment with.)

I grow most of my Phalaenopsis hybrids in glass cubes. Currently I use large orchiata but was thinking I could switch to these large cubes for them and keep a more even hydration through the cube than happens with the bark. My train of thought was that the larger voids would allow more air spaces for the roots - what do you think?
 

Ray

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Ray,
Have you tried any of the grodan chunks, the larger pieces? (I have bought a bag of both sizes to experiment with.)

I grow most of my Phalaenopsis hybrids in glass cubes. Currently I use large orchiata but was thinking I could switch to these large cubes for them and keep a more even hydration through the cube than happens with the bark. My train of thought was that the larger voids would allow more air spaces for the roots - what do you think?
I have not, but as long as you can avoid having the cubes stack face-to-face, you might be alright.

I planted a couple of very similar divisions of a mottled-leaved Paph in a ~67/33 mix of the mini Grodan cubes and LECA, but one is blended, while the other is in layers. It’ll be interesting to see if they differ in root growth.mixed_vs_layered.jpg
 
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eds

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Thanks Ray. I'll give it a go and report back. With how I grow my Phalaenopsis think it might provide more even moisture than the bark currently does.
 

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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.
Hi Ray I want to use Traditional culture using 50/50 LECA & rock wool as you write. I would like to know how often you wet your orchids on average. I understand that it is very generic, but I need to understand if they should be watered more frequently or less than the traditional bark. Thank you
 

Ray

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First of all, with the exception of one Phal. venosa that is current in bloom, everything in a LECA -rock wool mix has been changed to a 67%/33% mix. I didn’t see any issues, but the 50/50 mix got a little too compact for my liking.

To answer your question, so far I have watered twice a week, for the most part.
 
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Ray, your data and thoughts have led me to use a four-layer parfait approach with my Rand AirCone pots. Hydroton at the bottom, layer of Grodan cubes, another Hydroton layer, and a top layer of Grodan. By volume in the pot I think I am about 2/3 LECA and 1/3 Grodan. I think this gives me the aeration that is important but lets me fertigate every 4-7 days, depending on the season. I think your concept that the Grodan is working as a barrier to evaporation makes sense. The roots are more in the LECA getting aerated. I am doing this with all pot sizes and Phrags, Paphs, and Cattleyas.
 

Ray

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Terry, when you consider the way paph roots spread laterally in the forest leaf litter, I think layering is a pretty good idea.

Several here have demonstrated the validity of "tray culture", so perhaps they go hand-in-hand.
 

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One question about the pots in SH. Where are the holes to drain?? Only in the bottom?? I have read that there are people who do not make holes in the bottom and make them in the sides at about 2cm to leave a reservoir of water. I understand that this method can rot the roots, but when in doubt I prefer to ask, because in the transplant that I have planned this spring and seeing your comments I am going to encourage myself to change to SH, thank you!
 

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One question about the pots in SH. Where are the holes to drain?? Only in the bottom?? I have read that there are people who do not make holes in the bottom and make them in the sides at about 2cm to leave a reservoir of water. I understand that this method can rot the roots, but when in doubt I prefer to ask, because in the transplant that I have planned this spring and seeing your comments I am going to encourage myself to change to SH, thank you!
I invented the solid-bottomed, “holes in the side” pot, and semi-hydroponic orchid culture, as a means of caring for my plants while I travelled 6-7 days a week (and occasionally 3-4 weeks at a time) for my job.

When you move a plant into S/H culture, select a container that has enough volume above the reservoir, so that when you pot up your plant, the roots do not extend down into it. If they do, those parts will drown and rot. However, once a plant has gotten established, the roots will grow down into the reservoir, and that’s perfectly fine, as the root cell structure will be optimized by the plant as they are added. READ THIS
 

JLOG

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I invented the solid-bottomed, “holes in the side” pot, and semi-hydroponic orchid culture, as a means of caring for my plants while I travelled 6-7 days a week (and occasionally 3-4 weeks at a time) for my job.

When you move a plant into S/H culture, select a container that has enough volume above the reservoir, so that when you pot up your plant, the roots do not extend down into it. If they do, those parts will drown and rot. However, once a plant has gotten established, the roots will grow down into the reservoir, and that’s perfectly fine, as the root cell structure will be optimized by the plant as they are added. READ THIS
Thank you Ray! Very interesting and valuable info to me 👌👌👏👏 and thanks to show me the web page, I can’t buy in Europe k-lite either quantum, and I see it can sent by international shipping 🥳🥳
 

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Ray, your data and thoughts have led me to use a four-layer parfait approach with my Rand AirCone pots. Hydroton at the bottom, layer of Grodan cubes, another Hydroton layer, and a top layer of Grodan. By volume in the pot I think I am about 2/3 LECA and 1/3 Grodan. I think this gives me the aeration that is important but lets me fertigate every 4-7 days, depending on the season. I think your concept that the Grodan is working as a barrier to evaporation makes sense. The roots are more in the LECA getting aerated. I am doing this with all pot sizes and Phrags, Paphs, and Cattleyas.
It would also get the necessary air (in the LECA) to go with the water being retained by the rock wool. That seems to be a central principal of orchid growing--getting that balance right. However, I also believe this ratio varies by genus. Since there is no reservoir, it would not work with those that like more water with their air, like Masdevallias or Miltoniopsis.

Ray, I'm wondering if you've ever tried a layer of rock wool cubes on top of a S/H setup to slow the evaporation from the top layer of LECA?
 
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My Miltoniopsis are doing great right now in Hydroton with a top layer of Grodan cubes. Fertigating every 4 days summer to every 7 days now. I also grew and bloomed a Masdevallia hybrid in Hydroton with a Grodan top layer. I threw it out because I didn’t like the flowers but at the time it was flush with healthy roots. I think Ray has nicely shown that the round balls of LECA optimize air space in a pot which are essential for roots. Everything else is about having frequent enough water and nutrition. I think the Grodan lengthens the time between needed fertigation.
 

Ray

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Ray, I'm wondering if you've ever tried a layer of rock wool cubes on top of a S/H setup to slow the evaporation from the top layer of LECA?
When I grew in a greenhouse, it was not at all necessary, as I kept the humidity high enough that evaporation from the medium did not outdo the wicking. Here in NC, out on my deck, the RH is usually high enough that it’s not a problem, either, but indoors now is a bit of a challenge.

I prefer to simply water more often to compensate, but have played with any number of “top dressings”, ranging from gravel, to sphagnum, to rock wool, to clear plastic disks.
 
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