Rock wool is an inorganic material that has many uses in orchid culture. It is manufactured by melting basalt and spinning it into glass fibers, forming a mat, and then – in the form I prefer and recommend – it is cut into cubes. Grodan BV out of the Netherlands, is the biggest producer, and their “Mini-Cubes” are what I have experimented with.
Rock wool is very lightweight, but holds water extremely well in the “web” created by the interlocking fibers. I have found that upon top-watering a pot full of the Grodan Mini-Cubes, they absorbed and held about 5x their weight in water! Fortunately, the uniform size and shape of the cubes, coupled with the physical structure of the glass fibers, allows them to maintain their shape, resulting in an open volume of about 45%.
Grodan Mini-Cubes can be used alone, where they are a great substitute for sphagnum. When I moved to coastal North Carolina, I grew all of my phalaenopsis is sphagnum, but had to replace it at least annually (twice a year was better). The non-decomposing nature of the rock wool cubes means I get a similar water-holding capacity as sphagnum, but without the risk of compression, resulting in root suffocation.
Grodan Mini-Cubes can also be added to LECA to extend its water-holding capacity. This is a particular advantage to growers of plants in Semi-Hydroponics in dry, home environments. If the water evaporates too quickly, the top layer of LECA can be too dry; by adding some rock wool cubes to the mix, they act as “mini-reservoirs” throughout the column, keeping the LECA more uniformly moist. I have used a 50/50 mix (photo above), but I suspect it may not be necessary to add that much to the LECA.
What’s nice about the one-centimeter cubes is that, being about the same size as the LECA, mixing them does not reduce the open volume, so the mix stays nice and airy.
Pro Tip: When mixing Grodan Mini-Cubes with LECA, saturate them first so they become closer in weight to the clay particles, allowing the to mix more uniformly, rather than "floating" to the top.
Even if you don’t want to repot your Semi-Hydro plants right away, you can place a layer of the wet cubes on the top surface of the medium to slow the evaporation from the LECA.
Grodan Mini-Cubes may also be added to traditional media to enhance the water-holding capacity – something that can be a boon to folks who have to leave their plants for extended time periods during vacations and business travel.
To give you an idea of the water-holding capacity of the cubes and a mix with LECA, the pots above were saturated via repeated overhead waterings, then weighed periodically. I later added a pot of LECA (same pot; not Semi-hydroponics) and treated it the same. The following shows the rate of water loss from the medium:
In a room where the temperature ranged from 75° to 81°F and had a relative humidity between 60% – 67%, the straight LECA dried out completely in four days, while both the Mini-Cubes and the mix still retained more than 60% of the absorbed water at 5 days.