I have been growing orchids now in semi-hydroponics for approximately 18 months, and have learned a lot. There are a lot of positive aspects to the method — easy watering, seemingly less disease, good air to the roots — but there are also some difficulties. These difficulties are difficult to dig out of the literature (read: net), as most of the people promoting the methods are vendors who have a financial interest in minimizing the difficulties and promoting the positive aspects of the method. I have found it to be good for several of my orchid types, but not so good for others. I will try to discuss a few of these not-so-good points in this post.
First, let me say that I will continue to use this method for my cattleya-type orchids, and those orchids that I already have in this media. The plants seem to grow well, especially if given enough light. Watering is easy and drainage trays work. I was initially using too much fertilizer, and not flushing well, and this was a big problem. The plants were not growing well, or flowering. But I have evolved to a twice-a-month flush with clear water, and together with adjusting the fertilizer, this seems to have solved this problem.
One of the advantages of semi-hydroponics is that the media (LECA pellets) wicks water from the bottom reservoir of the pot toward the top of the pot, creating an ever-damp environment for the roots of the plant. The preferred watering method, which fills the pot to the top and drains through the two holes at the bottom, draws air down through the media to the roots as it drains, creating an airy/damp media for the roots to grow in.One difficulty, and this can be deduced from many posts in the literature, is that the LECA pellets retain salts from the water. As the water evaporates (as some always does) the mineral salts are retained in the pellets. Over time, this can create a toxic environment for the plant roots if not monitored correctly. One way to reduce this is to flush the pots regularly with clear water, removing some of the dissolved salts through the drainage process.
Another difficulty is that the wicking works differently on different sized pots — the volume of the pot and the size of the reservoir affects the amount of wicking that is done. Most of the semi-hydroponics literature recommends that holes are drilled approximately 1″ from the bottom of the pot. In the case of the small pots, this implies that the volume of the reservoir is approximately 30% of the volume of the pot. In the case of medium-sized pots, the reservoir is approximately 20% of the volume of the pot, and 10% in the case of a larger pot. So, different amounts of water are wicked to the top of the pot, depending on the pot size. If sufficient water is reaching the top of the pot, it evaporates, and leaves the minerals contained in the water — and one gets salt buildup. The LECA pellets, which also retain salts from the water, deposit the mineral-salts from evaporation on the surface of the pellets at the top of the pots.
I find that the medium-sized 24-32 oz semi-hydroponics pots exhibit this. I frequently get pellets on the top layer that have visible salts on the surface of the pellets. No rinsing with clear water will remove this salt, and periodically I must remove the salt-encrusted pellets, and replace with new ones. In general, this is not a big problem, and I do it infrequently. In this case of the larger pots, I rarely see this problem.However in the case of the 16 oz small pots, the problem is even worse. The reservoir is large and close to the top of the pot, resulting in too much water being wicked to the top of the pot. In this overly damp environment, some of the water is actually wicked up the plants themselves. This results in mineral salt deposits on the plant components, the roots, the bottoms of the pseudobulbs (note the first photo), and even the leaves, and this creates a difficult growing environment for the plant.
Also, the overly wet medium created in the small pots encourages root rot, which is contrary to one of the reasons that many of us switched to semi-hydro in the first place.
How does one solve this problem? One needs to experiment with different-sized pots to find the correct ratio between the volume of the reservoir and the volume of the pot. I believe that this is a difficult problem, as it also depends on the person’s growing environment, light, water, etc. I experimented with drilling holes at different levels in the pots (depending on pot size) [the pot in the second picture has a third hole drilled to limit the size of the reservoir in a small pot]. This seemed to work somewhat better, but requires more frequent watering as the size of the reservoir is reduced. In general, I have largely solved this problem by not using the smaller pots. Small plants are grown in bark, bark mixes or sphagnum moss, and plants ready for S/H are grown in the medium- and large-sized semi-hydroponics pots. This seems to work reasonably well.
One additional problem is that semi-hydroponics is not for everything. First, the transfer into the new media is a substantial shock to the plant, and should only be done after a new plant is sufficiently acclimatized to its new environment, and is ready to grow roots. Waiting for the right time may take several months, and should only be done with a healthy plant.
I have also found that, for me, semi-hydroponics is not a good environment for orchids with thin roots. Cirrhopetalum Makoyanum, my Masdevallias, Kefersteinia Pellita, all have been transferred out of semi-hydro, and back into sphagnum moss, and are now growing well — they were all growing in small semi-hydroponics pots initially. I also have most of my Odontoglossom Hybrids growing in bark. They are mostly young plants, and I may transplant them into S/H when they need repotting.