If you’ve hung around the orchid growing community long enough, you’ve probably heard the statement “good roots means a good plant.” While accepting that at face value, little do folks look into the details of that statement and think about what it means to their culture of orchids. Let’s start with some root “basics:”
1. Root cells serve to regulate the flow of water and nutrients (as water-soluble ions) into the plant, and to regulate gas exchange (mostly carbon dioxide out and oxygen in, but any green roots will carry on photosynthesis, so the opposite flows are also present).
2. The rates at which those occur are determined by physical characteristics of the root cells and by the relative concentrations of the components on either side of the cell wall.
3. A root cell, once grown, is apparently “locked into” the configuration that was appropriate at the time it grew, and cannot change to accommodate a new environment. New cells must grow that are attuned to the new environment. That is why one may see the decomposition of the old root system if the plant is moved into a different culture, such as a bark-based medium to Semi-Hydroponics.
4. The functionality of cells in a particular environment is not “all-or-nothing,” but is a matter of degree.
Based upon those tenets, it becomes clear that some of the long-established “rules” are quite well-founded:
1. When repotting, for example, always make sure that new growths (hence new roots) are forming. Doing so gives the plant the opportunity to grow cells that are appropriate for the environment created by the new medium, and to develop a new root system that will fully support the plant as the old roots start to fail.
2. Likewise, it is important to keep the plant in a low-stress environment for four to six weeks after repotting so the plant can get established. If the new roots have not yet grown enough to fully support the plant, and the old roots are not able to function at 100%, this makes a lot of sense. This is an especially important issue when making a drastic change in media, such as bark-based to mounted, into inert media, or into water. Interestingly enough, aerial roots do quite well when submerged in water, as they can apparently regulate water flow better than those that were submerged in the medium.
One further thing that relates to media choice is decomposition and gas exchange. We know that decomposing medium can take down a plant quite quickly, but an aspect of that which is not often considered is its effect on gas exchange: as organic media decompose, they tend to compact and stay quite wet, and it it quite plausible that it’s the compacting of the medium – and not the wetness or decomposition themselves – that is the culprit in the rapid loss of the plant.
As plant respiration occurs, carbon dioxide exits the roots so it can be swept away into the air. If the medium is compacted and wet, the carbon dioxide will not dissipate, but can react with the water to form carbonic acid, which really kills plant tissue quickly. The localized high concentration of carbon dioxide near the roots also limits the plant’s ability to release more, essentially “choking” the gas exchange process.
It seems likely that the open, airy structure of the ceramic PrimeAgra medium – permitting lots of gas exchange – is why constantly-wet roots don’t rot in semi-hydroponic culture. Some experts feel that a lot of flushing with fresh water (carrying lots of oxygen) is beneficial to the plant, so the watering regimen used in semi-hydroponics may also play a role in plant health.
Further, the fact that there is no potential of medium decomposition means that we have eliminated another potential source of stress on the root system, and one response to stress is the generation of phenolic compounds. Designed to be released into the root environment in order to fend off external diseases and competition from other plants, the compounds are also toxic to the orchid’s own roots. Reducing the need for self defense reduces the possibility of self-destruction.
One last comment: the vast majority of the orchids we grow are epiphytes or semi-terrestrials such as paphiopedilums. Accordingly, their roots are designed to function in open air, attached to tree branches, or just under the surface of the leaf litter on a forest floor (could those “hairs” on paph roots be there in order to provide air space between the roots and the litter?). Potting a plant in any medium is for our benefit, not the plant’s, so we had better make sure that the conditions within the medium are not detrimental to the functioning of the roots.