Root Rot – Causes & Cures

As a new orchid grower, one of the first pieces of cultural information we are given is that we should make sure to let the potting medium dry out between waterings, as the plants will otherwise get root rot. Some will even include the explanation that the constant moisture favors the growth of fungus, and that is what attacks and rots the roots.

Unfortunately, that is simply untrue.

If the mere presence of water was the cause of root rot, then there would be no possibility of growing orchids in hydroponic-, semi-hydroponic-, or water culture, all of which are quite viable techniques, or of wild plants surviving during the many months some experience in monsoon seasons, which is often when they thrive. If we take a different approach and blame it on fungi, then how would we expect orchids to grow in the wild, in natural environments that are no doubt overrun with all sorts of fungi, bacteria, and the like?

Instead, we should blame it on suffocation and poisoning, and consider what happens when we water a potted plant.

When we water, some (most) of the liquid simply runs through the pot, some of it is absorbed by the potting medium components, while more of it is held by surface tension in between the medium particles. Referring to the articles on Air Management and Particle Packing, we understand that the smaller the spaces between the particles, the more easily that so-called “bridging water” is maintained, allowing it to more completely cut off the air flow pathways.

If the air flow to the root system is stifled long enough, the gas exchange is compromised, and the roots die due to a combination of suffocation from the lack of oxygen, and poisoning by its own waste gases.

Once the root tissue dies, the natural resistance to pathogens is eliminated, and the roots will rot.

If you think about it, that mechanism is likely the misinterpreted basis for the “let the medium dry out” myth: as the plant absorbs moisture, and more of the liquid evaporates, those bridging water droplets go away, opening up the air flow pathways and allowing the root system to “breathe” again.

Rather than suffocating our plants at each watering, the better approach is to prevent the extensive occurrence of bridging water in our potting media, and keeping the spaces larger is the key:

  • Avoid mixing particles sizes. The smaller pieces fill the gaps between the larger ones, reducing the sizes of the void space.
  • Do not pack the potting medium too tightly. That is particularly important with sphagnum moss.
  • Repot into fresh medium on a regular, timely basis. As organic medium components age and decompose, they break down into smaller and smaller pieces, creating a dense and suffocating root environment.
OK, so we have root rot. Now what do we do?

Usually we let it happen by waiting too long to repot, letting a poor plant suffocate its roots in a dense, mucky mess.  Of course, it’s often our most valuable or favorite plant, so how do we get the plant to recover?

First, keep in mind that as a “natural creature”, the plant has a survival mechanism, so it wants to recover. Then, with that in mind, consider that our job is give it the highest likelihood of doing so.

The keys to recovery are high humidity, warm temperature, and subdued light.

Consider the following scenario:

  1. A plant with no roots has no way to take up water, but can lose water through leaf stomata and cell wall permeation. As the plant dries, it gets weaker and weaker, making it less likely to have the energy to recover. That drying rate is directly related to the relative humidity, so high humidity sustains the plant while it tries to grow new roots.
  2. Plant growth rates are directly related to the temperature, and warmer equals faster. You want the plants’ metabolism to be faster, in hope that it will recover before the negative effects of desiccation kick in.
  3. Low light levels seems counter-intuitive, as plants need light to create the sugars they “burn” to function, but doing so requires water as well, further depleting the plants’ internal supplies. By keeping the plant in subdued light (not dark), you minimize that loss, again, extending the time that the plant can grow those new roots.

It may be difficult to maintain those conditions, so one thing to consider is the “Sphag-n-Bag” treatment to create a good, controlled environment (see below).

Another option – one that adds to the probability of survival, but is not a substitute for providing the conditions above – is the application of a rooting hormone. Brand is not critical, but freshness is (the hormones degrade with time, temperature and light exposure). We recommend KelpMax™ growth stimulant. The most effective treatment method for a root-free plant is a soak:

  1. Fill a container with water at about 80°F (~27°C)
  2. Add about a tablespoon of KelpMax™ growth stimulant per gallon of water (~1.25 ml/L)
  3. Totally immerse the plant for about an hour.

It is not necessary to repeat that, but you may begin adding the hormone to your irrigating solution once root growth has begun. Do not fertilize until the plant is established in its new pot and fresh medium.

“Sphag -n-Bag”

OK, you’ve managed to rot the roots off of your plant, but you don’t want to lose it. What can you do? Many orchid growers resort to the old “sphag-n-bag” technique. By the way, this is good for getting new imports established, too.

First, let’s consider the needs of the plant: It needs water to survive – water is the life’s blood of the plant. It provides turgidity to the tissues and cells, it is a chemical component used in the production of the sugars during photosynthesis, and it is used by the plant to control the osmotic pressure of ions inside- and outside of the cells. So if you have no roots, how does the plant get water?

Misting is of little value, as the plant cannot take up a substantial amount of liquid water through its leaves, and the brief period that the humidity is raised by periodic misting is likely insufficient to be of much benefit. The key is maintaining high humidity, not so much because of the easier absorption of water vapor (which is the case), but because a saturated environment prevents further loss of water from the plant tissues while it attempts to grow new roots. Basically, the “sphag-n-bag” concept uses a small bit of damp sphagnum as a moisture supply, and a plastic bag as a “micro-greenhouse” in which the elevated humidity can be easily maintained.

That’s simple enough, but a big mistake that many folks make is placing the plant in direct contact with moss that’s too wet.

The technique I use involves preparing the plant, then setting up the “rescue” environment:

  • Plant Preparation
    • Remove any old organic media that may be sticking to the roots. If it’s difficult to remove, just wait.
    • Using a sterile blade, trim off any dead or damaged roots.
    • Add about one tablespoon of KelpMax rootstimulant to a gallon of 80°F water. Do not add fertilizer to the soaking solution, as that will tend to draw water out of the plant.
    • Soak the plant for about an hour or more.
    • Remove any remaining organic media – it’s bound to come off after the soaking.
  • Setting up the Environment (this is the easy part).
    • Get a clear plastic bag large enough to comfortably hold the plant.
    • Place a small amount of damp sphagnum moss in one corner of the bag. You may substitute a piece of folded-up paper towel for the moss, or just add a teaspoon of water to the bag.
    • Place the plant in the bag, oriented as if growing normally, and not in direct contact with the moss or any free liquid.
    • Seal the bag. If it’s a Zip-Lock, that’s easy, if not, fold over the top several times and hold it with a paper clip.
    • Place it in a warm, shady location and walk away.Placing the plant in that warm, shady location (not dark) is important: The warmth will induce some growth activity, but the shade serves to moderate the vegetative growth while the plant develops new roots. Furthermore, warmth results in a higher moisture content of the air in the bag, and the shade prevents the bag from becoming a broiler, as it would become if direct sun hits it.

In a few weeks, the plant is likely to have developed a new root system, at which point it can be repotted.

Another idea for those of you who grow your plants in Semi-Hydroponic™ culture: Pot up your suffering plant and then place the plant – pot and all – in the bag. It will get all of the benefits of the “sphag-n-bag” environment, but has the bonus of growing its roots into the medium without the need for repotting afterward.

Using Science & Logic to Improve Horticulture