Bark is commonly used as a growing medium for epiphytic orchids. Should you sterilize it and other growing substrates prior use or reuse?
You should sterilize used bark and moss only when it was previously with an orchid with rot issues, pests, or fungal infection. Otherwise, new bark from the bag should not be sterilized, boiled, or baked as the heat can weaken it, accelerate decomposition, making it more vulnerable to infection.
In this article, we will show you when you should sterilize it and some practical ways to sterilize bark, tools, and pots.
The word “disinfect” is often used interchangeably with “sterilize” and even “sanitize”. Although they are related, they are not exactly the same.
When you disinfect something, you remove most or all harmful microorganisms, leaving behind the harmless ones. Boiling, steaming, and baking are commonly used disinfection methods, and chemicals such as chlorine, alcohol, and hydrogen peroxide are used as disinfectants.
In contrast, sterilization kills off every microorganism, leaving the object completely free of microbial life, whether harmless or not. The principal sterilizing agents include steaming under pressure, hydrogen peroxide glass plasma, and ethylene oxide gas.
In the context of cleaning orchid growing medium and tools, the most commonly used is hydrogen peroxide which is a disinfectant that removes only or mostly the bad microorganisms.
What most people call sterilization is actually disinfection. And for simplicity, this article will do the same.
With tools especially scissors or cutters, you should always sterilize them, especially if you have used them for other plants or purposes. Sterilizing or disinfecting tools help prevent the spread of diseases and pests between plants.
With growing media such as bark or sphagnum moss, the choice to sterilize depends on the status of the orchid. If the orchid suffered any form of rot or insect pests, the growing media it had contact with should be sterilized before reuse.
Sterilizing prevents the spread of diseases to other plants and new orchids, particularly if the previous orchid had rot issues due to bacterial or fungal infections (e.g. stem rot, crown rot). In such cases, the pathogen will still be present in the bark, and sterilization is necessary to get rid of it.
Furthermore, bark may easily harbor insects like scale, springtails, fungus gnats, and symphyla. Fungus gnats spread diseases when they bite into the plant, while symphylan feeds on young roots.
Sterilizing is the only way to eliminate pathogens completely and insect pests.
However, you should not sterilize new growing medium straight from the bag or if it was with an orchid that was not sick. Many manufacturers ensure that the bark isn’t mixed with soil or other organic matter that will contaminate it during processing.
This is because when you sterilize, you remove even harmless bacteria, leaving the bark clean, but at higher risk of getting colonized by potentially harmful pathogens from the environment.
Additionally, some forms of sterilization like boiling and baking can weaken the bark, making it decompose faster than normal. It can also remove some additives like dolomite which the manufacturer may have added to the bark.
Most orchid bark straight from the manufacturer is disinfected, but not sterile in the strict sense because it does contain certain microorganisms.
But that doesn’t mean that pathogenic microorganisms are present in the bark. In most cases, the producer would ensure that whatever microorganisms present in the bark are either harmless or beneficial to the plant.
If you need to disinfect bark, here are some practical ways to disinfect it.
Boiling water is an effective and easy way to disinfect orchid bark. Boiling water has a temperature of 212 °F (100 °C) which is hot enough to kill or inactivate viruses, bacteria, fungi, and even insect larvae. (WHO, 2015)
Simply put the orchid bark in a container, and pour boiling water over it. Cover the container and leave it to cool down. Doing this will kill off any pathogens and insects in the bark.
Baking orchid bark in an oven is another effective way to disinfect bark. It works in a similar way as boiling, using heat to kill pathogens and microorganisms.
You’ll first need to moisten the bark so that it doesn’t dry out or burn. When that’s done, place the media in the oven to bake between 180 °F to 212 °F ( 82 °C to 100 °C).
You can also use your microwave to sterilize orchid bark, as it also uses heat to kill the pathogens.
Get a microwave-safe container and put the orchid bark in it. You do not need to cover it up. You’ll again have to moisten the bark before putting it in the microwave.
Turn up the microwave to the highest setting for 90 seconds, after which use a thermometer to check the temperature in the middle. What you want is a temperature between 200 °F and 212 °F ( 93 °C – 100 °C).
Repeat the process as many times as it takes to get a temperature within the 200 °F to 212 °F range.
Admittedly boiling or heating bark in an oven is an easy and effective way to kill pathogens and pests. However, it also has disadvantages.
Boiling bark is like cooking food. The heat weakens/softens the bark, making it more prone to decomposition. That is because the bark is organic, and after heat, it’s easier for microorganisms to break it down. And you’ll find that the bark doesn’t last as long as it’s supposed to.
Moreover, boiling opens up the bark chips, making it more water retentive, which isn’t very good for orchids, as their roots need to be dried between watering.
If you need to sterilize bark, it would also be important to sterilize the pots before putting in a new plant.
Normally, heat would have been an option. But the challenge with plastic pots is that you can’t put them in boiling water, as the heat will also melt the plastic. And of course, you can’t put it in the oven either.
But besides heat, there are other ways to sterilize plastic pots (and tools). These usually involve washing with detergent and soaking in bleach solutions. Here’s a step-by-step process for disinfecting plastic pots.
- Pour some hot water into the pot and add some dishwashing soap. The water shouldn’t be so hot that it can scald your hands.
- Use a scouring pad and the soapy water to scrub the insides of the pot. The aim is to remove any remaining bits of soil, bark, or plant material as possible.
- Rinse with clean water and pat dry with a towel.
- Make a solution of one cup of bleach and one gallon of water. Put the pot into this solution and allow it to sit there for a while.
- After an hour or two, take the pot out of the bleach solution and rinse with plain water.
- Wash again with dish soap and hot water to ensure the bleach is completely gone.
- Rinse again and pat dry with a towel
- Place the pot on a clean surface to air dry.
Sterilizing a clay pot is similar to sterilizing plastic pots. But there are some additional steps since clay is porous, so bleach and soap might not get all the viruses, fungi, and bacteria.
- Repeat steps 1 – 7 for sterilizing plastic pots. When that’s done, preheat your oven to 400 °F and put the clay pot inside.
- Let the pot bake for 30 minutes. The heat will kill off any pathogens that the bleach misses.
- Remove the pot from the oven and let it cool down.
- When the pot cools, be sure to soak it in a tub of clean water to rehydrate it before planting in it. If you don’t do this, the clay will suck up the water from the potting mix when you water your plant.
Sterilizing scissors and other tools can be done by putting the tools in boiling water.
Rubbing alcohol will also work as a disinfectant, and it doesn’t require any mixing or soaking. You can just spray alcohol over the tools or wipe it using cotton with alcohol.
It’s important to note that rubbing alcohol and isopropyl alcohol are not the same thing. Rubbing alcohol is 70 to 90 percent isopropyl mixed with water.
It must be noted that 70% isopropyl is more effective a disinfectant than the 90% concentration. The reason is that the higher water content in the 70% slows evaporation and allows for better penetration into the cells of the pathogens.
Orchid bark typically lasts for about two years before you’d need to replace it. Instead of throwing away the old bark, you can add it to your compost pile. You can also use old bark as mulch for your vegetable or flower garden.
Not all orchid bark should be sterilized.
The need to sterilize bark depends on whether it has been infected with pests like symphylans and fungus gnats or if the orchid growing in it had suffered rot issues or fungal infection.
If the orchid was healthy and for first-time use, there’s no need to sterilize bark, as sterilization can affect the lifespan and, in some cases, can make it prone to pathogens.
Auburn University (2020). Ethanol/Isopropyl Alcohol FAQs.
Biome Exotics. Everything You Need To Know About New Zealand Orchid Bark.
Centers for Disease Control and Prevention. Introduction to Disinfection & Sterilization Guidelines | Guidelines Library | Infection Control | CDC. Cdc.Gov.
Salamanca, L. R. (2015). Sanitation is critical to prevent plant diseases Part 2: Field sanitation. Michigan State University Extension.
World Health Organization. Boiling Water.