By Jessica McKeil

For the hydroponic grower, sanitation and sterilization are two familiar words. Sterilization, particularly, prevents unwanted pathogens from moving from one grow to the next. It’s why an in-depth sterilization procedure is necessary, especially for the substrate.

Sterilizing substrates serves several purposes. First, it reduces waste from one round to the next. Second, it reduces costs by reusing the medium more than once. Third, it eliminates dangerous remnants of bacteria, fungi, and algae before you start afresh. 

It’s this final point that we’ll be focusing on: sterilizing substrate to lay the foundation of a healthy indoor ecosystem and healthy plants.

Notes on Sterilization

There are two ways to approach sterilization: chemical or heat. Both are suitable for use with hydroponics, and the decision will largely boil down to the amount of substrate needing sterilization. Smaller home-hydro systems can likely get away with applying heat in small batches, while larger systems will need to deploy the chemical option.

Chemical Options

Chemical sterilization relies on strong cleaning agents like bleach and food-grade hydrogen peroxide (H2O2) to kill off unwanted microorganisms.

Both cleaning chemicals should be handled with care, especially before dilution with water. For example, if you are using household bleach, the standard dilution is 10%. Submerge the substrate for at least 30 minutes. 

The other option (and often preferred) is food-grade (35%) hydrogen peroxide, sourced from commercial cleaning supply distributors, hydroponic stores, and the like. Most growers dilute food-grade hydrogen peroxide down to 3% for use in hydroponics. Read the label for specific safety and dilution instructions.

Heat Options

With enough time, high temperatures will kill all living microbial life. It’s another option for sterilizing substrate, which avoids using chemicals but is difficult for large home systems.

According to the Encyclopedia of Food Safety, sterilization occurs with dry heat between 250 to 264 °F (121 to 129 °C), while moist heat sterilizes between 349 to 449 °F (176 to 232 °C). For the home grower, boiling, microwaving, and baking are the three most common means of sterilizing with heat. 

Substrates Best Suited for Reuse

Some growing mediums are better equipped to handle repeated cleaning and reuse, including clay pellets and rockwool cubes. Other options, like coco coir, are beloved by growers specifically because they foster a thriving microbial ecosystem. They are more challenging to reuse and sterilize between grows.

But, let’s look at the two options that take well to reuse. Clay pellets are perhaps the most sustainable option out of all inert substrates because they are robust and stand up to repeated washing and handling. They are also one of the easiest substrates to clean, as they can be boiled, baked in an oven, or chemically sterilized between uses.

Rockwool cubes are also surprisingly reusable. Plus, they are small enough that it’s possible to sterilize with quick spurts in the microwave or through soaking in a hydrogen peroxide-water mixture. 

How to Sterilize  Rockwool Cubes

First, clean out as much plant matter as possible from the rockwool cubes. Rinse with water to flush out lingering roots, leaves, and other remnants. Dispose of any cubes that may have started to disintegrate. 

Option #1: Microwave

Soak rockwool cubes in water, place them in a microwave-safe bowl.

Microwave for 5 minutes on high.

Remove from the microwave and allow to cool before using.

Option #2: Hydrogen peroxide

Place all rockwool cubes in a large, clean bucket.

Fill the bucket with a 3% hydrogen peroxide solution until cubes are covered.

Soak for 24 hours.

Drain and rinse with fresh water several times to flush out excess hydrogen peroxide. 

Air dry for another 24 hours.

How to Sterilize Clay Pellets (LECA Beads, Hydroton)

Fill a large tub or bucket about halfway with the dirty clay pellets. A larger container with a drain or colander is best, but choose a container best suited to the number of pellets you are sterilizing. Work in batches if needed.

Fill with water and stir the pellets, working out old roots and dead plant material. Drain and repeat until all visible organic material has washed out.

Again, fill the container with water until the clay pellets are submerged. Add hydrogen peroxide to the proper dilution. 

Stir the mixture and allow to soak for 24 hours. Weigh down the pellets if needed.

Drain and rinse with water several times.

Air dry for another 24 hours, ensuring the hydrogen peroxide has evaporated. 

Reduce, Reuse, & Sterilize

In a hydroponic system, each round must start with an inert substrate. Just like hospitals take care to sterilize all equipment between patients. As a cultivator, you’ll want to do the same for your plants.

You can keep buying new, but over time these costs will add up. Taking the time to sterilize substrate keeps both your plants and wallet happy. 

Jessica McKeil is a cannabis writer based in British Columbia, Canada. She has a passion for cannabis tech and scientific breakthroughs, which has led her to work with some of the industry’s biggest brands.