Can You Sterilize and Reuse Your Grow Media?

Clean means green, and green means go, in the garden. Whether you’re growing indoors or outdoors, adopting good sanitation and sterilization practices will save you headaches.

That’s doubly true if you’re trying to save a dollar by recycling, re-purposing, or reusing supplies that are sometimes considered consumable rather than durable in nature.

Take your grow media for example. If your set-up uses media of one kind or another, it won’t take long to realize you can save money by reusing soilless potting mix, expanded clay, and other options. In fact, the prospect can be downright tempting from an economic standpoint.

There are risks, though. Not all types of media are compatible with reuse strategies, and those that are need more than a quick dunk in a bucket of water. Still, if you’re game, read on.

Sanitizing Your Grow Medium

You’re probably familiar with good sanitation practices. In the garden, they help keep your tools clean and offer some valuable peace of mind. Sanitizing doesn’t kill all types of bacteria, viruses, fungus, and spores, though.

When reusing media, sterilization is the only foolproof option for killing microorganisms. Depending on your goals and objectives, you may want to use a less extreme solution, but when doing so, there’s always a risk that persistent, dangerous microorganisms might resurface.

Sterilizing Your Grow Medium

Sterilizing is more than sanitizing. It is the nuclear option to discourage plant pathogens. Sterilization kills all destructive microorganisms that may have glommed onto or encroached into grow media during use. The process doesn’t kill microorganisms going forward, but it does restart the clock.

For plant enthusiasts, there are two basic, reliable sterilization methods: chemical measures and heat (moist or dry) at levels high enough to be lethal. When using heat, pasteurization—which is achieved at lower temperatures than sterilization—may be an option for some soilless growing applications.

Chemical Options for Sterilizing Grow Mediums

Popular chemical media cleaning agents include hydrogen peroxide, alcohol, quaternary ammonium salts, and bleach. When using a chemical agent, application and dwell time are only part of the process. It’s also important to remove any residue by rinsing again and again—and again. After that, depending on the type of media involved, thorough drying might be necessary.

These are some broad guidelines:

Bleach - 10 per cent solution for a minimum of 30 minutes followed by multiple rinses.

Hydro cleaning agents containing quaternary ammonium salts - Effective when used in accordance with the manufacturer’s instructions.

Hydrogen peroxide - A diluted 35 per cent hydrogen peroxide solution is required for sterilization.

This product is not found at your local drug store. It is sold through food service retailers and may be available through hydroponics outlets.

At high concentrations, H₂O₂ is an effective bactericide, virucide, and fungicide. (Always use safety precautions when using concentrated hydrogen peroxide.)

Heat Treatments for Grow Media

When using heat to kill pathogens, temperature and duration are critical factors in processes that can be pretty diverse:

Oven - Heating is a popular method for pasteurizing and sterilizing soilless potting mixes. It will also kill seeds, insect eggs, and larvae. For oven pasteurization, the goal is to reach and sustain a temperature of 180°F for 30 minutes.

Pasteurization is less aggressive than sterilization, which requires processing at a higher temperature of 212°F. Pasteurizing can be easier to tackle than sanitizing, though, and may be enough for the gardener who hasn’t had notable pest problems in the past.

"If you plan to heat treat media in your kitchen, be forewarned. The procedure can produce a sour odor that lingers."

For example, pasteurization is effective at eliminating the fungus-like organism pythium, responsible for fusarium wilt, but is less successful at combating some pathogenic fungi known to cause damping-off and numerous forms of root and stem rot.

If you plan to heat treat media in your kitchen, be forewarned. The procedure can produce a sour odor that lingers.

Microwave - You can pasteurize moistened media like potting mix or vermiculite by microwaving it for three to five minutes, depending on your microwave’s power rating. This only works for small batches of up to two pounds or so.

Microwave pasteurizing has the advantage of being faster than oven treatment, and a microwave can be temporarily relocated to reduce odor problems. Sterilizing in the microwave can be problematic, however.

Microwave ovens heat quickly, but they sometimes leave cooler spots that are hard to detect or eliminate.

Be aware that these methods are equal opportunity options. They kill beneficial as well as destructive organisms.

Solar - Outdoor solarization is another way to pasteurize media that uses heat—in this case, sunshine. In this method, media is moistened, covered with clear polyethylene tarp material, and left to steam in the sun on a hot day.

A sustained temperature of 150°F for 30 minutes is hot enough to kill many common microorganisms. You can test the temperature using a candy thermometer. Adding additional layers of tarp can increase the temperature somewhat.

Covering the soil for a longer duration can be helpful, too. One day is effective, but leaving the tarp in place for a week to a month can yield even more impressive results. To sterilize media outdoors, solarization should be coupled with a chemical agent.

Nutrient Washing

The big advantage to most modern growing media is that it is inert. It provides a blank or relatively blank slate. That way it’s easier for the gardener to control nutrient composition and concentration through the plant cultivation process.

The problem with reusing media is that any lingering nutrient residue can spoil the “recipe” designed for a specific project or produce toxic concentrations of some chemicals. You can often see this fertilizer salt buildup as a white powder crusted on some types of used media.

An important step in recovering media for reuse is to eliminate residual nutrient where necessary. This is usually done through repeated rinsing with pH-neutral water; however, in some cases, the addition of a chemical chelating agent may be necessary.

Just as each growing media is unique, each type is best cleaned by a particular method:

Tips for Chemically Sterilizing Clay Pebbles

Let’s take a look at a small sterilizing job using clay pebbles. Reusing this media is a good example because it is often recommended as the environmentally friendly and economically sensible choice. Note that you can’t reclaim clay indefinitely, though; it degrades after a few reuses.

For this example, we’re sterilizing with hydrogen peroxide. Bleach is another common chemical option.

"Reused grow media is only a money saver if you can cultivate healthy plants with it."

Also, before you begin any media sterilization procedure, remove loose debris and scrape or lift away any large pieces of stubborn organic matter you see.

For hydroponic media, you can speed up this process by using an enzyme product designed to aid in the breakdown of organic matter in hydroponic systems. It softens dead roots, making them easier to wash or scrub away.

Just be sure to read the label, as some enzyme products are not recommended for use with hydrogen peroxide.

General Directions:

• Fill a large sieve, colander, or strainer half full with clay pebbles. Use the largest mesh that will still retain the media itself.
• Rinse with tap water to remove organic matter. If using a sink or tub, install a drain strainer to catch debris.
• Stir pebbles and keep rinsing/stirring until the water runs clear and no surface debris is visible. This can take five minutes or longer. Set aside.
• Fill a large bucket with enough water to cover the pebbles generously (but hold back on adding the pebbles just yet). Make a note of how much water you’ve added.
• Optional: Install an air stone to the water to promote better circulation. Put on gloves and eye protection.
• Add concentrated H₂O₂ to the water using the dilution directions for sterilization listed on the bottle. Stir.
• Add pebbles carefully. At this point, the slurry will begin to foam. That’s the hydrogen peroxide doing its work. If necessary, use the colander as a weight to keep the pebbles submerged.
• Soak for 15 minutes or in accordance with the hydrogen peroxide instructions. Remove the air stone and pour off the water.
• Rinse thoroughly to remove hydrogen peroxide residue. In case there are lingering traces of hydrogen peroxide, it’s a good idea to let the pebbles sit overnight before reusing them.
• This procedure can be scaled up as needed. It can also be used to sterilize perlite; just use a finer mesh sieve.

For more tips on expanded clay pebbles, check out The Dos and Don't of Expanded Clay Pebbles.

Tips for Treating Rockwool

Viable rockwool cubes can be presoaked in water and pasteurized in the microwave for 30 seconds to a minute.

Sleeves and other large sections can be scrubbed gently and sterilized with a bleach or hydrogen peroxide solution and rinsed thoroughly afterward.

Tips for Reusing Coconut Coir

There is some debate about reusing or reconditioning coconut coir. One advantage of coco as a substrate is that it promotes the growth of beneficial bacteria and fungi like trichoderma and mycorrhizae that subsequent sterilization would destroy.

Coco enthusiasts advocate for the reuse of media without sterilization, but only if there is no history of problems with destructive microorganisms.

There are other challenges, too. It can be difficult to remove dead roots from coco, and used coco will be less porous and have a higher water-holding capacity as it ages.

Since some root particulate will likely remain, regardless of the cleaning strategy employed, it’s a good idea to add an enzyme product to the end of a grow cycle that will help speed organic deterioration so residual roots are less of a problem.

Loss of porosity can be dealt with by supplementing reconditioned coco with new material or with another product like perlite, especially if the coco is very fine or is reused more than a few times. High residual salt concentrations may also be present in recycled coconut coir, thus requiring EC testing and distilled water flushing before reuse.

Not all types of growing media can be reused. Some like starter plugs and small rockwool cubes weaken under the onslaught of growing plant roots and are best discarded after a single use. Check with the manufacturer for recommendations.

Even if reuse is not encouraged, additional information may be available that can help you evaluate the risks. A word of caution here: Reused grow media is only a money saver if you can cultivate healthy plants with it. That’s an important judgement call.

For more tips on sterilizing your grow room equipment, check out Two Methods to a Clean Grow Room: Sterilization and Sanitation.