Hydroponic Grow Media | 11 Options & Care

Choosing Your Hydroponic Grow Media  • A Review of Common Types

Options, Care & Maintenance Considerations

Instead of soil, hydroponic growers use a range of grow media as a substrate that anchors plants and helps get water, nutrients and oxygen to the roots. Below we compare the most common options and cover upkeep such as flushing and sterilizing, so you can pick a medium that suits your setup.

The right medium depends on your system's design, size and location, and on what you grow. Questions to ask about any medium you're considering:

• Is it easy to handle and pot up?
• Can you get it locally at a reasonable price?
• For seed starting, does it give seeds good contact?
• Does it hold plants steady?
• Where temperatures swing, does it insulate the roots at all?
• Is it free of toxins, contaminants and pathogens that could hurt germination, slow growth or spread disease?
• What's its cation exchange capacity (CEC)? That's how well a medium holds and releases positively charged nutrients (cations) from the solution without throwing the solution out of balance. Most micronutrients and a few macronutrients in fertilizer carry a +1 or +2 charge.
• What's its water holding capacity (WHC)? If your system needs the medium to stay moist between irrigations, pick one that holds a lot of water.
• On the flip side, what's its air-filled porosity (AFP)? This is the share of the medium's volume taken up by air rather than water at a given time. A good medium keeps roots well aerated, or at least doesn't hinder aeration.
• Are you growing organically or with integrated pest management (IPM)? If so, consider an organic medium such as coco coir that can host beneficial microbes.
• Can it be reused or recycled?

All of these affect your results. Test several media before scaling up to find what works for you, since different crops do best with different systems, conditions and media.

Plugs vs. Soilless Mix in Hydroponics

Baby salad greens started in plugs and moved up into expanded clay pellets. Photo © Nagualma Hydroponic Farm, Cannes, FRANCE

Some media come loose in bulk for filling trays, bags or buckets. Others come as ready-made plugs, blocks, discs or slabs.

Plugs let you sow straight into small propagation plugs with a pre-made hole for each seed, then set the whole plug into a larger block as the plant grows, with no need to disturb the roots. Moving up this way prevents or reduces transplant shock. Many commercial growers go from plugs to larger blocks and then to the final medium, such as pots, trays, slabs, bags or buckets of soilless mix.

11 Different Types of Hydroponic Media

Here are some of the most widely used hydroponic media.

Burlap

Burlap is woven from jute fiber and comes in rolls or sheets. Choose untreated burlap sold for agricultural use. It breaks down naturally and can be composted after use.

Coco Coir

Coco coir is a byproduct of the coconut industry, made from the fibrous outer husk, and is usually sold as plugs, blocks or slabs. It soaks up water and holds it well between irrigations while still letting roots breathe. It also resists some fungi and can host beneficial microbes. With disease kept in check, it can hold up for several seasons before it needs replacing. Some coir is high in salts when you buy it, depending on how it was processed, so look for products labeled low-EC (low electrical conductivity) or low-salt. If EC is a concern, rinse the coir with water before planting to bring it down. Coir isn't inert and carries potassium, so a nutrient solution made for coir can help keep nutrient ratios right. Unlike rockwool, it's renewable and biodegradable.

Expanded Clay Pellets or Pebbles

Expanded clay pellets, sometimes called grow rocks, come in sizes from about 1 to 18 mm across. Small air pockets inside give excellent drainage and aeration, but they hold little water, so they need frequent irrigation. They last many crop cycles before breaking down, provided disease is controlled when you reuse them. They're bulky, though, and nutrients can settle out onto the surface, so wash them before reuse. Rinse new pellets well to remove clay dust that can clog filters and emitters.

Gravel

Gravel is cheap, easy to find and clean, and drains freely. It's heavy and absorbs no water, so it isn't used much, but it can fill in where other media are hard to get or for limited uses. It won't work in systems that depend on the medium to keep roots moist, but it's fine in drip or flood setups. Use inert, non-calcareous gravel (no limestone), and make sure your benches and containers can handle the weight.

Hemp Fiber

Grow mats of cleaned industrial hemp fiber are close to neutral in pH and fully biodegradable and compostable. Hemp holds a lot of water, keeping seeds moist for germination and supporting strong roots, which makes it a good fit for hydroponic microgreens.

Perlite

Perlite is volcanic glass that's heated until it pops and expands. Horticultural perlite comes in fine grades for seed starting, coarse grades for propagation and blends for production. It's used alone or in soilless mixes. It's very light, wicks moisture well and keeps roots well aerated, which suits wick systems, but it holds less water than many media and can dry out fast between irrigations. It has essentially no cation exchange capacity, so it doesn't buffer the solution, and since it floats it may not suit ebb-and-flow systems. Dry perlite is dusty, so wear eye protection and a dust mask, or wet it before handling.

Phenolic Foam

'Rex' lettuce seedlings started in peat and phenolic-foam plugs for a floating raft system. Photo courtesy of a retired seed sales representative and longtime grower.

Phenolic foam is a synthetic material, related to florists' foam, sold as plugs and widely used for seed starting. It's made to balance water holding with root aeration. Many products can be used for more than one season; check the maker's information for expected life and whether it composts or biodegrades. Some types break down in roughly 18 to 24 months.

Rockwool

Rockwool is made by melting rock such as basalt with other minerals at very high temperature and spinning the melt into fine fibers, which are then cooled and pressed into plugs, blocks or slabs. It's sterile, holds water and air well, and is one of the most widely used hydroponic substrates. If rinsed thoroughly and kept disease-free, it can be reused for several years. It doesn't biodegrade, so plan for proper disposal. Binders can give new rockwool a high pH, so soak it in a slightly acidic solution (about pH 5.5) before use and keep a close eye on system pH. The fibers irritate skin, eyes and lungs, so wear a dust mask, goggles and gloves when handling it.

Sand

Sand is cheap and easy to get. Coarse enough sand holds water and lets roots breathe; aim for particles at least 2 mm across. If water drains through freely without pooling, the size is probably right. Perlite is sometimes mixed in to improve aeration. Avoid limestone sand, which raises solution pH, and remember wet sand is very heavy.

Sawdust

Sawdust absorbs and holds water well. Choose a medium particle size: moisture spreads more evenly through fine sawdust, but very fine particles can starve roots of air. Avoid treated wood and western red cedar, which is toxic to plants. Logs sometimes pick up salt in transport, so check the sawdust with an EC meter and flush it well with fresh water if salt levels are high. Sawdust breaks down fast, so expect to replace at least part of it after each crop.

Soilless Mixtures

Many soilless mixes are available, often built from vermiculite (an expanded mica mineral), peat moss, pine bark, sand and/or perlite. Cornell University's well-known Peat-Lite mixes, for example, combine equal parts peat and vermiculite (Mix A), equal parts peat and perlite (Mix B), or half peat with a quarter each of vermiculite and perlite (Mix C). These mixes break down and compact over time and need periodic replacement. Peat harvesting from bogs also raises environmental concerns.

Salt Buildup in Hydroponic Media

Plants rarely use every nutrient in the solution, so salts can build up in the medium over time. Keep buildup in check, because it can push pH up, interfere with water and nutrient uptake, and eventually damage plants. Running EC higher than needed makes it worse.

To clear salts, flush the medium and container with plain water, about twice the container's volume, letting it drain out freely. How much you need depends on the medium; porous ones like perlite, clay pellets and sand flush more easily.

Recommendations range from weekly to monthly flushing; for home growers, about once a month is usually enough. Also flush if plants show signs of nutrient toxicity. If plain water doesn't fix it, try a commercial flushing product with chelating agents.

Sterilizing Your Hydroponic Media & Cleaning Your System

Regular system cleaning cuts down on nutrient residue on trays and equipment, kills pathogens, and clears out plant debris and broken-down media that can clog pumps and filters.

How often to clean depends on your system, your crops and your disease pressure. Commercial operations often sterilize with steam or chemicals; small growers and home gardeners usually use one or more of these methods:

• Oven pasteurization: Heat vermiculite, perlite, clay or sand in an oven at 180°F (82°C) for 30 minutes.
• Solarization: Wet the medium, cover it with clear plastic and let the sun heat it. For full effect, leave it covered for at least 6 weeks during the hottest part of the year.
• Bleach: Soak in a 10% bleach solution for 20 minutes, then rinse several times with water.

Learn More

Hydroponic Growing Resources

• Hydroponics primer article
• Growing healthy hydroponic plants article
• Pest and disease control in greenhouses and hydroponics article

General Hydroponic Growing Resources

• Hydroponics. University of Florida: IFAS Extension.
• Cornell Controlled Environmental Agriculture.

References

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3. Leonhardt, K.W. & W.W. McCall. 1982. Hydroponics. HI Cooperative Extension Service.
4. Peterson, D. 2018. Techniques, varieties fuel success with hydroponic strawberries. Vegetable Growers News.
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6. Resh, H.M. 1993. Hydroponic Tomatoes for the Home Gardener. Santa Barbara: Woodbridge Press Publishing Company.
7. Rorabaugh, P.A. 2015. Introduction to Hydroponics and Controlled Environment Agriculture. UACEAC.
8. Stewart, K. 1992. Hydroponics for the Home Gardener. Ontario: Key Porter Books.
9. White, B. 2004. Alternative hydroponic substrates. Greenhouse Product News.