The Do’s and Don’ts of Expanded Clay Pebbles

Derived from small pieces of clay processed at extremely high temperatures, expanded clay pebbles have become a go-to grow medium for many hydro growers over the last 10 years, offering numerous benefits to gardeners when used properly.

Expanded clay pebbles resemble oddly shaped, semi-rounded pieces of clay-colored popcorn. The high-heat popping during the manufacturing process gives the medium its large, airy macro-pores (spaces between each piece of expanded clay), in addition to its micro-pores, which are contained within each piece of expanded clay.

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The medium provides plants with hefty amounts of oxygen for fast-growing roots, and it holds enough moisture within its micro-pores to allow for healthy water and nutrient retention from the roots all the way to the top nodes. This spacing also allows for excellent drainage when using top drip or ebb and flow growing methods while still allowing roots to draw water out of the clay’s pores.

Throughout my years of tinkering with and exploring hydroponics, I have found myself using this medium time and time again. For me, it took trial, error, failure and eventual success before I came to fully understand the potentials of using expanded clay.

Here’s what I’ve learned so far:

Major Do’s for Expanded Clay Pebbles

Rinse and Soak Before Use

For those familiar with expanded clay, the simple practice of rinsing to remove debris and dust from the factory may seem obvious.

Rinsing will provide a clean start that won’t cause murky problems in your reservoir down the road, and the lesser-known process of soaking the pebbles is crucial to truly maximize yields with expanded clay.

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Soaking for 6-24 hours, preferably with an air stone, before planting allows water to percolate through the clay’s micro-pores, completely saturating the media. After an adequate soak, you will notice the media is heavier.

In an airy media like expanded clay pebbles, you want to ensure the roots don’t have to travel too far to find water, or wilting will occur. As a general rule of thumb, make sure a 3-inch radius of pebbles around your plants is always fully saturated.

Add a Small Amount of Nutrients

After rinsing the media, place it in a container and fill with a nutrient solution with an electrical conductivity of no more than 0.4. If you don’t have a ppm/EC meter, use your base grow nutrient at one-quarter strength. You may also choose to use a hydroponic plant enzyme to ensure a clean transition when you transplant.

Seedling in rockwool transferred to expanded clay. - Yarygin / Shutterstock

Mister and Cycle Timer Method

Starting seedlings using only expanded clay is doable, but I usually cheat and start in rockwool, which easily transfers into the clay. If you want to start seedlings in nothing but expanded clay, though, it can be done.

Obtain small starter net pots, which are usually between 1.5 and 2.5 inches in diameter. Fill with pre-soaked clay pebbles. Place seeds on top and cover with one or two loose pebbles, depending on the type of seed being planted. The pots can then be placed in a humidity dome with a spaghetti line affixed to mister fittings.

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You will want to have the pump feeding the misters connected to a cycle timer. Aim for short bursts of mist lasting 4-10 seconds, depending on the head shape and size of the mister. Have each burst occur every 2-3 hours. The pre-soaked medium, in conjunction with humidity in the air from misting, will easily force germination.

If you’re lacking the equipment to set up misting cycles for your seedlings, you can manually top and bottom feed every day until germination occurs. Ensure the media does not dry out until you are moving plants to the final system.

Crush Media Method

Crushing clay pebbles using a hammer or something similar breaks up the media into smaller pieces, reducing the size of macro-pores substantially and increasing the media’s water retention, which is ideal during the sensitive germination stage.

Simply place the desired amount of pebbles into a garbage bag or something similar and smash them. In this state, treat the crushed pebbles as a traditional potting mix until you are ready to transplant into the final system.

Ensure you don’t crush the medium too finely—you don’t want it to fall out of your net pots.

Expanded clay pebbles in a large net pot. - Firn / Shutterstock

Cloning Method

If you plan to start clones using clay pebbles, there are two methods growers routinely use. There is the low transplant technique—usually used in deep water culture—and the top drip method.

The low transplant technique is simple, you only need to remember two things: provide humidity up top, and plant your node down low in the pot. In other words, only fill your net pot about one-third to one-half full and then plant the stem 1-inch or so below the pebbles.

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This allows the portion of the pebbles submerged in the deep water culture reservoir to act as a wick for the couple inches above the water line where the clone has been positioned.

In the top drip method, usually used for drip systems, starting deep water culture systems or starting ebb and flood systems, clones are placed directly into their permanent grow site.

Ensure drip emitters are placed close enough to the freshly placed cutting and there is a sufficient flow rate. With newly planted clones, whenever possible, I always aim to provide at least three emitters with a more open flow rate of about 1 gallon an hour.

If the pebbles are properly soaked and the emitters are fastened correctly, clones take root quickly and easily.

Periodically Rinse

It is hard to claim that any hydroponic medium is 100 per cent inert because most of them have a specific pH value and cation exchange capacity (CEC). When describing a media as inert, most growers are really just referring to its lack of ability to provide any real measureable nutrients, or the ability to take away nutrients from the plant.

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With expanded clay pebbles, watch out for the clay’s high CEC value. Its high CEC value means the clay has the ability to bind with and hold nutrients for longer. Because of this, you might eventually notice an encroaching, whitish substance on top of your pebbles.

This white substance is salt residue left over from the solution. If left to build up for too long, it could eventually lead to phytotoxicity, which chokes out and starves the plant of water or nutrients.

As a preventative measure, be sure to routinely pull your plants out of the system (if growing in pots) and rinse from the top down using fresh, pH-adjusted water (use pH Up or pH Down products if you need to). If you’re not using pots, simply leach (flush) the entire system using fresh, pH-adjusted water. This will avoid any toxic salt buildup in the system.

ivan_kislitsin / Shutterstock

Major Don’ts for Expanded Clay Pebbles

1. Do not allow the pebbles to ever dry out.
2. Do not use the pebbles in a regular pot without a dedicated source of water.
3. Do not reuse the pebbles without properly sterilizing with peroxide or isopropyl alcohol.
4. Do not use the pebbles in place of soil outdoors.

Read also: Why You Shouldn’t Use Construction-Grade Clay Pellets

Additional Uses for Expanded Clay Pebbles

Expanded clay pebbles can be used in various other ways around the garden. I often find myself adding them to other soilless mediums to increase the overall porosity and drainage ability of a medium. (For some history on soilless gardening, see How Soilless Agriculture Differs from Soil-based Agriculture.)

Once the pebbles have been used up, you can add them to outdoor garden beds to increase the bed’s organic matter content and provide more aeration to the top six inches of your soil.

By employing some of these tried-and-tested approaches to growing in expanded clay pebbles, you too can experience the numerous benefits contained in such a wonderful, easy-to-use media.

Wondering if you can reuse your clay pebbles? You can! Read more about it in our article, Sterilizing and Reusing Your Grow Media.