Clay Pebbles and Rockwool Cubes: How to Use Them in Hydroponics

Clay pebbles and rockwool are the two classic hydroponic growing media, and neither feeds the plant. Pebbles drain almost at once and suit systems that water automatically; rockwool holds a lot of water and is a common way to start plants for any hydroponic system. This guide covers what each is, how to prepare and reuse it, which systems it suits, and how both compare with coco.

What clay pebbles and rockwool are

Clay pebbles, also called LECA or clay balls, are expanded clay pellets: clay fired in a kiln until it puffs up into light, porous balls. They are inert and don't rot, so they can be washed and reused. Water runs straight through and air follows it in, but very little stays behind, which is why Canna say Canna Aqua Clay Pebbles, used as the only medium, suit automated, recirculating systems only.

Rockwool, also called stone wool, is basalt rock melted and spun into thin fibres, mixed with a binding agent and pressed into slabs; propagation cubes are cut from those. Ours is Cultilene's Cultiwool. Its page says the cube fibres are spun in two directions, which keeps the cubes flexible and strong when wet. Rockwool holds a lot of water, with air still in the structure. The Cultiwool pages describe it as sterile, and like pebbles it is inert.

The clay pebbles and rockwool we sell

How to prepare clay pebbles and rockwool

• Rinse clay pebbles. New ones are dusty: rinse until the water runs clear, or the dust settles as sludge in the tank and can block pumps and drippers.
• Soak rockwool. New rockwool tends to push pH up, so it is soaked in a mildly acidic solution before planting. For cuttings, Athena's schedule gives a pre-soak of EC 1.0 at pH 5.6; for seeds, growers commonly use pH-adjusted water or a very weak feed. As a rule of thumb, keep Athena's stronger rockwool pre-soak, pH 5.0-5.5 and EC 2.5-3.0, for the cubes and slabs a rooted plant moves into. Athena then feed at pH 5.8-6.2 through veg and flower (see the Athena feed chart). The Intense Kickstart page gives 2 ml per litre, with a root stimulant, for pre-soaking transplanting cubes. You'll need a pH meter and pH down.
• Drain, don't squeeze. Wet rockwool right through and let it drain; squeezing crushes the air out of the fibres. Handle it damp: the dry fibres and dust can irritate skin and throat.

Which systems they suit

• Flood and drain. On the IWS Flood & Drain we find a base layer of pure pebbles under a clay and coco mix works best; pure pebbles on their own are for the smaller systems. Which inner pot suits which is in flood and drain systems.
• Recirculating drippers. Origin dripper systems take soil, coco, rockwool or pebbles, but we always recommend pebbles: more air at the root zone and a simpler set-up.
• Slabs on drippers. The rockwool route: starter cube, transplanting cube, slab. Soak the slab, then sit the rooted cube on top with a dripper in it. House & Garden Aqua Flakes is made for run-to-waste rockwool and NFT, and Canna Hydro for run-to-waste systems on inert media.
• NFT. A rooted transplanting cube sits in the tray of an NFT system, with no other medium.
• Under soil and coco. Growers commonly put a layer of pebbles in the bottom of the pot, which the Canna page says improves drainage and aeration. Our Autopot pages say soil always goes on a 40-50 mm bed of them (see the Autopot guide).

In pebbles and rockwool, use a hydroponic feed, not a coco one. Canna Aqua is made for recirculating systems (feed chart), and Intense 60/40 is a two-part feed developed for the clay and coco mix. Tank pH and EC are covered in flood and drain systems.

Pebbles, rockwool or coco?

Our coco coir guide compares how much water each holds.

Reusing pebbles and rockwool

Pebbles: pick out the old roots, then wash the pebbles. Clean the system too: the Liquid Oxygen page says it sterilises systems, pots and reservoirs (at the rate on the label). Canna's Aqua chart doubles Cannazym, the enzyme additive that breaks down dead roots, to 50 ml per 10 litres for reused substrate. Don't use the two together: the Cannazym page says enzymes should not be used with liquid oxygen (hydrogen peroxide). The same applies to Silver Bullet Roots, which its page describes as silver ions and hydrogen peroxide.

Rockwool: cubes stay with the plant, so they are used once. The Cultiwool page describes slabs as reusable, but a used slab holds old roots and feed salts, so growers commonly start each run on new ones. Start with fresh media after any root disease (more on root rot in flood and drain systems).

Which should I choose?

• A system that floods or drips and recirculates: clay pebbles, topped with a clay and coco mix in flood and drain.
• Run-to-waste drippers or NFT: rockwool. Cubes on a slab under drippers, a cube on its own in NFT.
• Hand watering: coco, not pebbles.

Quick answers

How do I use clay pebbles?

Rinse them until the water runs clear and fill the pot. Start the plant in a rockwool cube or plug, then bed the rooted cube into the pebbles. On their own they need a system that waters automatically.

How do I use rockwool cubes?

Soak them in pH-adjusted water or a weak feed, let them drain without squeezing, then put the seed or cutting in the hole and keep the tray in a propagator. When roots show through the sides, move the cube into a soaked transplanting cube, or bed it into rinsed clay pebbles. See propagators and cuttings.

Is stone wool the same as rockwool?

Yes, two names for the same spun-rock material. Buy the kind made for growing, not building insulation.

More Growers' Guides

• Coco coir guide
• Flood and drain systems
• Cal-mag and reading deficiencies
• Perlite and vermiculite

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.