Rockwool

Rockwool cubes hold water and air in even proportion, so you can steer dry backs with precise irrigation shots. Propagate in A-OK or Macro plugs and veg in a 4 inch block. Finish in a 6 inch Hugo or on a 36 inch slab.

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Rockwool

Rockwool Blocks, Plugs and Slabs for Hydroponics

Rockwool cubes are stone wool spun from melted basalt rock and pressed into plugs, blocks and slabs that hold water and air together. This collection is Grodan's range, from A-OK and Macro starter plugs to Gro-Block, ParGro and Hugo blocks and full-length slabs. New to the medium? Read our grower's guide to stone wool in hydroponics....

Rockwool cubes infographic: starter plugs, 4 and 6 inch blocks, slabs, saturation and dry back targets

Stone wool belongs to our soil and growing media range. Growers who want a buffered organic medium can compare coco coir.

Types of Stone Wool

Starter Plugs and Mini Blocks

Grodan A-OK plugs come 98 to a sheet and fit a 10 x 20 flat. Macro Plugs have a slit down the side, so a rooted cutting from an aeroponic cloner lays in without broken roots. Delta and Improved mini blocks bridge the gap to a larger block.

Gro-Blocks, ParGro and Hugo

Our stone wool guide picks the GR10 4 x 4 x 4 or GR6.5 block for veg. The 6 x 6 x 6 Hugo replaces a one-gallon coco bag with similar volume. On flood tables, the ParGro Biggie stands above the roughly 4 inch capillary rise, so its top dries between shots. Improved blocks add a wetting agent for even water content.

Slabs and Loose Stone Wool

ParGro and Gro-Slabs run 36 inches long and carry three or four plants per slab. Uni-Slabs at 9.5 x 8 x 4 inches take a single 4 inch block. Grow Chunks fill pots or pack around blocks.

How Growers Steer Stone Wool

Saturate blocks to about 85% water content before transplant. The guide suggests small shots of about 3% in veg and larger 6% shots in flower. Keep water content above 25%, or blocks can turn hydrophobic. Sensors for water content and EC make those targets repeatable.

Frequently Asked Questions

What are rockwool cubes made of?

They are made from basalt rock and chalk, heated to about 3,000°F and spun into fibers. The fibers are compressed into plugs, blocks and slabs with an airy structure. Grodan's fibers run largely vertical in plugs and blocks, which helps water drain down and air enter from the top.

Do you soak rockwool cubes?

Yes. Stone wool leaves the factory alkaline, so condition plugs and blocks in pH adjusted water and let them drain first. Then saturate blocks fully before transplant. Grodan's crop specialist puts a saturated Hugo block at about 2,650 grams, so weighing a sample block confirms it took up water.

What size rockwool cubes to use?

Start seeds and cuttings in 1.5 inch A-OK plugs, then move them into a 4 inch block for veg. Run a 6 inch Hugo or ParGro Biggie for larger plants or flood tables. For flower, set the block on a slab so roots grow out of the base undisturbed.

Is rockwool safe for plants?

Yes. Stone wool is inert and sterile, so the EC you mix is the EC the roots see. It adds no nutrients and carries no pathogens from a previous crop. Wear gloves and a mask when handling it, since the loose fibers can irritate skin and airways.