Grodan Improved Gro-Block | 3in - Small - GR4 - 3in x 3in x 2.6in - Pre-Cut Holes - Shrink-wrap/Strip

Small-Format Stonewool Block for Early-Stage Growth, 3in GR4

Moving a rooted cutting or seedling into its next stage of growth calls for a block sized to match that plant's current root mass, not one built for a much larger transplant. Grodan Improved Gro-Block in the 3in Small GR4 format is an engineered stonewool block built on Grodan's hydrophilic fiber technology, arriving with a pre-cut hole already sized to receive a starter plug or small rooted cutting.

This GR4 block ships shrink-wrapped in strips, a packaging format suited to growers who want blocks pre-separated and ready to place without additional handling.

Key Features & Benefits

• 3in Small GR4 Format — sized for a plant's early stage after propagation, before it needs a larger block.
• Pre-Cut Hole — ready to accept a starter plug or small cutting without cutting a hole on site.
• Hydrophilic Fiber Structure — distributes water and air through the block for consistent root-zone conditions.
• Chemically Inert Media — doesn't alter feed solution pH or nutrient balance, leaving that control to the nutrient program.

Who Should Use This 3in GR4 Block

Growers moving rooted plugs or small cuttings out of propagation and into their first standalone block, before stepping up to a larger format, are the fit for this GR4 size.

About Grodan

Grodan has manufactured engineered stonewool media since pioneering the technique in 1969, and today operates as part of ROCKWOOL Group, applying that same fiber process across its GR-series Gro-Blocks.

Frequently Asked Questions

What is the GR4 block size used for?

GR4 is Grodan's 3in Small format, sized for a plant's early growth stage right after propagation, before it's ready for a larger block. It's typically the first standalone block a rooted plug moves into.

How do I use a GR4 block with a pre-cut hole?

Pre-soak the block with pH-adjusted water, then insert the rooted plug or cutting into the pre-cut hole so roots can grow down and out into the block. Keep the block consistently moist rather than letting it fully dry between waterings.

Can GR4 blocks be placed on a larger slab later?

Yes, GR4 blocks are commonly used as a step between propagation and a larger Gro-Block or slab, with the whole small block moved onto the next surface once roots have filled it. That step preserves the root ball rather than disturbing it at transplant.

Does the pre-cut hole fit any starter plug?

It's sized for a standard Grodan starter plug or small rooted cutting. Check the exact plug dimensions against the block's hole if you're using a different brand's propagation plug.