Grodan Improved Gro-Block | 4in - GR40 - 8in x 8in x 4in - No Hole

Large Solid Stonewool Block for Custom Root Zones, GR40

A commercial operation running its own propagation method sometimes needs a large stonewool volume without a factory-cut hole dictating plug size or placement. Grodan's GR40 block, at 8in x 8in x 4in, is a large solid cube manufactured from the same engineered fiber as the rest of the GR-series, left open for growers to cut, dibble, or seed to their own specification.

At 8in x 8in x 4in, GR40 is one of the larger single blocks in Grodan's lineup, sized for commercial operations that need substantial root-zone volume in one unit.

Key Features & Benefits

• 8in x 8in x 4in Format — one of Grodan's larger single-block sizes, offering substantial root-zone volume.
• Solid Block, No Hole — left open for a grower to cut, dibble, or seed according to their own propagation method.
• Hydrophilic Fiber Structure — distributes water and air evenly through the block's larger volume.
• Chemically Inert Media — leaves pH and nutrient control entirely to the grower's feeding program.

Who Should Use the GR40 Block

Commercial growers running a custom propagation or transplant method, who need a large stonewool volume without a factory pre-cut hole, are the fit for the solid GR40 block.

About Grodan

Grodan has manufactured engineered stonewool media since pioneering the process in 1969 and, as part of ROCKWOOL Group, offers large solid formats like GR40 alongside its pre-cut block lines.

Frequently Asked Questions

Why choose a solid GR40 block instead of a pre-cut Gro-Block?

A solid block lets a grower dibble, cut, or seed at their own spacing rather than being limited to a factory-sized hole, which matters for operations running a specific propagation plug or seeding method. It also suits growers transplanting an oddly sized rooted cutting that a standard pre-cut hole wouldn't fit cleanly.

How large is the GR40 block compared to other Gro-Blocks?

At 8in x 8in x 4in, GR40 is one of the larger single-block formats Grodan offers, well beyond the small propagation-stage GR4 or GR6.5 sizes. That larger footprint gives roots more lateral room than a Jumbo or Hugo block provides in a single unit.

How do I create an opening in a GR40 block?

Pre-soak the block with pH-adjusted water, then use a dibbler or cutting tool sized to your plug or seedling to create the opening you need. Working the block while it's evenly moist keeps the stonewool fibers from tearing during the cut.

Is GR40 made from the same stonewool as Grodan's smaller blocks?

Yes, it's manufactured from the same engineered hydrophilic fiber structure used throughout Grodan's GR-series; only the external size and hole configuration differ. That consistency means water and air move through GR40 the same way they do through a GR4 or GR10 block.