Grodan Improved Gro-Block | 4in - Small - GR6.5 - 4in x 4in x 2.6in - Strips - Pre-Cut Holes - Shrink Wrapped

Intermediate Stonewool Block for Transplant Growth, 4in GR6.5 Wrapped

A young plant that has outgrown its propagation plug still needs a block matched to its current root mass rather than one sized for full-term vegetative or flowering growth. Grodan Improved Gro-Block in the 4in Small GR6.5 format gives that plant an intermediate stonewool block, built on Grodan's hydrophilic fiber structure and supplied with a pre-cut hole ready to accept a rooted starter plug.

This GR6.5 block ships shrink-wrapped in strips, a format that keeps blocks pre-separated and protected during storage before they go on the bench.

Key Features & Benefits

• 4in Small GR6.5 Format — an intermediate size between propagation plugs and full vegetative or slab-based growing.
• Pre-Cut Hole — sized to receive a standard rooted starter plug without on-site cutting.
• Hydrophilic Fiber Structure — manages water and air distribution through the block for even root development.
• Strip Packaging — blocks ship connected in strips for easier handling and bench placement.

Who Should Use This 4in GR6.5 Block

Growers stepping a rooted plug up from propagation into an intermediate block, ahead of a full vegetative or slab stage, are the fit for the GR6.5 size.

About Grodan

Grodan built its reputation on engineered stonewool media after pioneering the technique in 1969, and as part of ROCKWOOL Group today, applies that same fiber engineering to intermediate formats like the GR6.5 block.

Frequently Asked Questions

What growth stage is the GR6.5 block designed for?

GR6.5 is Grodan's 4in Small format, sized for a plant that has outgrown its propagation plug but isn't yet ready for a full-size vegetative or flowering block. It bridges that gap in root volume.

How do I transplant a starter plug into a GR6.5 block?

Pre-soak the block in pH-adjusted water, then set the rooted plug into the pre-cut hole so roots can extend down into the block. Keep the block evenly moist rather than letting it dry between waterings.

Can GR6.5 blocks sit directly on a Gro-Slab?

Yes, GR6.5 blocks are commonly placed on top of a larger slab once roots have filled the block, letting roots continue growing down into the slab. That transition doesn't require disturbing the established root ball.

What's the difference between the shrink-wrapped and non-wrapped GR6.5 strips?

Both use the same block size, density, and pre-cut hole. The difference is only in the outer packaging, with one strip shrink-wrapped for storage and the other left unwrapped.