Starting Seeds in Grodan

The most important thing when using Grodan stone wool is that you need to saturate the product fully before using it. When you don’t fully saturate it, some of the stone wool can stay dry and will be hard to get wet at a later stage (hydrophobic). A way to check if the product is fully saturated is to weigh the product and it should be above a certain weight. As an example a Grodan Delta DM6G Grow Block of 10 x 10 x 6.5 cm (4” x 4” x 2.5”) has a volume of 650 ml, so in theory if it is completely filled with water it should weigh 650 grams. However, there will always be air in the Grodan rock wool so we take a water content (WC) of about 85% after initial saturation, meaning the Grow Block should be approximately 550 grams or slightly greater.

I recommend using nutrient solution at 5.5 pH when saturating the product so that nutrients are immediately available for the plant. For the germination itself nutrients are not needed, so some people saturate the plugs with just water and then add the nutrient solution immediately following germination. However, fairly quickly after germination nutrients are needed for further development. So to sure it is easier to saturate the plugs with nutrient solution. Most growers use 1/2 rate nutrient solution for starting then full rate at transplant.

Below are characteristics of some of the most common types of Grodan used in hydroponic production.

Grodan AO cubes 25/40 10/10

AO cubes are recommended for lettuce, herbs, and other greens in a raft system as the tapered base makes it easier to fit into the raft faster.

AO 25/40 mm (0.98 x 1.58 in)
L25 x W25 x H40 mm
200 (20 x 10) cubes per pad
6,000 cubes per case
30 pads per case

Top = 25 mm
Bottom = 19 mm
Height = 40 mm
Hole = 10 x 10 mm
Weight dry (1 cube) = 2 g
Weight saturated (1 cube) = 18 g
Also available: AO 36/40 = 1.5 in; 98 cubes/pad; 2,940 cubes/case
AO 50/40 = 2 in; 50 cubes/pad; 1,500/case

Grodan AX cubes 25/40 10/10

AX cubes are recommended for lettuce, herbs, and other greens in a NFT system as the wider base gives the plug more stability in the gutter.

AX 25/40 mm (0.98 x 1.58 in)
L25 x W25 x H40 mm
200 (20 x 10) cubes per pad
AX-cube 6,000 cubes per case
30 pads per case

Top = 19 mm
Bottom = 25 mm
Height = 40 mm
Hole = 8 mm (1/4”)
# per pad 200 (20 x 10)
Weight dry = 2 g
Weight saturated = 18 g

Grodan Kiem Plugs in French Trays

Kiem Plugs in French Trays
# per tray = 240 (12 x 20)
Height = 2.1 cm
Dry weight = 1 g

Kiem Plugs
Height = 27 mm
Diameter: 20 & 22 mm
Weight dry = 1 g
Weight saturated = 9 g

Grodan Block Delta DM4G

DM4G
7.5 x 7.5 x 6.4 cm (3 x 3 x 2.5 in)
Weight dry = 25.3 g each
Weight saturated = 273 g each
Volume = 368.7 cubic centimeters
85% WC = 313.4 g

Grodan Block Delta DM6G

DM6G
10.2 x 10.2 x 6.4 cm (4 x 4 x 2 1/2 in)
hole = 1.5/1.6 in (36/40 mm)
Weight dry = 45.6 g each
Weight saturated = 494 g each
Volume = 655.5 cubic centimeters
85% WC = 557.2 g