Rockwool is a popular starter medium that has been used a long time with hydroponics. Smaller rockwool cubes are used for cuttings or seed germination and then transplanted into hydroponic systems. Larger cubes or slabs can be used to grow larger plants. Rockwool works well as a grow medium because it’s great at both maintaining moisture and allowing air to the roots.
Here’s how to use rockwool to germinate seeds.
Supplies needed For Rock Wool Seed Germination
- Rockwool
- Bowl of water
- pH meter or test strips
- pH lowering agent or lemon juice
How to Germinate seeds in Rockwool
An important note before we get started. When handling rock wool, it is important not to squeeze it or it may alter the airflow to the roots. Lack of airflow in the grow medium can cause root rot.
Rockwool does not come pH balance and comes in a little high on the pH scale. Because of this, the pH will need to be adjusted before inserting your seeds.
Correcting The PH Of Your Rockwool
The first step is correcting the pH of your rockwool.
Rockwool typically arrives with a pH of around 7-8. Since hydroponic plant roots need to be kept in a pH range of 5.5-6.5, the rock wool will need to be soaked in pH-balanced water to bring them down to the correct pH range.
Place your rockwool in a bowl of water. Next, adjust the ph of your water using some ph lowering product or add a little lemon until the water is in the range of 5.5-6.5. Rockwool will need to soak in the pH balance water for about an hour.
You’ll need to have an area ready to place your cubes after they are done soaking. The rockwool will need to stay moist during seed germination so I find using a domed tray that acts as a mini greenhouse works well and requires less watering.
Germinating Your Seeds In RockWool
Now that your rockwool is pH balanced, it’s time to add your seeds.
Most rockwool comes with holes already made, but if not you will need to make a hole in the top of the cube. You can do this by cutting a 1/4 inch deep slit or hole in the top of the rockwool.
I suggest placing a few seeds in the rockwool to increase the chances of getting a healthy sprout. If you only plant one seed and it doesn’t germinate you’ve lost some time. If several sprout you then have the option of choosing the healthiest-looking seedling. The seeds should be placed at the bottom of the hole to escape the light. Put the rockwool in an area that will stay dark and warm, between around 70 to 80 degrees. The cubes lose a little moisture over time so you will need to maintain the moisture needed for the plant to grow. I use a spray bottle with a low dosage of nutrients and moisten the rockwool about once a day.
Sprouting seeds
After a few days, you’ll hopefully see few sprouts start to pop up out of the rockwool. You’ll want to find the heartiest-looking seedling(s) to keep and clip the rest. It’s important to clip and not pull or you may disrupt the root system of the other plant you want to keep.
When the root system starts showing out of the bottom of the cube, it’s time to transplant it into your hydroponic system.
