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. It works well as a grow medium because it’s great at both maintaining moisture and allowing air to the roots. Rockwool is not pH balanced and will need to be adjusted before getting started.
Preparing Rock Wool For Seed Germination
The pH of the rockwool will first need to be brought down. This can be used with either a pH lowering product or a lemon. When handling rockwool, it is important not to squeeze it or it may alter the air flow to the roots. Lack of air flow causes root rot.
Get a bowl of water and something to measure the pH of the water. The goal is to get the water to a pH measurement around 5.5. To do this you want to add a small amount of your pH lowering agent or lemon juice until you reach the correct measurement.
Next, you want to soak the cubes in the treated water. It needs to be soaked for 10 seconds.
How to Germinate seeds in Rockwool
You need to have an area ready to place your cubes after they are done soaking. I find using a domed tray that acts as a mini greenhouse works well for germination and need less watering. After the rockwool is done soaking, take the cubes out of the water and line them up for seeds or cuttings. 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 of 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. With most vegetables, I use 2 or 3 seeds and with herbs, I use a few more. The seeds should be placed at the bottom of the hole to escape the light. Put the rockwool in an area that will stay between around 70 to 80 degrees.
After a few days, you will hopefully see few sprouts start to pop up out of the rockwool. 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 a low dosage of nutrients and moisten the rockwool about once a day. After several days, 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 to show out of bottom of the cube it is time to transplant into the hydroponic system. Be careful not to place the rockwool below the grow medium your transplanting it in or water will puddle, increasing the chances of root rot.