How to Use Rockwool in Hydroponics

Rockwool cubes might be the most popular hydroponic growing medium. It’s so popular because it takes up minimal space, provides some incredible plant growth and doesn’t cost a lot of money. Here’s a video that shows the step-by-step process for using rockwool for your hydroponic system.

What You’ll Need

  • Rockwool cube starter plugs
  • Tray & cover
  • Seeds
  • Vermiculite
  • 1 toothpick
  • Paper towel

Step 1: Presoak the Cubes

soaking rockwool cubes
Step by step images are from the above video by TheGrowGeek.

Rockwool cubes come completely dry, so it is crucial to soak them prior to use. Set your cubes in a tray that has some of your nutrient-rich solution in it. You an also just use regular water for this part if you want, but we suggest using the nutrient solution if you can. The cubes can be totally submerged or partially. This doesn’t take long and you’ll know your done because bubbles will stop rising.

Place the cubes into a sink after they’re finished soaking to drain any excess water for about 15 minutes or so.

Step 2: Plant Your Seeds

planting seeds in rockwool

Set your pre-soaked cubes into some sort of tray that has a removable cover. You can purchase a special “propagation” seed tray, or you can buy any plastic tray/cover such as a cake transporter or some other baking-style tray/cover system.

Pour some seeds onto a paper towel so they’re easier to see and handle. Use the moistened end of a toothpick to pick up the seeds and shove them about 1/4″ down the center of each cube. Push the seed into the side of the inside wall and remove your toothpick. Repeat this step until you’ve planted all the seeds desired. You may want to spray some water to moisten the planted seeds a bit more. A spray bottle is perfect for this step.

Step 3: Add Your Vermiculite

add vermiculite to rockwool cubes

Time to cover the seeds up with vermiculite. Just use a small pinch and sprinkle the vermiculite into each of the holes where your seeds are. Make sure you moisten the vermiculite to help it settle.

Step 4: Cover Your Tray & Let Sit

Now it’s time to start the germination process. Cover the tray and put it on your heat tray in your propagation station for about a day. Check on it daily and add water to the tray if necessary to maintain proper moisture levels.

Keep this daily process up until your plant begins to show signs of growth

This 2nd video shows what to do once you see some growth from your cubes:

Step 5: Separate Your Cubes

Use any type of knife to slowly break your cubes apart into separate cubes.

Step 6: Place Your Cubes Into Net Pots

Your cubes should be moist still and easy to work with. This makes it quite easy to stuff each cube into it’s own plastic net pot.

Step 7: Recycle!

It’s not a good idea to reuse your cubes with new seeds, but they are great for composting. Feel free to take the used rockwool, shred it with your hands, and add it to potting mixes or garden beds so they don’t go to waste.

Where to Buy Rockwool Cubes

Like most things in life, rockwool cubes are available on Amazon, but also at specialty gardening stores or websites. The same also applies for vermiculite which we’ve talked about earlier in this article.

How long do I soak rockwool cubes?

Before use, soak your freshly opened rockwool cubes for about 1 minute, or until you stop seeing bubbles floating. Once you’re in the germination stage, moisten the cubes each day as the seeds begin to grow.

What is Rockwool made out of?

Rockwool is rock that has been heated to a very high temp and spun out into the consistency of wool. The best part of rockwool are these two factors:

  1. It’s able to wick and hold moisture
  2. It’s ability to hold air

The key here is that you don’t want a grow medium that allows for too much water or too much air – rockwool helps you achieve both.


Using rockwool in your hydroponic garden helps to provide a nice mix of water and air to your plant roots, making it a great option for hydroponic gardeners. If you plan on using a lot of it and working with it for extended periods of time, you may want to wear gloves, protective eye wear, and or a breathing mask. We weren’t able to confirm that rockwool is harmful, but when in doubt, safety first.

Good luck and happy growing!