Starting Seeds in Rock Wool

When going hydro, my favorite medium to start seeds in is definitely rock wool. Rock wool (also known as mineral wool) is produced by blowing a stream of air through molten rock. Rock wool is excellent at holding moisture and delivering it to your plants while simultaneously allowing their roots to breath.You can get rock wool cubes or slabs in many sizes, from little 1″x1″x1″ starters to the 8″x8″x8″ “Big Mama” block. Below is the process I follow to start seeds in rock wool.

What You’ll Need

  • Rock Wool 1″ or 1.5″ mini blocks. The size I like to start in is the 1.5″ GRODAN Mini Blocks, which come with a little hole punched in the top that’s perfect for pushing a couple seeds into.
  • A pH meter. You can get these pretty cheap online. You’ll also need solution to calibrate your meter with.
  • pH Adjustment solution. You will likely need to bring your pH up or down, and the easiest way to do it is with a kit like this.
  • Something to mix your water in.
  • A tray with a dome or something similar to keep the wet blocks in a very humid environment

1. Soak the Rock Wool Cubes in a 5.5pH Solution to Prepare Them

First, fill up a bucket or container with water. You won’t need a whole lot; just enough to cover however many rock wool cubes you’re preparing.

Next, you’ll need to bust out your pH meter and calibrate it. I calibrate my meter just about every time I use it, since it’s a cheap meter and calibration only takes a few seconds. Grab your calibration solution and pour some into a container or the cap of your pH meter. Stick your meter into the solution and see what it reads.

My meter was reading 7.12 in the calibration solution, which is close, but I’d rather it be bang-on. All you need to do for this particular meter is adjust a little screw on the back of it until it reads what it should. My calibration solution is 7.0, so I set the meter to this value.

Next, dip your calibrated meter into your water and check pH. Mine was reading 6.38, so it’ll need to come down to get to the 5.5pH that GRODAN recommends. To do this, I’ll need to add pH down.

Tap water pH of 6.38.

Add your pH Down (or up, or whatever you need) just a little bit at a time, stir it in, then check pH. It doesn’t take much, so be sure to add only a few drops at a time.

I bought a bag of little syringes that I use for measuring nutrients and pH fluid.

My trusty stir stick. It’s a piece of laminate that never made it to the garbage after I finished our flooring.

5.54 – That’ll do.

Once you have your pH dialed in, you can toss the rock wool cubes in the mix. They only need to be dipped for 10 seconds or so.

When you remove the rock wool cubes, DO NOT SQUEEZE THEM! This ruins their structure. Instead, shake them gently. You’ll want to put them into a tray like the one shown below – even better yet, a tray with holes along the bottom to allow excess water to drain through to another tray below. If you don’t have a tray, you can toss them into a sealed ziploc bag or something, and just make sure to open the bag each day to cycle the air inside.

2. Plant Your Seeds

Now it’s time to add your seeds. I bought these seeds that have a colorful clay coating on them and they’re awesome. It makes them way easier to handle.

I’m toying with the idea of trying to grow a cabbage in a 5 gallon Kratky setup under a dedicated COB or two – we’ll see how that goes 😀

Toss a couple seeds into each of the grow blocks to ensure germination. If they both sprout, you can later chop the weaker seedling and allow the strong one to continue growing.

Drop ’em in the hole.

Make sure the seeds get pushed in far enough. This screwdriver is a little overkill for the job, but sometimes you’ve just got to make do.

Once the seeds are in, pinch the hole closed.

If you’re germinating different stuff, be sure to label it somehow. Can’t say this hasn’t bitten me in the ass before.

3. Seal in the Humidity and Place Your Cubes Somewhere Warm

If you’re using a tray, throw the dome on and place the tray somewhere warm. I put mine on a shelf above my lights which put off just enough heat to raise the temperature around them by a few degrees. If your seeds take awhile to germinate, make sure the rock wool cubes are staying moist, but not soaked. You don’t want them to sit in water. The cubes will take awhile to dry out as they hold moisture really well – just keep an eye on them and moisten as needed.

In 3 days, all my kale and lettuce sprouted using this method. I’m thinking the basil and swiss chard will likely be close behind.

Kale sprouts.

More Merlot lettuce.

As soon as you see sprouts, you’re going to want to get them out of the humid tray and get them under lights immediately. Once the seeds have sprouted, you have a lot of flexibility in what you do next. You can place these starter cubes on top of or inside of bigger rock wool slabs/blocks, or into net cups with expanded clay pellets, or into soilless mix or coco coir or whatever. I’ll be moving these into 3.5″ net cups with clay pellets – I’ll be sure to share how that goes as well.

And that’s all there is to it. If you’re struggling to get seeds to germinate, give rock wool a try! Have a different go-to method? Leave it in the comments below.


  1. Have you ever considered switching from rockwool to something more environmentally friendly and less dangerous? Something like a starter plug?

    • LEDGardener

      June 7, 2017 at 6:44 pm

      To be honest, I wasn’t aware of the negative side of it until I did some more reading after seeing your comment. They’re damn convenient and work really well but knowing more about the effects, I think I’ll try something different next. Cheers.

      • Great to hear!! Check out “Rapid Rooters.” They are amazing! Save the tray and purchase some plug refills.

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