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Germinating marijuana seeds in rockwool cubes

Germinating marijuana seeds in rockwool cubes

If you’re ready to start growing and want to find out the best seedbanks that ship to the U.S. click here.

After acquiring growing equipment and cannabis seeds I now had everything I needed to start growing marijuana. The first major step in this process is germinating the seeds or getting the plant to sprout from the seed.

The Plan: Use Rockwool to Sprout and Grow

There are many numerous methods germinating seeds. Popular methods include using paper towels, solo cups or just directly inserting seeds into soil.

I decided to use rockwool cubes as my germination / sprouting medium. It seemed like many people had success using rockwool to start their homegrows. In addition, I liked how after they sprouted it’s easy to transport to soil (the cube has a solid structure) without disturbing the root structure which stresses the plant.

My plan for growing the marijuana plant using rockwool was as follows:

  • Insert the cannabis seed in the rockwool cube (1.5 inch size)
  • Let it germinate and sprout
  • Transfer the cube into a pot with soil
  • Let it grow and flower on my balcony
  • Harvest the fruits of my labor

When formulating this plan I was a little hesitant as I hadn’t seen that many people using this process. Most people if they use rockwool stay with rockwool in a hydroponic indoor growth setup. I didn’t find many people who took the rockwool cube and transferred it to soil after it sprouted and wasn’t sure if this would work.

From this Question & Answer post someone asked if you could use rockwool outside and answered:

Sure, you can grow autoflowering plants using rock wool cubes outside or inside. If you are using the large cubes you can put them into a fabric container, plastic, etc., but they do need a place for the water to run out of them when it rains and when you water.

Granted this was only one answer and didn’t mention if it’s OK to transfer rockwool directly into soil. A little further digging and I found this snippet from another guide which said:

Another method is to use 3″ rockwool cubes to start seedlings in, then put 20 of them in a litter pan, cover it with another pan, and transport this to the grow site. The cubes can be planted directly into soil.

That was good enough (I’m so trusting about stuff I read online) for me so I decided to proceed.

Below will show you what the whole process looks like in video form:

Preparing Rockwool Cubes for Germination

The downside to using rockwool is the need to prepare the cubes before you insert your seeds. Rockwool has a pH of 7.8 and plants like a pH range of 5.5 – 6.8. If you were to simply insert the seeds into rockwool soaked in regular water the pH would be too high and will cause the seed not to sprout or kill the seed.

To prevent this from happening you use an acid like pH Down (phosphoric acid) which can bring down the pH level of the water you plan to soak your cubes in. This is why it was important to buy a reliable pH meter.

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Calibrating the pH Meter

The pH Meter I bought was new so even before I could start soaking my cubes I needed to calibrate it first.

Bringing Down the pH

Now I had to mix a proper pH water solution to soak the cubes in. I filled a plastic food container with Brita filtered water (don’t think this is needed, but why not?) and added 0.25ml (a few drops) of pH Down. I stopped when I got to a pH level of 6.1.

IMPORTANT NOTE: Use pH Down sparingly as it’s a strong acid and will quickly bring down the pH level if you use too much. I ended up going too low and had to add more water to bring the pH back up. Better to be more conservative with this stuff.

Soak the Rockwool Cubes

With the rockwool cubes properly pH adjusted to 6.1 I placed 5 rockwool cubes into the solution for 60 minutes. As for the exact time to soak I found conflicting advice. Some guides said to soak for a whole day while others said 15 minutes. I took a gamble with 60 minutes.

Remove Cubes and Let Drain

After 60 minutes I removed the rockwool cubes and let it drain in the sink for 10 minutes. I was surprised how little water came out and how firm the cubes still were after being soaked. I thought from pictures that rockwool would be like a sponge, but it holds its structure surprisingly well.

Another tip I learned is to not squeeze out or shake out the water from the cubes. You want the cube to be damp as the wetness is what helps trigger the germination process.

Saving Some of the pH Adjusted Water

Instead of tossing away the reduced pH water I saved some in a separate container. As you wait for your seeds to sprout you may need to re-water the rockwool cubes if they get dry. Re-using this water saves you from having to mix up another batch of reduced pH water.

Insert the Seeds and Store

Now that the cubes were properly prepared it was time to insert the marijuana seeds and store them in an ideal sprouting environment.

Inserting the Cannabis Seeds

I placed the cubes into the seedling starter tray and then inserted each seed about a quarter of an inch into the 1.5 inch cube. The seeds are really tiny so to help I used tweezers to place them inside the cubes.

Here’s a video of me inserting one seed into a cube:

Place Humidity Lid and Store

With the seeds now inserted I placed the humidity dome / lid onto my starter tray (came with the set I bought). The reason for the lid is it helps keep the humidity high which assists with the germination process.

In addition to humidity in order to trigger the sprouting process experts advice to place the seeds in a dark environment. This mimics being in soil and coaxes the seed to sprout. To do this I placed the tray onto a shelf in a storage closet.

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Wait for Sprouting

Once setup it could take anywhere from a few days to a week or more before the seed sprouts. Just check on it each day to see the progress. You probably won’t need to add additional water as rockwool does a good job at retaining water. However, if you touch the cube and feels dry it doesn’t hurt to add some of the pH-adjusted water you saved from earlier.

Below you can see a time-lapse of a marijuana seed germination process.

FAQ on Cannabis Seed Germination

If you are using the rockwool cube germination method and have followed the procedure correctly it could take 2 days or up to a week for it to germinate. If it’s over a week there could be issues.

Don’t get too deep. No more than a quarter of an inch.

Find a dark place or room to store the seeds. In order to germinate the seed needs to be in darkness so avoid places where light could leak in.

You need to make sure the pH of the water you used is in the correct range, 5.5 – 6.8. Depending on your pH meter it could be off as well if not properly calibrated or if you are using a cheaper pH meter. In these cases try removing the seed and re-soaking the rockwool cube in another batch of pH-adjusted water

There are multiple ways to germinate seeds. The rockwool cube method describe here is just one method. In addition, another popular method is the “paper towel” method where you place the seeds into a water soaked paper towel.

Successful Germination, Now What?

Congrats! If you succeeded in getting your cannabis seeds to sprout you are now on your way to growing a healthy marijuana plant.

Checkout our other marijuana grow phases posts to make sure you continue to succeed.

How to Germinate Marijuana Seeds the Easy Way

There are many opinions about how to germinate marijuana seeds.

Some people advocate putting cannabis seeds in between two wet paper towels.

Others say put them in a glass of water.

Such methods force you to handle your seeds when they’ve just sprouted.

The seeds have to be carefully plucked from the paper towels or water, and just as carefully placed into a growing media.

The extra handling creates opportunities for germinated seeds to be damaged.

The simplest methods that work well are always the best methods.

That’s why I germinate marijuana seeds in the media they’ll grow in.

If the final destination for my seedlings is soil, soilless mix, or coco coir, I put the appropriate material in a JiffyPot and water them from the bottom with reverse osmosis water at 6.1 pH.

You may notice it’s impossible to get accurate pH readings on zero parts per million reverse osmosis water.

I add beneficial microbes such as Voodoo Juice, Piranha, and Tarantula until I get enough parts per million to make my pH readings accurate.

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The added benefit of using beneficial microbes products is they protect cannabis roots and root zones, and speed up root development.

If the final destination for my marijuana seedlings is a pure hydroponics system such as deep water culture or an ebb and flow hydro system using rockwool, I plant my seeds into a rockwool cube.

Not the smallest size ones either. I like the larger sizes.

Look here for detailed information that helps you germinate marijuana seeds using rockwool cubes.

Note that rockwool must be pre-treated before use to get its pH where you want it.

Look here for instructions on how to condition rockwool.

The best depth for planting marijuana seed is 3/8 th to ½ inch below the surface line.

The best temperature range to germinate marijuana seeds is 74-76°F, but they can germinate up to 80°F. Warmer than that is too warm.

If you don’t have precision temperature control where your seeds are germinating and temps are too cool, you could use a seedling heat mat.

Be careful to control the seed zone temperature when using a seedling heat mat.

It’s best to get a seedling heat mat with built-in thermostat.

Don’t Drown Your Cannabis Seeds

The biggest mistake growers make when they germinate marijuana seeds is overwatering.

The rockwool cube, soil, soilless mix, or coco that you germinate marijuana seeds in shouldn’t be soaking wet.

The media should be totally damp, but not soaking wet.

If the media is too wet, seeds might rot or drown.

If you have healthy seeds, they should germinate in 2-5 days.

Old or inferior seeds can take as long as 10-11 days.

If they’ve been moist that long and haven’t germinated, they’re most likely not going to.

If your seeds are old or otherwise not fresh or reliable, look at this kit that can help them germinate.

As soon as your seeds germinate, put them under a Hortilux PowerVeg lighting fixture, or some other T-5 or low-power LED, for at least 2-3 weeks until you’re sure your seedlings’ root systems and leaf structures can handle full HID, LED, or sunlight.

Rather than using the paper towel or submerse in water methods, this method we’ve given you to germinate marijuana seeds is direct, easy, and doesn’t require you to handle, transplant, and possibly damage a sprouted marijuana seed.

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