Newbies may find it a challenge to germinate marijuana seeds, here’s my experience sprouting them in rockwool cubes. How To Start From Seed With Rockwool Cubes In this lesson, we will learn how to start your own seed with Rockwool Cubes. Below is a detailed guide to success with Rockwool, and a step-by-step Learn How To Germinate Cannabis Seeds in Rock-wool Cubes for Hydroponics Today on Cannabasics #115 Gear from video on Amazon: https://www.amazon.com/shop/ruffhousestudios?listId=PJ2A6JYQSUWJ (Paid Affiliate Links Help Support RuffHouse Studios!) Seed Vault: https://www.cannabis-seeds-store.co.uk/
How To Plant Weed 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.
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.
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 Start From Seed With Rockwool Cubes
In this lesson, we will learn how to start your own seed with Rockwool Cubes. Below is a detailed guide to success with Rockwool, and a step-by-step video tutorial can be found at the bottom of the page.
Items you will need:
Rockwool Cubes have a PH of roughly 7.8. This is pretty alkaline, yet our plants prefer to grow in a slightly more acidic environment (between 5.5 – 6.5). In order to prepare our Rockwell cubes for the seeds, we need to soak them in some PH adjusted water, that way they have everything the seeds need to germinate and sprout; water and a slightly acidic environment.
Step 1: Hydrate And Stabilize The Rockwool Cubes
Get a bowl or some other container that is big enough to fill with water and have room left for your Rockwool cubes. Your average salad bowl will work fine for 3 Rockwool cubes, if you are planning on doing more than you will need a larger container.
Fill the container/bowl with water from your tap. You may also choose to use water filtered through a britta or reverse osmosis (R/O) water, I’ve had success with all 3 of them so whichever you have on hand will work fine.
Using either a PH test kit or a Ph meter, determine the Ph of the water. Water comes out pretty alkaline, usually around 7.4, so you will need to acidify it a little bit to bring that Ph down to the desired level. Aim for as close to a Ph of 5.5-6 as you can get.
To accomplish this, use either Ph down chemicals, or lime juice (as it’s acidic). Add these to the water in small increments (VERY SMALL), and test the water to see where the Ph is. Continue doing this until you have a Ph of 5.5-6.
Important: Do not let the PH of the water go below 5. A Ph this low will damage the fibers of the Rockwool Cube
Now that we have the Ph adjusted water, it’s time to stabilize and hydrate the Rockwool cubes in it. Insert the Rockwool Cubes into your container and let them soak for roughly 1 hour. Once the hour is up, the cubes will be big and fat with water. Take them out of the bowl of water and put them somewhere you don’t mind getting a little wet. Save the remaining water for step 3.
DO NOT SQUEEZE THEM TO DRAIN ANY WATER
Rockwool Cubes are designed to maintain the correct water to air ratio and squeezing them may damage their structure. Some of the marijuana forums advocate doing everything from squeezing them completely dry, all the way to flinging them around like paintbrushes in order to get excess water out. None of that is necessary, leave it as it is. It will stay moist for several days without needing to be watered this way as well.
On top of that, Rockwool is like asbestos, you don’t want to be squeezing it or breathing it or generally touching it any more than you need to. Here is a good article on some of the health concerns of Rockwool. I use it because it is what works best, but be cognizant to the fact that it is a potentially dangerous substance to be making contact with so don’t do anything more than you need to with it.
Here is what it should look like:
Step 2: Plant Your Seeds
Most Rockwool cubes come with holes in them, if yours did not, than create a hole in one side that is approximately a quarter inch (0.75 cm) deep.
They should look like this:
Take 1-2 seeds and insert them carefully into the holes. Use a toothpick or similar object to push them down to the bottom, as you want them to be at the bottom of that hole. Rip or push a piece of the Rockwool over the hole (you don’t have to fill it completely), so that the seed can germinate in a dark moist environment.
Now, if you can, place them in a tray with a dome on it. This will help create a little humidity in there which seedlings like. This is not mandatory, but it helps. Whichever you choose, take your cubes and put them in a cool dark place, and leave them alone. The temperature should be roughly 68 degrees F, though my house stays at about 72 and they do fine there. I usually place them above my refrigerator and just leave them for a day or two. My lettuce seedlings sprouted with a quickness the last time I tried, and by the 3rdday they had grown so tall that I had to take the plastic dome off of my container because they were bumping up against the ceiling.
Step 3: Leave Them Alone And Let Them Grow
If you put more than one seed in your cube (just in case one didn’t make it), than you probably have several seeds sprouting up in each cube at the end of ~3 days. Once the first true leaves emerge, we want to select for the strongest one (the one that grew the tallest), and cut off the tops of all other seeds that are growing next to it. Do not pluck them out, as you may uproot it’s neighbors. Simply cut it off as close to the hole as you can without messing with the stronger one that you plan on keeping alive.
Depending on how hot it is (and other factors) you may need to water your cubes 1-4 times a day. Use the Ph adjusted water when doing so (that’s why I had you save the leftovers from step 2). If you already threw that water out, go make another batch of Ph adjusted water and keep it in a separate bottle or container for watering. Note: Do not over water, in fact while some say to water 1-4 times a day, I did it only once a day when I got home after work.
Some people claim they use a diluted nutrient solution to water their Rockwool cubes with during germination. Do not do this, as my experience has always been negative. Note the picture below, where I did an experiment by adding a very diluted grow nutrient to the Rockwool cube on the far left. It died within an hour or two, and the others went on to live happy lives. In my opinion, they do not need nutrients until they get into your hydro system.
Do not add any nutrients to your Rockwool Cubes. The one on the left got nutrients, the other two did not.
Step 4: Transplant Into Their Final Destination
About 2-3 weeks after germinating, you are ready to transplant these babies into the hydroponic system of your choice. A good rule of thumb to go by is that you want to transplant them once the first roots begin poking out of the Rockwool cube. Don’t wait too long though, as eventually the roots will begin tangling around the cube since it is their only source of water. You want to catch them right as they pop out, so that when you transfer them into your hydro system the roots will grow down into the system, and not just try to feed off the Rockwool cube alone.
If you have any questions, suggestions, or comments, please leave them below and I will do my best to answer all of them.
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