How to Start Marijuana Seeds in Rockwool?

Rockwool is an excellent medium for cultivating cannabis. You’re free to take your favorite strain throughout the entire marijuana life cycle with Rockwool. It is easy to learn how to start marijuana seeds using this growing medium. In fact, Rockwool is advisable for both expert and novice growers. Before knowing how to begin, you must understand first what Rockwool is.  

Rockwool – What Is It? 

This good growing medium is suitable for insulation. It works like the stone wool and mineral wool. Rockwool is an inorganic tough material with the properties that are important to a growing hydroponic root region. 

The contemporary horticulture-grade Rockwool substrates are better than the previous versions. They are mainly for growing cannabis. According to, Rockwool is a cotton candy-like fiber spun that came from the basalt rock.  

Rockwool is the trade name for mineral wool. Rockwool blocks, slabs, and cubes are available in different sizes and shapes. They are staples of every hydroponics store.  

The Most Suitable Hydro Systems to Use for Rockwool 

Rockwool often comes along with advanced hydroponic systems like DWC and NFT. But this is a versatile substrate that works effectively with top feeding from drip irrigation up to hand-watering. Rockwool slabs are the number choice of high-volume cultivators. You can also cultivate a huge plant in the oversized rockwool block.  

How to Start Marijuana Seeds with Rockwool? 

Do you want to use Rockwool? Here is a step-by-step guide on how to start marijuana seeds using Rockwool: 


Here are the items you need for this task: 

  • Bowl or water container 
  • Rockwool cubes 
  • pH balancing chemicals like lime juice, pH Up, or pH Down 
  • Cannabis Seeds 

Rockwool cubes have a normal pH level of 7.8. That is alkaline, but cannabis plants often grow within the lightly acidic environment with a pH level ranging from 5.5 to 6.5. Soak the rockwool cubes in the water with adequate pH level. This way, the rockwool cubes will have what the seeds need so they can grow and sprout. 


Step#1. Hydrate and Stabilize the Cubes 

Get a bowl or water container. Make sure it is huge enough because rockwool cubes need more water and space. You can use a salad bowl for three cubes but use a bigger bowl if you plan to do more.  

Fill the bowl or container with tap water. You can also use water filtered through reverse osmosis. You can use a pH test kit or meter to know the water’s pH level. Water may often come out as alkaline, so you must acidify it a bit to make the pH level right for your rockwool cubes.  

To get it done, use pH down chemicals or substances like lime juice. Add a small amount of it to the water before testing the water’s pH level. Pour more drops of lime juice until you get the ideal pH level.  

Avoid letting the water’s pH level to drop further. If the pH level is below 5, the water is no longer safe for the fibers of your rockwool cubes. When the water is ready, you can start hydrating and stabilizing the rockwool cubes. Put them in the bowl or container and allow them to soak for around an hour.  

When time is up, the rockwool cubes should be fat and large due to the water. Get your rockwool cubes from the container or bowl and leave them on a clean surface. Save the used water for step 3.  

Don’t squeeze the rockwool cubes because you need them to keep the right air and water ratio. Squeezing the cubes can damage them. The rockwool cubes will remain moist for many days without the need to re-water them this way again.  

Rockwool works like asbestos. Thus, you must never squeeze or touch them. Rockwool cubes work but can be dangerous so don’t do anything else aside from moistening them.  

Step #2. Plant Your Cannabis Seeds 

Many rockwool cubes have holes. If your cubes don’t have holes, make some in one side. The holes should be at least 0.75 cm deep. 

Get one or two cannabis seeds and put them in the holes. Use something like a toothpick in pushing them down to the base. Push or rip a piece of rockwool through the hole. You do not need to fill the hole completely. Doing this will help the seed germinate within a dim moist environment.  

Now, put them in the tray that has a dome. This will create a bit humidity inside which the seedlings need. It is not mandatory but will be very helpful. Whatever you choose, get your cubes and place them within a cool and dark place. Leave them there for a while.  

The temperature must be around 68 degrees Fahrenheit. Don’t worry if the temperature is 72 because that is still okay.  

Step #3. Leave Them Untouched and Allow Them to Grow 

If you put two or more cannabis seeds, then you can have more seeds sprouting up in every cube on the 3rd day. When the first leaves appear, look for the strongest one and cut off the top of other seeds which are also sprouting beside it.  

Never pluck the leaves out to prevent them from uprooting their neighbors. Just cut the strongest one, reaching the hole. Do it with extreme caution. 

Depending on the temperature, you must water the rockwool cubes one to four times every day. Don’t forget to check the water’s pH level. If the used water is no longer available, make a new solution and store it a clean container or bottle. Avoid overwatering your rockwool cubes. Though you can water them up to 4 times daily, watering them once each day is already enough.  

Some cannabis growers use a diluted nutrient formula in watering their cubes during the germination period. Never do that because it can cause harm to your rockwool cubes.  

Don’t add nutrients to your rockwool cubes. That is not advisable because you cannot make sure that all seeds will get the nutrients.  

Step #4. Transplant the Rockwool Cubes in Their Last Destination 

The germination period will last for a couple of weeks or more. So, wait for two to three weeks after that period before transplanting your rockwool cubes in a hydroponic system. Feel free to choose the hydroponic system you think is the best for your case.  

Likewise, you should think about transplanting your rockwool cubes when the first roots started to come out of your rockwool cubes. Don’t wait for a few more days because the roots can start tangling all over the rockwool cubes because only these materials provide them water.  

Be sure you catch the roots as soon as they come out. This way, when you transfer the cubes in your preferred hydroponic system, the roots of your growing cannabis plants will grow down in the system. 

How to Use Your Rockwool Cubes in Hydroponics 

Before you start growing your cannabis seeds in rockwool cubes, adjust first the water’s pH level. Soak the cubes in the water with 5.8 to 6.3 pH level for twenty-four hours. Use a pH pen or pH-perfect nutrients that will help maintain the right pH level.  

Rockwool Cubes for Seedlings and Clones 

Seedlings and clones will root in the cubes. Get a tray full of 12 to 24 tiny rooting cubes. Small rockwool cubes are ideal for cannabis growers who are not yet familiar with hydroponics.  

Clones and seedlings will let you the right period for transplanting your cubes. Watch out for the algae. You may cover your cubes with plastic when you see them turning green. The algae need light, so they will die if there’s no light.  

Slabs and Blocks for Larger Plant 

Transplanting your rockwool cubes is not easy. Follow the direction given above. Using a heavier medium like soil may be awkward. Using it may even cause harm to your plants. A heavier medium can cause stress on the plants that have transplant shock.  

Best Fertilizers for Rockwool 

Rockwool is inert. Thus, you can fully control the nutritional needs of your growing cannabis plants. Brand name marijuana-specific hydroponic nutrients under the ideal pH solution are your best choice. 

In choosing a fertilizer for your rockwool cubes, make sure it is a quality chemical fertilizer. Like watering, you must be too careful when fertilizing your cubes and seeds. Avoid over-fertilizing them.  

Feed your plants well but never overfeed them. Feed them based on how much they need each day. Don’t overfeed them to force your plants to grow fast.  

When shopping for fertilizers, avoid those products that look extremely thick. Organic nutrients are often too thick, so they are hard to flush from your growing medium. Also, organic fertilizers are common culprits of blockage in the water lines.  

Choose from those fertilizers that will dissolve easily. Check their quality before you buy. Read reviews on the web first and look for the most recommended brands and types of fertilizers that are suitable for rockwool cubes. You may get help from your friends or relatives who have been growing marijuana. 

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