germinating seeds in rockwool

How to use Rockwool Cubes for Growing, Seed Starting, and Cutting Propagation

Rockwool is a mainstay growing media for commercial hydroponics growers, primarily those who implement drip irrigation systems. Originally used as insulation and also known as mineral wool or stone wool, rockwool was developed in Denmark back in the 1970’s for gardening. It retains moisture well, it retains oxygen well, it never impedes root growth, it is chemically inert, and it comes in a variety of sizes and shapes. These benefits contribute to its popularity amongst growers, accommodating almost any plant they are growing.

Here is my favorite Rockwool brand.

A OK Rockwool Stonewool Hydroponic Grow Media Starter Cubes Plugs + THCity Gloves

Great moisture retention without drowning the seeds. Cheap and reliable starter cubes for seed starting and germination

What are Rockwool Cubes?

Similar in texture to steel wool, rockwool is a dense mat consisting of long strands of natural fibers. Basalt rock and chalk are combined and then melted at a very high temperature (approximately 3000°F) to form lava. Then people throw the lava into a spinning chamber to create the fibers in a process much like making cotton candy.

Due to the firing process, rockwool cubes are chemically and biologically inert. This creates an ideal growing medium for hydroponic growing systems.

After the fibers are spun, they are mixed with a binding agent and pressed into large mats. The mats are then cut into various sizes of cubes and slabs to be sold to customers as a growing medium for plants.

Rockwool cubes are popular with hydroponics growers, as well as conventional growers because of their beneficial structure. When the fibers are spun it creates a structure that is perfectly suited to retain water while holding more oxygen than typical soil mediums. The increased water holding capacity and oxygenation within the root zone is exceptionally beneficial when starting seeds and rooting propagation cuttings.

Another benefit to rockwool cubes is they are considered to be a natural product even though they are man made. This is because they originate from basaltic rock and chalk which are natural ingredients. Being considered a natural product makes them acceptable in organic growing systems, increasing their popularity.

Their inert nature also means growers can quickly adjust the conditions within the root zone to meet the needs of plants. A non-existent cation exchange capacity (CEC) prevents nutrients from being changed by the growing medium or tied up and made unavailable for plant uptake. The cubes can also be quickly rinsed with fresh water, leaching fertilizer salts out.

Rockwool cubes come in a couple of different sizes. The smallest ones work well for starting seeds and propagating stem and leaf cuttings; the larger cubes are used by growers to grow more compact plants.

Do’s and Don’ts of using Rockwool Cubes

One of the drawbacks to using rockwool cubes is the special care and consideration that needs to be taken when working with them. While they are a beneficial resource for hydroponics growing system, they do have some important drawbacks that need to be addressed.

  • DO take the time to properly prepare rockwool cubes before starting seeds or trying to root cuttings. Their natural pH is too high for optimum plant growth and will create problems and instability within the hydroponics system if left at that alkalinity.
  • DO wear protective gear to keep yourself safe when handling rockwool material. The fibers are irritating to skin, lungs, and eyes so it’s imperative to protect yourself. At the minimum, wear a dust mask to prevent inhaling fibers into your lungs. It’s also recommended to wear eye protection and long sleeves if there is a chance of prolonged contact with your skin.
  • DO sterilize the feeding solution before running it back through the rockwool cubes. Even though it is a manmade, inert material, it can still be prone to algae and bacterial growth that can contaminate the hydroponics system if the feed solution is recirculated without being treated properly.
  • DO dispose of the materials properly after you are done using the cubes. Unlike other growing mediums, rockwool is not composed of natural materials so it will not break down over time. If it reaches the landfill it will be there indefinitely. Instead of throwing them away, break up the pieces and work them into your garden or potting soil to help increase their water retention.
  • DO heat treat the rockwool cubes if you are going to reuse them for a successive growing season, or even numerous seasons. This can be done by steaming them, or pouring boiling hot water through them to kill off any bacteria or fungus that may be residing with the cube’s fibers. Some websites recommend using chemical treatments to sterilize rockwool cubes to use again but this can be dangerous unless you are absolutely sure you have rinsed all of the sanitizing chemicals out of the fibers.
  • DON’T squeeze the cubes when they are wet. If you need to remove some of the water from them after preparing them in a pH controlled solution it’s best to shake them gently. Rockwool cubes are known for their internal structure that gives them such great water holding capacity and oxygen movement; squeezing them compacts the structure and hinders the benefits they are so well known for.
  • DON’T forget rockwool is completely inert and can not provide any nutritional value to the plants growing in the cubes. Everything the plant needs has to come from the nutrient solution supplied to them.

How to Prepare Rockwool

Before using your rockwool in a hydroponics system it’s important to prepare it for use. Unlike other pH-neutral hydroponic growing media like Hydroton, and Coco Coir, Rockwool has a naturally high pH, typically around 8.0, because of the way it’s manufactured. During the process there is an abundance of lime that is deposited on the fibers; lime naturally neutralizes acidity, raising the pH.

A pH of 8.0 isn’t optimum for growing plants so it needs to be adjusted to a level that is better suited for growth. A higher pH will make many plant essential nutrients unavailable for plant uptake, causing deficiency symptoms. Most plants prefer to grow in slightly acidic conditions, and will benefit if the pH of rockwool cubes is brought down to a more acidic level.

There are different ways to prepare rockwool for use, none of which are difficult, but they can be time consuming. Due to this you will need to account for a couple of days to prep rockwool before being able to use it.

The first step in preparing rockwool cubes is to soak them in acidic water to lower the pH. The acidic water will dissolve the lime that is formed on the fibers during the manufacturing process and the pH will drop to a better level.

It is best to use distilled water because of its purity but tap water can work in a pinch if it is the only thing available. Adjust the pH slowly until it reaches the desired 5.5; it is critical the pH doesn’t drop below 5.0 since it will start todamage the rockwool fibers when it is too acidic.

After Then submerge the rockwool cubes in the water and allow them to soak for up to 24 hours. When they have finished soaking, remove them from the water and carefully add them to the hydroponics system and allow the system to run without any plants until the pH of the system stays between 5.5 and 6.0 — this means the cubes are stable and can be used.

This video will show you how to prepare rockwool properly.

What are the Uses of Rockwool Cubes

Rockwool cubes hold a tremendous amount of water for their size, which provides a buffer against power outages that make shut down pumps or timers. On average they also hold at least 18% oxygen between the fibers providing a tremendous amount of oxygen to the root zone, as well as making it incredibly difficult to overwater the plants.

Growers primarily use rockwool cubes as growing media for two main purposes in their hydroponics systems: germinating seeds and propagating new cuttings. Typically the 1.5” cubes are used for starting seeds or propagating cuttings. Some of the larger cubes (up to 4”) are used as a growing medium for compact plants as well, but on a much smaller scale.

Germinating Seeds

Starting seeds can sometimes be a stressful adventure. It’s a constant balance between keeping them wet enough to promote germination without having them so wet they dampen off and die. Rockwool cubes are popular for germinating seeds in because of their excellent moisture retention — they are great at helping to keep seeds or seedlings from drying out but don’t let them sit in a waterlogged environment.

Moisture is the critical factor in seed germination. Water enters the seed through the seed coat or tiny opening called a micropyle. The presence of water will activate the enzymatic reactions within the seed that begin germination.

Cloning New Plants

Propagating new plants is the process of taking an existing plant part (usually a leaf or stem cutting) and growing it into a whole new plant. This method results in an identical clone of the original and is cheaper than purchasing seeds and starting from scratch.

Maintaining a high humidity around the cutting is critical for successful propagation. Plants need to keep from drying out; without enough moisture the plant will go into self-defense mode and will stop trying to develop new roots.

Growing new plants through propagation techniques needs to happen in a sterile environment, free of bacteria or fungus. Rockwool cubes are completely inert because of the heating process they undergo during manufacturing. This ensures they are sterile and free of any harmful microorganisms that could hinder propagation.

Step by Step Instructions Using Rockwool for Seed Plantings and Propagating Cutting

  1. Prepare the rockwool cubes for use, making sure to soak them in pH adjusted water to bring their pH down between 5.5 and 6.5.
  2. To Plant Seeds:
    1. Insert 2 seeds in the hole on the top of the rockwool cube. Use a toothpick or other similar object to press the seeds down to the bottom of the hole.
    2. Pinch the hole closed.
    3. Place the cubes in a nursery tray and cover with a humidity dome to lock in moisture.
    4. Maintain at 70 – 80°.
    5. Keep rockwool cubes moist by watering sparingly every couple of days or misting with a spray bottle when they start to dry out.
    6. Remove from humidity dome and place under lights as soon as seeds sprout.
    7. Cut the tops off the seed in each cube (if you planted 2 per cube) that isn’t the strongest or tallest. Do not pull them out as it may dislodge the healthier plantlet at the same time.
    8. Transplant when plantlets reach 2-3” in height.
  3. To Propagate Cuttings
    1. Water the stock plant well the night before beginning the propagation process.
    2. Remove a 3-4” leaf stem cutting from the main stem of the plant, cutting it off as close to the main stem as possible without damaging the node.
    3. Dip cut end in rooting hormone.
    4. Plant the cutting in the rockwool cube making sure it doesn’t poke out the bottom of the cube.
    5. Fill a nursery tray part way full with perlite or vermiculite.
    6. Set rockwool cubes on top of growing media.
    7. Cover nursery tray with a humidity dome to lock in moisture.
    8. Maintain close to 80°.
    9. Crack humidity dome when roots begin to emerge, gradually increasing the day after.
    10. Remove the humidity dome a couple of days after roots first appear.
    11. Transplant when roots begin to poke out the bottom of the cubes.


Rockwool cubes are an indispensable product in the hydroponics growing industry. Naturally occuring basaltic rock and chalk are melted down at really, really high temperatures in a molten lava. This lava is then put into a spinning chamber to create long fibers that are later formed into blocks or cubes for easy handling. Rockwool cubes are popular because they retain moisture and oxygen well and they never impede the development of roots. With a variety of sizes and shapes available, they are adaptable to almost any grower’s setup.

Rockwool is a mainstay growing media for commercial hydroponics growers. I'm going to find out its use, how to prepare, and use it for growing, and propagation

Growing cannabis seeds in rockwool: starting seeds in rockwool cubes

It’s easy to learn how to grow marijuana seeds in rockwool cubes in a few simple steps. We’ll show you the best way for starting seeds in rockwool, how to prepare rockwool cubes, when to start feeding seedlings, the best nutrients for rockwool, and when to transplant rockwool seedlings.

Starting marijuana seeds in rockwool cubes is an easy and effective way to encourage rooting. Rockwool is a reliable growing medium for pot seeds, and it’s suitable for transplanting to all types of growing including soil, hydroponics, and coco coir.

Germinating seeds in rockwool cubes

Here are the basic steps for germinating seeds in rockwool:

  1. Soak rockwool cube in one pint of water that is pH’d down to 5.5. No extensive soaking is required, but make sure water gets fully absorbed. Include rooting stimulant (such as 2 ml per pint of water of Canna Rhizotonic) to encourage root growth once the seed germinates.
  2. Remove from water and shake gently to release excess water.
  3. Add marijuana seed to the hole in the rockwool and push to the bottom gently with a toothpick.
  4. Place under a gentle grow light in a humidity dome to create a warm, moist environment.
  5. You seed should begin to germinate in the rockwool within a few days.

How to prepare rockwool cubes for marijuana seeds

Preparing your rockwool cubes for marijuaja seeds is fairly easy. Most rockwool cubes made for starting seeds will already have tiny holes in them. If yours doesn’t, bore one out by gently twisting a wooden skewer about 3/4 down (but don’t push it all the way through. When you add the seed, you want your seed to sit in the bottom of the hole.

Your next step is to soak the cube in a light nutrient solution. We like to use Canna Rhizotonic to stimulate rooting, it’s worked well. We’ll also add a very light mix of Canna Coco A & B after rooting has occurred – perhaps a 25% to 40% of the recommended feed chart dosage for vegging plants.

What is the proper pH when watering rockwool cubes for cannabis seeds?

Since rockwool cubes naturally have a very high pH, you need to offset this by lowering the pH of your watering solution. This is the most critical part of growing pot seeds in rockwool.

The proper PH for watering rockwool cubes when trying to germinate cannabis seeds is 5.5. Try to keep the PH of your nutrient solution as close to that as you can. Use PH down (or substitute lemon or lime juice) to lower the PH once you’ve added your nutrients.

If mixing a small amount of nutrient solution, use a dropper to apply one drop at a time. Mix gently and test with a PH meter. If you go too far, add a drop of pH up until you hit the 5.5 mark.

Soak your cube in the 5.5 pH water 15 seconds. You dont need to soak the cubes too long, they quickly will wick up the moisture. Remove from the water, gently shake away excess. Now you’re ready to add your seed.

Be mindful of your pH. If your pH is too low (below 5.0) then the cubes will begin to break down – so keep it at 5.5.

Where should weed seed be placed in rockwool cube?

The marijuana seed needs to sit in the bottom of the hole in your rockwool cube. Drop seed in hold and very gently nudge it down with a toothpick or wooden skewer. Be gentle!

How long does a marijuana seed take to root in rockwool?

Marijuana seeds grown in rockwool cubes usually germinate within a few days. If they still haven’t germinated after a week with proper moisture maintained in the cubes then your seed might be a dud. Once your seed germinates you’ll see two small leaver poke up out of the whole. These two leaves, called cotyledon leavers, are quickly followed by additional leaves that have the classic shape associated with cannabis plants. As the seedling grows taller the roots will continue to grow well. Eventually you’ll see white roots start to poke out of the sides.

Some growers use 2 seeds per rockwool cube, letting each sprout for a few days. They’ll then thin out the weaker seed by snipping it’s stem at the base of the cube (don’t pull out, it may damage the other seed’s roots). The benefit is that the grower is left with the strongest, most robust plants. This method is great if you have a lot of seeds, but for the average small home garden grower it is a bit wasteful. Particularly since if you’re using regular non-feminized seeds you’ll likely get 50% males, so you’ll want to grow each out individually so you can maximize the resulting female plants. .

What type of lighting should be used for starting seeds in rockwool?

Seeds like germinating in warm temperatures, so the main function of the light when you start marijuana seeds in rockwool is to provide some warmth. Using a clear plastic humidity dome provides an optimal environment.

We like to use a large CFL lighting fixture designed for growing seeds and clones, placed an inch or two above the top of the dome. T5 fluorescent tube lights which are great for covering a tray of rockwool clones. You can also use LEDs, very low wattage HID lighting set far above the cubes, or even a sunny window. Avoid high-powered HID lighting such as 600-1000W Metal Halide or High Pressure Sodium lights, they are far too intense for young seedlings.

Tips for germinating marijuana seeds in rockwool

Rockwool is very alkaline so you need to go 5.5 with the PH to balance it out. You’ll notice a few leaves emerge, at this point your marijuana seed has successfully popped in the rockwool cube. Monitor the rockwool for dryness and aim to keep it damp, adding additional PH 5.5 water, rooting stimulant, and a very light nutrient mix.

Avoid compressing the rockwool cube after soaking in water in step 1. You want to keep the fibers loose to allow the roots to easily penetrate them.

Using a heating mat underneath the tray of cubes can help speed up germination of weed seeds.

Make sure to monitor your rooted weed seeds closely. Look for signs of nutrient deficiencies in the new leaves, which could be due to either too light nutrient mix or potentially nutrient lockout caused by improper PH. PH issues can be addressed by soaking the cube in 1 pint PH 5.5 water with light nutrients and Rhizotonic. Adding the plant to the permanent medium will allow you to introduce a properly PH balanced environment for the roots to expand outside of the rockwool cube.

When to transplant rockwool seedlings

As your new marijuana plant sprouts from the rockwool cube it will begin growing roots. These roots will begin to poke out of the sides of the cube soon after the seedling emerges. At this point you’re ready to transplant rockwool seedlings.

One advantage to growing weed seeds in rockwool is that they can be easily transplanted across most common growing mediums, such as soil, coco coir, and hydro methods such as Deep Water Culture (DWC). Starting seeds in rockwool for soil grows allows you to just pop to rooted cube in the soil or soil-less medium. If you’re growing in coco you can learn how to transplant seeds in coco coir here.

What is rockwool (definition)?

Rockwool is made by melting a mixture of chalk, rock, and sand at very high temperatures. Air is blown through the molten mixture similar to making cotton candy, resulting in a mineral wool with long fibers that are capable of holding a lot of liquid. This makes rockwool a great medium for germinating cannabis seeds and growing hydroponically.

Rockwool is a man-made substance and it doesn’t break down, so it’s not the best for the environment. Avoid breaking up the cubes and breathing in the small fibers, they’ll damage your lungs.

Best nutrients for starting seeds in rockwool

The best nutrients for starting seeds in rockwool are light mixes of of standard hydroponic nutrients. Most of the commercially-available hydroponic nutrient companies will work fine; it comes down to personal preference, budget, and other considerations such as whether you want to grow organically.

Go very light, start with a 25% strength mix. Work your way up to 50%. You don’t want to overdo nutrients when germinating in rockwool, the new seedlings’ roots are very delicate. Several companies make special nutrients specifically for germinating seeds, but most hydro nutes mixed at a light dose will work fine. Including a rooting stimulator is ideal to help grow out a strong root system as quickly as possible.

When to start feeding seedlings in rockwool

The best time to start feeding seedlings in rockwool is as soon as the roots begin growing. One you see the first set of cotyledon leaves plant emerge from the cube you should provide a light dose. As the roots poke out the sides of the cube a stronger root system is searching for nutrition so you can increase to a 40-50% mix of vegging nutes.

Since rockwool is a hydroponic growing medium the plants will need to get all of their nutrition directly from your nutrient solution. But the new roots will be fragile so you’ll need to monitor your new seedlings closely to make sure you’re not over or under-doing it.

Watch your leaves for nute burn which can be seen when the tips yellow. If the whole plant is a very light green or it starts to yellow then up the nutrient mix accordingly.

How to care for seedlings in rockwool

The most important considerations for caring for seedlings in rockwool is to go easy. Avoid harsh light. Use light nute mixes. And don’t let the rockwool cubes fully dry out.

Check moisture levels daily. Lift the cubes to monitor water levels. Seedlings grow best in light moisture since they’ll grow roots to seek out more water. But if the rockwool cubes dry out fully you risk killing the seedlings. The most important tip for beginners is to check them daily. It’s better to have a wet cube then a dry one.

Does rockwool grow mold?

Rockwool mold issues are usually related to an overly humid environment. Fragile seedlings absorb moisture from the air through their leaves, so using humidity domes help maintain proper conditions. But if your humidity dome is completely covered inside with condensation you risk mold. Open vents on top if the dome, and if need be wipe inside down with a dry paper towel.

If you notice white fuzz forming where the seedling stem exits the rockwool cube then take action immediately!

Algae on rockwool cubes

You may also notice dark green forming on your cubes – in that case you have algae. The moisture rockwool holds provides a nice environment for it, but algae on rockwool cubes isn’t a huge deal. You can prevent algae on rockwool by covering the surface to block light from hitting the cube, without light the algae cannot live.

Starting seeds in rockwool for soil

One of the advantages of rockwool is the ease in which you can transplant to other growing mediums. If you’re starting seeds in rockwool for a soil grow, follow the steps above to germinate. Once roots poke our the sides you’re ready to transplant. You can just place the whole cube into a small hole in your growing container. Don’t try to break up the cube or you risk damaging the fragile root structure.

My weed seeds rooted in rockwool – what next?

Once your weed seeds have rooted in your rockwool cube you’re ready to plant them or add them to your hydroponic setup. Knowing when to transplant rockwool seedlings is easy to judge – once the roots poke out from the cube you’re ready to go.

If you’re new to growing and are looking for an easy medium to grow in, you can use soil or you can consider using coco coir. Coco provides the quick, robust growth of hydroponics while being somewhat forgiving.

Can rockwool be reused?

Rockwool can’t be reused for seedlings since in order to remove the new plant you’d need to break the cube apart. The cubes are very inexpensive, don’t be a cheapskate.


Growing weed seeds in rockwool cubes is an easy way to start your cannabis plants. Marijuana seeds typically germinate within a few days in rockwool cubes. It’s important to properly pH the nutrient solution to 5.5 to balance out the higher pH of the rockwool, this provides the slightly acidic conditions that marijuana thrives in.

It's easy to learn how to grow marijuana seeds in rockwool cubes in a few simple steps. Rockwool is a reliable growing medium for propagating weed seeds, and it's suitable for all types of growing including hyrdroponics and coco coir. Here's how you do it.