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How to plant cannabis seeds groblock

Grodan Gro Block Improved Mini Block 1.5″ Strip



Grodan partnered with the Wageningen University and Research facility in The Netherlands and top growers throughout North America to identify the optimal stone wool substrate and irrigation strategies for specialty crops. The result of months of scientific research and trials is Grodan Gro-Blocks Improved with Grodan’s Advanced Wetting Agent.

These leading-edge blocks have the optimal fiber structure and physical properties for crop uniformity, more even Water Content (WC) and Electrical Conductivity (EC) distribution throughout the entire block, and the greatest steering capabilities available. Water distribution is more uniform throughout the entire block which is the basis for a well-defined root system with higher capacities for water and nutrient uptake to produce robust and vigorous crops.

New and improved wetting agent and hydrophilic technology greatly enhances the water characteristics of Grodan Gro-Blocks Improved giving even better water distribution over the height of the block. This results in greater plant development and steerability and more efficient use of water and nutrients.

Optimized wool structure allows for faster rooting-in and nutrient solution replenishment, all while the Gro-Block Improved maintains its structure, firmness, and rigidity throughout the growing process.

Note: This item is for commercial use and is packaged as bulk, without retail wrapping or labeling.

Fail-Proof Guide to Germinating Seeds in Hydroponics
Plus, How to Care for Hydroponic Seedlings…

We have a cannabis seedling germination page which has everything you need to know about all the different germination methods, but this tutorial is different. In this hydroponic seedling tutorial I’m going to share exactly how I do my seeds from beginning to end in a DWC/bubbleponics setup!

Just follow these instructions and you’ll end up with healthy, fast-growing plants that germinate in just a few days. It’s pretty much fail proof!

Learn How to Start Seedlings So You Can Grow Hydroponic Cannabis Plants Like This!

Supplies Needed

1.) Get Cannabis Seeds

There are a few different ways to get cannabis seeds, with the most common being ordering seeds online and growing seeds you find in weed that you buy. Learn how to research and find the right strain.

Here’s a picture showing several healthy and viable cannabis seeds

2.) Germination for Hydroponics

I’ve tried a bunch of different germination methods over the years, and the technique I prefer is for hydroponics is starting with the “Paper towel method” to germinate, putting the germinated seeds into Rapid Rooters, and installing the Rapid Rooters directly into reservoir. Lots of other germination methods as well, but this has worked best for me!

Paper Towel Method

This method is hard to mess up if you follow the instructions!

  1. Place your seeds inside a folded wet paper towel, and place it between two paper plates (or regular plates) so that they don’t dry out.
  2. Check on your seeds every 12 hours but try not to disturb them. When they’ve germinated, you’ll see the seeds have cracked and there are little white roots coming out.
  3. They should germinate in 1-4 days, though some seeds can take a week or longer (especially older seeds).
  4. Keep them warm if possible. One thing you can do to get seeds to germinate a little faster is to keep them in a warm place (75-80°F). Some people use a seedling heat mat but in most cases that’s unnecessary.

These seedlings were sprouted using the paper towel method!

3.) Place Germinated Seed in a Rapid Rooter

The Rapid Rooter should be cut open lengthwise

Gently place the germinated seed inside, root down

Most seedling plugs will go back into place easily, and you’ll barely be able to tell it’s been opened

4.) Prepare Hydro System for Its New Guest

If you haven’t put your hydroponic system together yet, now is the time! Make sure your pumps are all running, and that you’ve made a reservoir with seedling-strength nutrients. You need a home to put your new plants!

Hydro Tips & Hints

  • Air bubbles – have lots and lots of bubbles in your water reservoir. That means your air pump needs to be on all the time for the full grow. The main benefit of hydro is your plant roots are getting an unlimited amount of both water and oxygen. This is achieved by dissolving a lot of air into the water via an air stone and air pump. In order to get the fast growth, you want a lot of bubbles! A highly-oxygenated tank is also far less likely to get root rot, or suffer from other unwanted organisms growing in the reservoir! – This supplement contains a specific bacteria that was first found in rice paddies in Japan in the 40s! It’s been common in Asia for years but only in the last several years has it been available in the US from a company called Botanicare. I highly recommend, even insist, that all hydro growers get this cheap-but-effective supplement to keep plant roots healthy!
  • Add seedling level nutrients from the beginning. A lot of growers, especially soil growers, will tell you not to add any nutrients for the first few weeks of the plant’s life. That makes a lot of sense in soil, because there are lots of nutrients contained in the soil itself for your young cannabis seedling, and giving more right at the beginning can end up giving way too much for such a young plant. However, in hydro, the only nutrients your seedling gets is what’s in the water, plus what little was contained in the seed itself. Because of that, I highly recommend giving seedling-strength nutrients to your plants from when you first fill your reservoir. Seedlings grow a LOT faster with light levels of nutrients than if you only give plain, pH’ed water at first. from the beginning of your plant’s life to end the of your plant’s life

5.) Install Rapid Rooter and water the seedlings until roots reach the water reservoir – Turn on light to keep seedlings warm for best results!

Make sure to always keep the Rapid Rooter moist but not soaking wet.

If you have a top-feed, place the tube near the bottom of the net pot so the water isn’t soaking the seedling’s roots. You just want water dripping out the bottom so the root can use it for oxygen and water until it’s fully established in the reservoir.

Add your Rapid Rooter(s), and fill around the edges with extra clay pellets to hold each one in place.

Since your seed has already sprouted and been in placed into the right growing position, it’ll often pop its head out within just 12-24 hours! Sometimes you see just the leaves, but occasionally you actually see the seedling push the shell above ground. I keep the grow light on even before the seedling appears. It helps keep it warm and guide it toward the light.

When this happens the shell usually falls off on its own as the seedling grows!

The Rapid Rooter in this picture is a little too wet, which makes the seedling prone to “damping off.” If you ever notice the Rapid Rooter actually looks wet or shiny, it’s too much water. Try turning the top-feed off every few hours, or hand-watering the seedling at first. Too much moisture can kill!

Don’t use a humidity dome on seedlings unless it’s very dry where you live. If you do use a dome, consider keeping a vent open and watching the humidity. A young seedling doesn’t require high humidity, and they tend to get “wet feet” and stop growing in constantly wet conditions.

Now that your seedling in in the tank, it’s time to learn how to….

6.) Take Care of Hydro Seedlings

Here are tips for taking the best care of hydroponic marijuana seedlings:

  • Leave roots alone as much as you can with young seedlings in a hydroponic setup. It takes them a little while to get all established in the tank, almost like a fish, and during that time seedlings are much more sensitive to their roots being touched or being moved around. If at all possible, try to let the seedling grow in the same place without being moved for at least a few weeks until you put them in their final home, or even just start them in their final home!
  • Avoid reservoir changes for a few weeks if you can – Going along with what I said before about leaving the roots alone, I’ve found that young seedlings often don’t respond well to reservoir changes. Instead of changing the reservoir, just top off with half-strength vegetative nutrient water until the plant is at least 3 weeks old. It won’t be using enough nutrients to mess with the ratios, and as long as you maintain the pH and use Hydroguard your young plant will be fine with being topped off.
  • Check the pH dailyto prevent nutrient deficiencies
  • Warm but not hot temperature– I recommend hydro growers aim for 75°F, and try to stay between 73-80°F.

These seedlings are a few weeks old, and the grower plans to move them all to the

This is a time-lapse video of a cannabis seedling sprouting and growing over 13 days.

Cannabis seedlings just getting their bearings – try to avoid moving or disturbing them until they are growing fast, with new leaves every day!

Big cannabis plants ready to switch to the flowering stage

I thought hydro plants liked it cold?

Just like in soil, cannabis plants in hydro tend to grow faster in relatively warm temperatures. This is a somewhat controversial statement because a lot of hydro growers prefer to keep their temperature lower in the grow space to help prevent root rot. In fact, there are some growers right now who are reading this and shaking their heads at me.

There’s good reason to believe that hydro plants would grow better with a cool reservoir. For example, the bad microorganisms that make root rot don’t survive well at lower temperatures. Additionally, water can physically hold more oxygen at lower temperatures, which seems like it would be great for faster plant growth. Because of this, lots of growers will AC their room to 60°F, and/or get a water chiller to cool their water reservoir to a similar temperature.

I do agree that if the temperature is above 80°F, your plant is a lot more likely to get root rot. However, I personally have not found that cool temperatures are adequate to prevent root rot. Even if the temperature is 60°F, you still need lots of bubbles and a “good bacteria” supplement like Hydroguard to prevent root rot in many cases.

I’ve seen several growers buy a water chiller and still get root rot. So I personally don’t believe cold temperatures are the best way to go to keep roots healthy.

The other reason I recommend to keep it warmer is because the plants just grow faster around 75°F in hydro. If your roots go from 60°F to 75°F, you’ll see the plants start growing faster in just a day or two, just like how plants in soil grow faster when it’s warm!

Just like in soil or coco, cannabis plants in hydro grow fastest when it’s a little warm, around 75°F!

Although there may be more oxygen dissolved in the water at lower temperatures, at least in my grow tent that apparently isn’t the limiting factor to growth, because plant growth speeds up at warmer temperatures.

I’ve found that if the grow space feels cool to you, it also feels cool to your plant most likely, and it may not be growing to its full potential. Some Sativa strains are particularly sensitive to the cold, though some Indica strains from cold climates will still thrive at lower temps.
Autopsy: Why Aren’t My Seeds Sprouting?

If your seeds still aren’t sprouting and growing properly, consider the following factors.

If there’s no germination at all…

  • Temperature may be too hot or cold – aim for 75-80°F
  • Too wet – seeds and seedling roots should always be moist, but should not stay wet
  • Too dry – if a root dries out the seedling can die!
  • Bad seeds – It might not be you, it could be the seeds themselves! How can I tell if seeds are viable?

If seeds sprout, but then stop growing…

  • Temperature is too hot or cold – aim for 73-78°F
  • Too wet – even though your plants are growing with root directly in water, new seedlings don’t like “wet feet”. They don’t like for it to be too wet near the seed for too long, so make sure your Rapid Rooter or growing medium nevers looks shiny or muddy, as that means there’s too much water! Young roots that stay too wet for too long start to get mushy and die. For this reason, it’s also usually recommended to avoid using a humidity dome with seedlings unless your air is dry. Although clones love humidity domes (they need water from the air because they don’t have any roots to get water), seedlings like it a little more dry or roots tend to get mushy.
  • Too dry – less common unless you live in a very dry area, but sometimes your medium dries out too fast if you’ve got a heavy-drinking, fast-growing seedling!
  • Too much light – if the seedlings get blasted with high levels of light right away, it can shock them. They may need some time to adjust to higher light levels. Simply starting your grow light a little further away that normal is usually enough.Think sunny window at first, and start ramping up after a week of healthy growth.
  • Not enough light – if seedlings are growing long and stretchy without growing new sets of leaves, it means it wants more light.
  • No light for more than a day – if the sprouted seed doesn’t get light within 24 hours after sprouting, it may die. Once seeds are sprouted, get them in a Rapid Rooter and under at least some amount of light as soon as possible!
  • Roots damaged – If somehow your roots got damaged, it can sometimes stop the seedling from growing

Unfortunately sometimes you will never know why certain seeds just don’t thrive! It’s all part of nature

Growing cannabis seeds in rockwool: How to start seeds in rockwool cubes

It’s easy to learn how to start seeds in rockwool cubes in a few simple steps. We’ll show you the best way to start seeds in rockwool cubes, how to prepare rockwool, when to start feeding seedlings, the best nutrients for rockwool, lighting for growing marijuana seeds, and when to transplant rockwool seedlings. This advice also works if you’re looking to start seeds in rockwool that aren’t weed seeds, the techniques are the same.

The Bluelab pH pen is the reliable pH pen to use to adjust the pH level when watering cannabis seeds in rockwool.

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.

How to start seeds in rockwool cubes

Here are the basic steps for germinating seeds in rockwool:

  1. Soak rockwool cubes 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 stimulants (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.

Your 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 it to sit in the bottom of the hole.

Soak cubes in a rooting rooting stimulant

Canna Rhizotonic is a rooting stimulator for cannabis plants that improves root development in cannabis seeds and clones.

Your next step is to soak the cube in a light solution of rooting stimulant. We like to use Canna Rhizotonic to stimulate rooting. It has worked well for both rooting seeds and clones, and can be used as a foliar spray once leaves appear. You can learn more about how much Rhizotonic to use here.

  • A 250ml bottle of Canna Rhizotonic is enough to get you going, but if you intend to use it throughout the vegetative growth stage the 1 liter bottle is a much better value. is another option to improve rooting in rockwool. At under $10 it’s a good value. is another organic rooting stimulant to consider. It comes in 250ml and 1 liter sizes. is a root booster that provides beneficial bacteria that encourages strong root growth. It contains 5 different microbe strains loaded with rooting hormones. It comes in 250ml, 500 ml, and 1 liter sizes.

Regardless which rooting stimulant you choose, make sure that it is suitable for marijuana plants. Some rooting stimulants aren’t intended for plants that will be consumed.

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 is 5.5.

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 for 15 seconds. You don’t need to soak the cubes too long, they will quickly wick up 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.

The Hanna pH pen is a good choice to properly pH water for rockwool cubes.

pH testers

You’ll need a pH tester to get your water to the proper pH. Here are the two pH meters we have used over the years:

    : Bluelab’s pH pen is a standard tool used by most growers. It is fully submersible and is very accurate. They’re usually in the $70-$100 range on Amazon. This is the only pH pen we’ll use these days. : At $30 on Amazon this is a great entry-level tester for new growers. It’s cheap and it works, we used these for years. You can buy new probes that screw in in case you let this one dry out. We ordered the probes directly from Hanna and the cost including shipping was nearly that of a new meter.
  • Hanna also makes a more accurate pH pen, it costs a bit more but it’s still less than Bluelab so it’s worth considering.

As with all pH pens, you need to make sure to keep the bulb wet so it doesn’t dry out, and calibrate it frequently to ensure accurate measurements.

Where should weed seeds be placed in rockwool cubes?

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!

What are the best rockwool cubes for cannabis?

There are multiple sizes of rockwool cubes available, but the 1.5″ x 1.5″ cubes or 1″ x 1″ cubes are the most economical for growing marijuana seeds.

1.5 inch rockwool cubes are perfect for germinating cannabis seeds.

If you check out Amazon you’ll find a bunch of options. Here are a few to consider:

Larger sized cubes such as 4 inches or 6 inches can be used if you intend to use rockwool for your entire growth cycle and don’t want to have to transplant.

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.

Marijuana seeds should germinate in rockwool within a few days, and you’ll soon see roots poking out the bottom.

Once your seed germinates you’ll see two small leaves 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 female seeds, but for the average cannabis 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.

But if you’re looking to start seeds in rockwool that aren’t as expensive, such as tomatoes, then using two seeds per cube is good practice.

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. Or cover cubes in plastic soda bottles with the bottoms cut off if you’re looking for a free DIY solution.

LED grow lights

A good LED grow light such as the SpiderFarmer SF-1000 is perfect for germinating cannabis seeds in rockwool cubes.

You can also use LEDs to grow seeds. Usually the cheapest LEDs you find on Amazon aren’t great for flowering plants due to low light intensity and lower quality diodes. But these lower cost lights are perfect for delicate seedlings.

A few small LED lights for seed growing to consider:

  • Duralux 80-watt 2×1 foot LED is a cheap model for growers on a budget
  • SpiderFarmer 2×2 foot LED grow light is a step up for small plants
  • Mars Hydro 2×2 grow light is another mid-level option

If you’re planning to use your LED light for your complete cannabis grow you should invest in a higher wattage model with a dimmer for when the plants are small.

CFL lighting

We like to use a large CFL lighting fixture (such as the 125 watt Hydrofarm Agrobrite CFL) designed for growing seeds and clones, placed an inch or two above the top of the humidity dome. 125 watt CFLs work great for clones too, and you can also use these for vegetation. For flowering you need more light intensity, CFL bulbs produce thin wispy bud growth.

Look for 125w bulbs or higher, and avoid the budget lower-wattage hardware store CFLs not designed for growing plants. They are marketed as grow bulbs but the usable light for plants is minimal.

You’ll need to replace CFL bulbs once a year, depending on how often you use them. The usable portion of the light decreases over time. If you use a timer to have an 18-hour on, 6 hour off light cycle you’ll reduce electricity usage while extending bulb life 25% compared to running it full blast.

T5 fluorescent lights

The Durolux two foot T5 grow light is perfect to fit over a tray of cannabis seeds or clones in rockwool cubes.

T5 fluorescent tube lights which are great for covering a tray of rockwool clones. Check out the Vivosun 2-foot T5 fluorescent light with 2 bulbs for a good T5 grow light option for germinating cannabis seeds.

There’s a nice value pack on Amazon that includes a Vivosun 2-foot T5 with two bulbs and a heat mat for $66, a good value if you need both a light and a mat.

Durolux makes a two-foot T5 grow light with 4 bulbs for about the same price as the Vivosun 2 bulb T5. It’s a cheaper brand but 4 bulbs provide better light coverage across a full sheet of rockwool cubes.

Like CFLs, you’ll need to replace fluorescent light bulbs every year. You can also buy LED replacement bulbs that fit into T5 fixtures. This is better if you intend to grow for a while since the LEDs never need replacing.

HID lighting: Metal Halide or High Pressure Sodium

This dimmable 400 watt iPower MH/HPS grow light should be raised higher above your rockwool to reduce light intensity.

Avoid high-powered HID lighting such as 600-1000W Metal Halide (MH) or High Pressure Sodium (HPS) lights, they are far too intense for young seedlings.

400 watt or less will work, but you’ll need to put the light several feet from the rockwool cubes to lower the light intensity. Watch that the cubes don’t dry out under the warmth of the light. A dimmable ballast is helpful for young plants, and once roots are established you can turn up to 100%.

Metal Halide is the proper spectrum for growing young seedlings and for the vegetation stage. If you’re trying to use a single HID light setup for the entire lifecycle of your grow, look for a dimmable MH/HPS conversion ballast. This will let you grow seeds at a reduced light intensity using MH, turn up intensity for vegging out, and then swapping to the HPS bulb when you flip into flowering. If you’re only buying one bulb, get the HPS.

Sunlight

You can also germinate seeds the old-school way, putting them in a sunny window. Just make sure to avoid extended direct sunlight when the plants sprout to avoid killing them off, as the rockwool cubes can dry out very quickly in sunlight (especially the smaller 1″ x 1″ cubes).

Consider using a hydroponic rockwool system to avoid your rockwool cubes drying out too quickly.

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.

The MIXC humidity dome is great for starting seeds and the clear plant trays let you monitor root growth.

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. Use a humidity dome to retain moisture and provide the proper environment for young plants.

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 in rockwool can be addressed by soaking the cube in 1 pint of 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.

Should I use a heat mat with rockwool cubes?

A heating mat placed under the tray can speed up germination of seeds in rockwool cubes. This Vivosun heat mat is inexpensive and works for both seeds and clones.

Heat mats speed up germination for cannabis seeds in rockwool. They’re generally inexpensive and are a worthwhile investment regardless of whether you’re growing in rockwool, soil, coco coir, or any other medium.

The Vivosun heat mat is a good value. It is 10″ x 20.75″, perfect for sitting under a standard sized plant flat with a humidity dome. We’ve used this mat dozens of times with good success.

A step up is the Vivosun heat mat and thermostat combo which allows you to fine-tune the temperature settings of your heat map.

If you need to buy the humidity dome and also need a heat mat, check out the Soligt tray with humidity dome and heat mat.

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. Once your rooting system is growing strongly out the bottom and sides you’re ready to transplant rockwool seedlings.

One advantage to growing seeds in rockwool cubes 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 soilless medium. If you’re growing in coco you can learn how to transplant seeds in coco coir here, it’s basically the same process.

Unlike some other methods, rockwool cubes let you closely monitor the growing roots, which makes it easy to know when to transplant rockwool seedlings into your final growing medium.

How to use rockwool in hydroponics

The HighFree 6 site hydro system is an entry-level rockwool hydroponics kit for starting small cannabis plants.

In hydroponic growing the plants’ roots get their nutrition directly from the watering solution. So you can use rockwool in hydroponics to start seeds, and you can move to larger sized rockwool cubes once rooted.

Using rockwool in hydroponics for the entire grow cycle isn’t a method used often by first time growers, as there isn’t as much wiggle room in having the cubes dry out vs. using something more forgiving like coco coir. But since the rockwool dries out easily and can also absorb a lot of water given the size, so you can grow some very strong root systems which in turn can generate large yields of cannabis per plant.

You can also use rockwool to start seeds and then transfer to other hydroponics systems. This includes direct water culture (DWC), coco coir, wick systems, nutrient film technique (NFT), ebb and flow, drip systems, or aeroponics. Once you use rockwool to start seeds and you see healthy roots poking out the sides and bottom you’re ready to transfer into your hydro system.

What is rockwool (definition)?

Rockwool is made by blowing air 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.

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 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 hydroponics nutrients meant for cannabis will work fine when mixed at a light dose. Including a rooting stimulator is ideal to help grow out a strong root system as quickly as possible.

Use a rooting enhancer such as Canna Rhizotonic once roots appear to encourage fast, healthy root growth.

When to start feeding seedlings in rockwool

The FloraFlex 6-site kit is a rockwool hydroponics system that automates watering for six 6″ x 6″ rockwool cubes.

The best time to start feeding seedlings in rockwool is as soon as the roots begin growing. Once 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 50% mix of cannabis nutrients.

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 nutrient 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 nutrient 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 than 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 of 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 knowing how to start seeds in rockwool is the ease in which you can transplant to other growing mediums such as soil. If you’re starting seeds in rockwool for a soil grow, follow the steps above to germinate seeds. Once roots poke out the sides you’re ready to transplant. You can just place the whole cube into a small hole in your growing container’s soil. Don’t try to break up the cube after the seed germinates or you risk damaging the fragile root structure.

My weed seeds have 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.

Summary

Growing weed seeds in rockwool cubes is an easy way to start your cannabis plants. The techniques for how to start seeds in rockwool are the same for cannabis seeds as they are for other types of seeds.

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. Proper lighting is needed, and once roots appear a light mix of nutrient solution will deliver necessary nutrition to the growing seedling. Once roots are established you can then transfer your seedlings.