Germinating Cannabis Seeds In Rockwool

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Whatever method is used to germinate, specific important considerations must always be considered. Don't bury the seeds too deeply and do not oversaturate the medium with water. Hi mates! Hey i really do need a BEST advice how to start my uniq seeds in high success rate! Some of my seeds may be damaged over years, but i need to… 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.

The Most Common Ways To Germinate Cannabis Seeds

News Americas, NEW YORK, NY, Weds. May 31, 2022: Marijuana seed germination is a simple process, but it is critical for starting the crop in the best possible way.

There are several methods for germinating cannabis seeds. You can try to germinate weed seeds in a glass of water or use any other method shared below. All of them are correct, but some are preferable to others.

We must always germinate the seeds before growing them indoors, outdoors, in hydroponic or organic crops. To accomplish this, you must follow a series of simple steps ideal for effective and rapid germination.

This post will discuss the various methods for germinating marijuana seeds. We will also provide you with a summary of the benefits and drawbacks of each technique. Let’s get started!

How to germinate cannabis seeds

The first step is to summarize the conditions that must be met for adequate germination. The first thing we must control is humidity or water.

We can use tap water without issue unless it has a very high EC (from 1).

In the meantime, a small amount of distilled or reverse osmosis water will suffice.

Many growers typically add a few drops of hydrogen peroxide (hydrogen peroxide) to the water. As a result, the water contains more oxygen. In addition, hydrogen peroxide’s antifungal properties are used. This reduces the likelihood of fungi appearing.

The pH level of the water must be close to 6. The temperature should be around 22oC, and complexes or root stimulators can be added if desired.

Best seed germination substrates

  • On the one hand, there’s Jiffy (pressed peat or coconut tablets)
  • Rock wool cubes
  • Paper napkins.
  • Tacos with Root Riot Spread

Once we have the water and the medium for germinating, we must determine the best conditions for growing.

It is recommended that they be kept in a dark and humid environment, but they must be well ventilated. It would help if you also held it at 25 degrees Celsius.

There are some small greenhouses on the market that simulate ideal seed-germination conditions.

Suppose we use kitchen paper as a germination medium. We must maintain these conditions until the seeds germinate, that is, until the radicle appears, which will later become the plant’s main root.

The seeds are planted when this article is about 1 cm long, and after a few days, a tiny seedling can be seen growing.

Next, we’ll go over the various methods for germinating seeds:

Substrates and media for cannabis seed germination

Now we’ll go over the most common methods for germinating seeds. Keep in mind that the media must be wet with the previously mentioned water.

1. Peat jiffy

Jiffy is pressed, and dry peat plugs. It would help if you soaked them for some time before transforming them into a cloth bag of the substrate.

Usually, seeds are sown directly in the jiffy without first germinating. In other words, the seedlings germinate now in the jiffies where they will grow. The plant is then transplanted into a pot to begin the growth phase.

Jiffy comes in various sizes and materials, such as peat or coconut fiber. Because this substrate retains a good amount of water, they do not need to be hydrated during the germination stage.

2. Rock wool

Another germination method is Rockwool cubes or plugs, which are widely used, especially when using hydroponic or aeroponic methods. This method cannot contaminate the system with peat or coco residues, as occurs when cultivating with a jiffy.

The seeds germinate quickly in this substrate, but You must control the humidity level of the rock wool because it dehydrates, unlike jiffy, which retains moisture. As a result, a daily check should be performed to ensure that the rock wool is moistened.

3. Root Riot Spread Tacos

Root Riot’s propagation plugs are made of a sponge-like material composed of composted organic materials. Its porous structure aids in maintaining the ideal air/water ratio for root development.

They contribute to the proper balance of water, oxygen, and micronutrients. This method of germination produces excellent results. It is used in the same way that rock wool blocks are.

4. Kitchen paper

Cotton is commonly used in place of kitchen paper. It consists of moistening the folded kitchen paper in a container. It is one of the most common methods for germinating seeds.

It is one of the most common ways to germinate seeds of any plant, and it is sometimes replaced by cotton. It entails moistening the folded kitchen paper in a container (for example, a plate). The more paper you add, the more moisture is retained, and the less the paper must hydrate during the process.

When the seeds are moist, they are placed on top and covered with a thin layer of paper (this step is usually forgotten). The plate is then covered with another of the exact dimensions, with a small gap between them to allow the seeds to “breathe.” As with rock wool, we will keep the medium – in this case, the paper – moist at all times.

5. Others

If the crop is to be grown outside, another option is to plant the seed directly in a pot or on the ground. This is not usually recommended because the seed may remain deeper than it should. We also push the seed deeper if we water after planting.

As a result, the soil must be watered before sowing the seed. It is also recommended to soak the seeds for one day before planting to promote seed germination.

Germinate marijuana seeds step by step

Marijuana seeds can be germinated using:

Germinate seeds in a jiffy (small bags of peat)

The jiffy must first be soaked in water for 15 minutes. When they begin to swell, They are drained slightly, and a 1cm deep hole is made. The seed is placed in this hole, with its crown facing upwards.

You can buy small plastic greenhouses with holes to allow them to breathe. The hole is filled with unpressed peat, and the jiffy is kept dry, warm, and ventilated. However, keep them away from drafts. Usually, seedlings emerge after three days. The plants took three days to hatch using this method.

Germinate marijuana seeds in Rockwool

To begin germinating seeds, we must first hydrate the Rockwool like we do jiffy. The excess water is then drained, and the seed is planted in the same manner as in the jiffy, one centimeter deep with the crown upwards and covered with rock wool. Because rock wool can dry out, we must keep it moistened daily if required. Small greenhouses are highly recommended because they keep the Rockwool moist and prevent it from drying out.

When the seedling emerges, it will be transplanted to our choice’s substrate or hydroponic system. Germination in Rockwool takes about three days on average.

Germinate seeds in Root Riot

Root riot’s seed germination procedure is very similar to Rockwool. Seedlings are born after four days using this method.

Germinate seeds on kitchen paper

To germinate marijuana seeds using this method, we take a couple of kitchen papers and fold them several times.

We moisten the paper in a deep plate or similar container with water. The seeds are then placed on top and covered with a layer of paper.

We use another deep plate with a small hole for air to enter as a lid.

It is frequently used in conjunction with one of the preceding methods. When the seed radicle appears, it is placed instantly, a cube of rock wool or a propagation block. After a few days, the seedling emerges.

Conclusion

Whatever method is used to germinate, specific important considerations must always be considered.

Best way starting seeds in rockwool?

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Charlie Green
Well-Known Member
Humboldt14
Well-Known Member

germinate them them move them to the rock wool cubes

Charlie Green
Well-Known Member

germinate them them move them to the rock wool cubes

Humboldt14
Well-Known Member

germinate them what ever method you prefer to use,

I use the paper towel method 100 percent success rate for me, then once the little white tail pops out of the seed, then move the seeds to the 1 inch rock wool with the tail facing down in the hole then lightly put a little piece of rock wool over the hole that the seed is in.

Dirty Harry
Well-Known Member

Soak the rock wool in 5.5 PH water for an hour or so. Squeeze out a little water from each piece. Put a seed 1/4- 1/2 inch in each one and put a cover or humidity dome to prevent from drying out. When they pop, keep them moist and under cover until they get a good start.
Any seed that doesn’t pop just didn’t pop. Pre-germination is just a say to see what seeds pop before you put them in what ever. Some of those will die before they pop out after being put into what ever.
I find it is less stressful on the seeds to just start them in what your going to grow in as they can be damage when moving them when they are just a sprouted root. but that is just MHO and it works for me. Your mileage may differ.

Humboldt14
Well-Known Member

you dont use a humidity dome for starting seeds and second rockwool cubes will not dry out. they retain allot of water, and with

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seedlings in them you would not have to water for about a week maybe longer.

i have a Hydro set up also so i have been starting seedlings in rockwool for years.

Dirty Harry
Well-Known Member

you dont use a humidity dome for starting seeds and second rockwool cubes will not dry out. they retain allot of water, and with

seedlings in them you would not have to water for about a week maybe longer.

i have a Hydro set up also so i have been starting seedlings in rockwool for years.

Just stating my opinion and am not going to argue about it. But trust me, rock wool dries faster than you think when it has seeds/sprouts in it. Cost me some $ to learn that lesson.

Charlie Green
Well-Known Member

Well i putted them in wet toilet paper and inside 2 saucers holding them locked inside. I didnt soak it with water i just made the paper wet. Now im waiting them to crack that shell open, then i will fast plant them into rockwool carefully (becouse later when the root is too long its more danger them to put in rockwool). So lets see what happens! I will start posting some pics also.

ganicsarebetter
Well-Known Member

germinate in paper towl folded up, leave a littel pocket, and moisten the paper towe, and keep moist. leave it in a tupperware contaier in the dark for 3-7 days, keeping the towel moist, and you should see the embryo.

another ethod that ive HEARD works is just to put in a shotglass over night, see what happens ins the seed absorbs the moisture until it pops. thats what ya want..

the paper towel method ALWAYS works. IMO

Dwezelitsame
Well-Known Member

i would have soaked mine over night first 75 percent of time is split the next day

from there go to paper towel all in dark on heating pad at about 80 degrees

once split i transfer to cup with holes in bottom covered with a baggie and rubber band for a little greenhouse effect once soil break i remove plastic once again all heating pad .. i normally use heating pad until first transplant at 3 or 4 weeks old

Fuzzbutter
Active Member

Pre-sprout your seeds in a moist paper towel and then plant in the rock wool once the spouts are over 1/2″ I did this and got 14/20 sprouted from which I had to pick 7 and they are all doing AMAZING.

It’s easy:
get 2 paper towels
fold them in half together
place the seeds in-between the folded area
dampen the paper towel (damp, not dripping, but not too dry either)
place the damp paper towel with seeds in a ziploc bag
put them somewhere dark for a couple days
ta-da!

Also, rock wool is naturally alkaline, so you’ll need to soak it in PH balanced water over night first before putting ANYTHING in it. My plants are currently in rock wool and doing great

Fuzzbutter
Active Member

you dont use a humidity dome for starting seeds and second rockwool cubes will not dry out. they retain allot of water, and with

seedlings in them you would not have to water for about a week maybe longer.

i have a Hydro set up also so i have been starting seedlings in rockwool for years.

Just stating my opinion and am not going to argue about it. But trust me, rock wool dries faster than you think when it has seeds/sprouts in it. Cost me some $ to learn that lesson.

He’s right, rock wool can and does dry out very fast. I had to work with professional hydroponics in my horticulture classes and we ended up losing a whole crop from a tard forgetting to properly set the water timer and the cubes dried up.

Serapis
Well-Known Member

I don’t know where the paper towel method started, I only know that it has been around far too long. I first learned of it over 30 years ago. It’s hogwash. does it work? Most of the time, but why risk handling a delicate seed and it’s new born tap root when you can simply place the seed in any moist medium and have it germinate? If the seed germinates in a paper towel, which surely can’t sustain the seed, won’t it surely germinate in moist rockwool?

Please don’t tell me God created weed that needed man’s assistance to sustain it’s self because there is no paper towels laying around in mother nature. LOL I’m sorry, but I have to laugh at the though of all the extra work towlies have to go through. Just plant the seed already, if it’s viable, it will grow, because amazingly, it is a weed. It’s hardy. The only time the plant has troubles is when we try to care for them.

Serapis
Well-Known Member

He’s right, rock wool can and does dry out very fast. I had to work with professional hydroponics in my horticulture classes and we ended up losing a whole crop from a tard forgetting to properly set the water timer and the cubes dried up.

Yep, you can skip the dome ONLY if you are flooding the tray and watering from bottom up, which is actually recommended. I understand Humboldt’s concern too, because moist seedlings in a cold environment run the risk of damping-off disease. Those trying to avoid damping-off disease, usually the same one’s who have lost crops to it, do not use dome tops unless absolutely necessary. So it’s not that one guy is more right than the other, it is just 2 different methods. Pick the one that best suits you or develop a 3rd.

Well-Known Member
Serapis
Well-Known Member

No, I sense a debate/argument brewing with this one. lol Also, we haven’t heard the other 27 ways to do it yet.

Well-Known Member

i know, i know. i think you nailed it pretty much with the viability aspect. i personally use root riot plugs but i have also had 100% success rate with striaght into coco and water. i don’t know why people make a big deal out of germinating seeds, it couldn’t be much simpler.

Serapis
Well-Known Member
Fuzzbutter
Active Member

I don’t know where the paper towel method started, I only know that it has been around far too long. I first learned of it over 30 years ago. It’s hogwash. does it work? Most of the time, but why risk handling a delicate seed and it’s new born tap root when you can simply place the seed in any moist medium and have it germinate? If the seed germinates in a paper towel, which surely can’t sustain the seed, won’t it surely germinate in moist rockwool?

It’s done because it gives a higher growth yield. This is a fact. In my horticulture class we were taught this by having to germinate seeds in different ways. Some were just dropped in soil, some rock wool, and others were pre-germinated like the above mentioned method. You get a higher yield because you are supposed to germ more seeds than you expect to sprout, once sprouted, so long as they are properly taken care of, you’re all set to go and you already know that your plants are started vs waiting to see it the seeds ever sprout. It’s very rare to lose a sprout from transplanting long as you know what you’re doing. Also, germinating with the paper towel method let’s you already see which plants are growing fastest and strongest so you can do an initial picking of stronger genetics to start with and possibly end up with better plants. I’m a wee bit stoned, so i hope this makes sense. I’ll re-read it later and edit if necessary. but I think that conveys what I’m trying to get at.

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.
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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.

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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.

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