Germinating Marijuana Seeds In Rockwool

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In this post we focus on explain the functioning of a hydroponic crop with rock-wool, pointing out the different steps to follow to get a successful c In this marijuana planting guide, we highlight several of the easiest and most efficient methods for successful marijuana seed germination. 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.

Growing cannabis in Rockwool

Rockwool is a product that was first conceived during the 20th century in Hawaii by observing natural volcanic action there. Upon this discovery, a Danish company named Rockwool began to investigate and develop ways to commercialise this technique, and in the late 1930’s released the first standardised stone wool product as we know it today.

To manufacture Rockwool, the raw material of basalt rock must go through several industrial processes, the first of which is melting of the rock in furnaces at extreme temperatures, up to 1600ºC, emulating the action of a volcano and leaving the rock in a natural state of liquid lava.

To obtain the fibres, an organic binder is added to the lava and the mixture is subjected to a mechanical process using centrifugal force, resulting in something resembling a woollen mattress. This wool is then compressed to varying degrees, the density and amount of air between the fibres depending on the projected end use, whether for acoustic or thermal insulation, fire-protection and, of course for it’s use in agriculture.

2. How to stabilise Rockwool

Rockwool Cubes and Slabs

Rockwool substrate is a product that needs to be treated before it’s use. Given that its initial pH level is quite alkaline, nearly 7 points, we should stabilize it to enable us to grow without mishaps from the very beginning.

We must immerse the slabs for 24 hours in a nutrient solution that contains a pH level of 4.5 with EC levels of 0.5-0.6 – ideal to start growing with seeds without worrying about nutrient deficiencies.

When we start to grow the pH level of the slabs should be at 5.5, if that isn’t the case, we should immerse them again during a few more hours checking the pH regularly until it is stabilised. An easy way to know if the pH of the slab is stabilised is to irrigate with water of 5.5 pH and measure the run-off with a pH meter. If the run-off is at 5.5 we can proceed to plant the seeds.

3. How to germinate cannabis seeds in Rockwool

Germination in rockwool

Once the Rockwool cubes have a stable pH as described above, we can proceed with the germination of the seeds we intend to use in our grow.

We will start the germination in two wet paper towels, placed between two dishes in order to maintain a constant humidity and facilitate the germination of the cannabis seeds. Once we see that the seed casing has cracked open and the root appears, we can very carefully transplant them into the stabilised Rockwool cubes and, once established, we can perform the transplant to the slabs.

To transplant the seedlings, we insert the small 3 x 3cm cubes into the 7.5 x 7.5cm Rockwool blocks taped to the slabs. As we do this, we must push gently and slowly until the small cube is well inserted into the larger block, taking great care not to break or damage any roots in the process.

4. How to root cuttings in Rockwool

The process of rooting cuttings in Rockwool is very similar to other cloning methods using jiffies or coco-coir (see how to take cuttings).

After the process of preparing the cuttings for cloning them, we place them in the Rockwool cubes (previously stabilised to pH 5.5) which should be moistened but not soaking wet, to avoid any problems with stems rotting.

Once the cuttings are placed in the cubes we can treat them exactly as if they were in jiffies or coco, monitoring the humidity every day and spraying the cubes to keep a constant moisture on them to ease the rooting. As time goes by it will be necessary to lower the relative humidity until we can finally remove the cover of the mini-greenhouse and start to acclimatise the cuttings so that they don’t suffer when transplanted.

Cuttings in Rockwool cubes

5. How to grow in Rockwool

As previously outlined, the growing medium must be stabilized at pH 5.5 with an initial EC of 0.5-0.6 during this first week of growth, not exceeding two waterings of 1 minute each, with a solution of around 60-70 ml per plant. In this way we will avoid water-logging and facilitate the root growth of plants in this highly absorbent and water-retentive substrate.

To ease the task of watering, using an automatic irrigation system will help enormously and is simple to assemble and use, as you can read in our post on how to install a hydroponic system.

We can maintain this irrigation schedule, varying according to conditions and demand, until the first week of flowering when we will start to gradually increase pH from the initial level of 5.5 , raising it slowly over the course of the week to a level of 5.8-5.9 suitable for this stage of flowering.

Hydroponic crop in Rockwool

From the second until the end of the third week of flowering, we will need to increase the frequency and duration of the irrigations, watering three times a day. The first watering should be of 2 minutes duration when the lights turn on, ensuring the plants start the day with energy. The second watering should be six hours later and 1 minute in duration, with an EC of 0.8-0.9 and a pH of 6.0.

Over the 4th week and until the end of the 5th week of flowering we will again have to increase the frequency of irrigation to 4 waterings at day. The first and the last irrigation should each last 2 minutes, and the remaining two waterings should be 1 minute, distributed evenly through the day. EC levels should remain at 1.0 to 1.2, always taking into account the condition of the plants. We should keep the pH at 6.0 allowing it to fluctuate up to 6.2 to lower it again to 6.0 with pH down.

During the 6th week of flowering it will once again be necessary to increase the frequency of waterings from 4 to 6 per day. The first and last irrigation should last 2 minutes each and the four remaining waterings will each be 1 minute. The EC should still be at 1.4 and the pH continues fluctuating between 6.0 to 6.2 as in the previous week.

In the 7th week of flowering period we will have to keep the same frequency and duration of irrigations; however, in this week, depending on the condition of the plants, we can increase the EC from 1.4 up to 1.6-1.7 and we can also let the pH fluctuate from 6.0 up to 6.3.

In the 8th week of flowering, plants will receive the same number of irrigations, at the same duration as in the previous week, although we can raise the EC to 1.8 depending on the condition of the plants.

In the 9th and last week of flowering phase we will have to increase the duration of all the waterings to 2 minutes, with the pH adjusted to 6.2 and with EC levels as low as possible. We can take advantage of this last week to apply a flush product to help us wash the roots, leaving the plants’ metabolism free of any remaining salts and nutrients and ensuring a far better, cleaner aroma and flavour in the end product.

Hydroponic cannabis grow using Rockwool

6. Recycling Rockwool

Rockwool can be a complicated product to dispose of, given that it isn’t biodegradable. However, today the technology is available to recycle surplus and used material and help to avoid the potential contamination caused by sending it to landfill sites.

The articles published by Alchimiaweb, S.L. are reserved for adult clients only. We would like to remind our customers that cannabis seeds are not listed in the European Community catalogue. They are products intended for genetic conservation and collecting, in no case for cultivation. In some countries it is strictly forbidden to germinate cannabis seeds, other than those authorised by the European Union. We recommend our customers not to infringe the law in any way, we are not responsible for their use.

How to Germinate Cannabis Seeds

Germination relates to the process of a new plant growing from a seed. It is the first step when adding to your cannabis garden. You can purchase the requisite seeds from a variety of sources. On the downside, it means that cannabis seeds vary enormously in quality. We recommend looking at reputable online seed banks to get your supply. However, please note that there are legal issues to contend with if you buy seeds. This is especially the case if you decide to buy them from a source outside the United States.

When buying seeds, opt for mature options with a dark brown appearance and a firm feel. Once you have them in your possession, make plenty of space for them to grow and thrive. Learning how to germinate weed seeds correctly is crucial to enjoying years of healthy plants and fruitful harvests. In this guide, we outline the ideal germination conditions and show you five different methods.

EDITOR’S CHOICE – Homegrown CannabisCo

Homegrown CannabisCo are the masters when it comes to seeds. Offering a massive variety of cannabis seeds that are well categorized, not only does this company create a resource for superb quality options including feminized seeds, it also provides extensive growing information for those looking for some support along their journey.

Ultimate Guide to Cannabis Seed Germination

In theory, germinating cannabis seeds is a simple affair. They only need three things: Air, water, and heat. The famed ‘paper towel’ method is incredibly easy as long as you follow the steps outlined below. Here is a quick overview of the best germination practices before we show you the various methods.

Water

Be careful not to over-soak your seeds. Hard seeds should be soaked for a maximum of 32 hours, although 24 hours is usually enough. Soaking too long can damage them. Marijuana seeds begin to sprout when they receive the twin signals of water and heat.

Once the right conditions occur, the taproot starts burrowing through the shell of the seed.

If the root breaks through the shell and there is no water, the seedling will die. Keep the roots moist once the seed sprouts, and make sure there is ample moisture at all times.

See also  Cannabis Seed Companies That Take Credit Cards

This is arguably the trickiest aspect of germination. You have to strike a balance between ‘warm’ and ‘hot.’ Spring temperatures are ideal in a ‘normal’ year. While cannabis seeds can germinate in colder weather, the process takes longer. Seedlings also germinate faster when there is plenty of humidity in the air. If you are concerned about low temperatures, invest in incandescent bulbs, and place them over the seed area.

First and foremost, seeds perform at their best when they are left alone! When you check them for the taproot, handle with care! Try to avoid touching the white taproot because it can easily break off.

Planting

You don’t have to plant germinated seeds too deep in the soil, or whatever growing medium you choose. 0.5” – 1” below the surface is plenty. Point the white root downwards into the earth to ensure the seedling is ideally oriented.

Are Your Seeds Good or Bad?

Always opt for dark cannabis seeds as they are the most likely to germinate when kept in the right conditions. White or pale-green seeds have little or no chance of growing.

You may have heard the age-old advice on checking for viable seeds. Apparently, if you can crush seeds between your fingers, they are ‘bad.’ First of all, they won’t be good seeds because you have destroyed them! Secondly, experienced growers know that even flimsy seeds can germinate when exposed to the right conditions.

How to Germinate Marijuana Seeds – 5 Methods

1 – The Paper Towel Method

This is the easiest method and requires cannabis seeds, paper towels, and two clean plates. A word of advice: Choose cheap paper towels because they are non-porous. As a result, you can lay seeds and roots on the surface and not worry about them getting stuck. If you use high-quality paper towels, the roots will grow into them!

It is a simple method, but also a risky one. You could damage the taproot while moving the sprouted seeds, or else the paper could dry out and kill the seeds. In any case, here is the process:

  1. Use up to four sheets of paper towel and soak them in distilled water. While you must soak the sheet, make sure there is no water dripping off.
  2. Place two of the paper sheets on one of the plates. Lay the seeds down at least 1” away from one another. Cover with the other two layers of paper towel.
  3. Cover the seeds with the second plate to lock in moisture. You have created a low-cost dome! Make sure you check the seeds often to see if they have sprouted.
  4. Keep the seeds in a room where the temperature is between 70- and 90-degrees Fahrenheit.
  5. Now you must wait! Seeds typically sprout within 1-4 days, although older seeds often take up to a week.
  6. When checking the seeds, make sure the sheets are saturated. If they are drying out, add more water.

You will know that germination has occurred because the seed will split, and a little root appears. Make sure you don’t touch the taproot when it sprouts or during the transplantation process.

EDITOR’S CHOICE – Homegrown CannabisCo

Homegrown CannabisCo are the masters when it comes to seeds. Offering a massive variety of cannabis seeds that are well categorized, not only does this company create a resource for superb quality options including feminized seeds, it also provides extensive growing information for those looking for some support along their journey.

2 – Direct Planting

In nature, a marijuana seed will germinate in the soil and emerge with its taproot growing into the earth. Therefore, you can plant cannabis seeds straight into your growing medium of choice. The main benefit here is that you don’t have to worry about ‘shocking’ the seedling while transporting it.

Your seedling should instantly adjust to the new environment and grow. When using this method, dig a hole 0.5” – 1” deep in soil that is moist but not saturated. Keep things warm with a heating pad or lighting.

3 – Starter Cubes and Seedling Plugs

This has been championed as the easiest germination method. It is effectively a foolproof method. All you have to do is place the seed into the cube/plug and add water. Assuming you put the seeds in a room with the right temperature, germination should occur automatically within a few days. There is a pre-made hole for the seeds, so it is a ‘set it and forget it’ method.

The main downside with this germination method is that such plugs are generally available in packs of 50. Waste is inevitable if you only plan on planting a few cannabis seeds. The plugs dry out in a week or so and become unusable.

You can also use Rockwool cubes as they are cheap and easy to find. However, they are a terrible burden on the environment and bad for your health. Rockwool also has a high pH (which means you must rinse the cubes first) and offers a low cloning and germination success rate.

4 – Overnight Soaking

This is as simple as option #3. It involves nothing more than placing the seeds in a glass of lukewarm water overnight. It is a good idea if you’re using old and hard seeds. The soaking process can breathe new life into them. When you place the seeds in water, they float for a few hours before sinking to the bottom.

The soaking process can breathe new life into old seeds.

If you use a transparent container such as glass, you get to see the white taproot break out! You shouldn’t leave seeds soaking in water for more than 32 hours. Otherwise, seeds that haven’t sprouted yet will drown. If the seeds haven’t germinated by the 32-hour mark, put them in a warm and moist place to complete the process. You should probably use the paper towel method at this point.

5 – A Germination Station

You can purchase a readymade version online. Alternatively, attempt a DIY station by placing a plastic dome over a plate that you then add to a heating pad. Professionally made stations are relatively inexpensive and work rather well. You can buy one for under $40, and their plastic tops ensure better humidity control. With top brands, all you have to do is choose your growing media and plugs to start growing.

Transplanting Germinated Cannabis Seeds

There is no room for a delay once your cannabis seeds have begun to sprout. Now is the time to transfer the seed to its growing medium. Most growers prefer to use small pots, to begin with. Make sure you fill enough pots with loose potting soil and use a pencil to poke a hole around 0.25” deep. Remember, you could break the taproot very easily. Transfer it using tweezers and drop the seed into the hole with the root facing down. Finally, cover it with a thin layer of soil.

For the first few days, use a spray bottle to water the seeds, because adding too much water can drown them. It is worth investing in a pH meter to test the soil regularly and make sure it has enough moisture. If all goes well, the seed should sprout from the earth within a week. If it hasn’t sprouted within ten days, it will probably die.

Turn on your grow light once you have planted the seeds. The heat improves germination rates and speeds up the process of the seedlings opening their first set of leaves. These leaves will remain yellow until exposed to a sufficient level of light in any case. When you plant multiple seeds, you will find that they grow at different rates.

Inevitably, some will fail, while others will flourish. You will have seeds that pop fast and proliferate. Don’t be disappointed if you have a few failures because that’s part and parcel of the growing process. Even when you get everything right, you will inevitably lose a few seeds, and it won’t be your fault!

EDITOR’S CHOICE – Homegrown CannabisCo

Homegrown CannabisCo are the masters when it comes to seeds. Offering a massive variety of cannabis seeds that are well categorized, not only does this company create a resource for superb quality options including feminized seeds, it also provides extensive growing information for those looking for some support along their journey.

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.

See also  How Often Do You Water Your Weed Seeds

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.

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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.

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