Growing cannabis seeds in rockwool: starting seeds in rockwool cubes


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

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

Germinating seeds in rockwool cubes

Here are the basic steps for germinating seeds in rockwool:

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

How to prepare rockwool cubes for marijuana seeds

Most rockwool cubes made for growing marijuana seeds come with a hold in them. If yours doesn't, create one using a wooden skewer.
Most rockwool cubes made for growing marijuana seeds come with a hole in them.

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

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

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

What is the proper pH for watering rockwool cubes for cannabis seeds?
The proper pH for watering rockwool cubes is 5.5.

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

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

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

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

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

Where should weed seed be placed in rockwool cube?

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

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

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

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

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

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

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

Tips for germinating marijuana seeds in rockwool

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

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

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

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

When to transplant rockwool seedlings

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

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

What is rockwool (definition)?

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

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

Best nutrients for starting seeds in rockwool

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

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

When to start feeding seedlings in rockwool

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

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

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

How to care for seedlings in rockwool

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

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

Does rockwool grow mold?

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

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

Algae on rockwool cubes

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

Starting seeds in rockwool for soil

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

My weed seeds rooted in rockwool – what next?

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

Learn how to choose the best growing medium for your weed plants

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. 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.

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Weed seeds in rockwool: how to grow marijuana seeds in rockwool cubes
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Weed seeds in rockwool: how to grow marijuana seeds in rockwool cubes
Description
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.
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Happy Pot Farmer
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