Germinating seeds in rockwool cubes will be a great process for your plant’s growing experience. Rockwool is a soil-free medium or substrate used for starting seeds SSSC’s Cannabis Seeds Germination Guide: In this article we will try and give you an overview over the different germination methods used for cannabis seeds and then tell you our preferred way of Rockwool cubes are a hydroponic growing medium often used to propagate plant cuttings, start seedlings, and clone. Learn about how this hydroponic media is used by growers.
Germinating Seeds In Rockwool Cubes
Germinating seeds in rockwool cubes will be a great process for your plant’s growing experience. Rockwool is a soil-free medium or substrate used for starting seeds. They can as well be used in hydroponics or stem rooting.
Rockwool offers so many benefits in almost any type of plant which makes it popular and best-loved among most growers.
This article attempts to enlighten you on germinating seeds in rockwool cubes and more, so read on to learn.
Table of Contents
Rockwool are made from spinning chalk and basalt rock which are formed into a thick mat of natural fibers. The combinations are made to look like the consistency of cotton candy. The spun fibers are then combined with a binding agent. The materials are pressed and formed into cubes which we know as rockwool cubes. Mostly, rockwool cubes can come in one or two inches sizes: although, you can also obtain bigger rockwool cubes of about four inches.
The smallest rockwool sizes are the most appropriate for starting seeds, leaf cuttings, and stem propagation. Bigger rockwool cubes are mainly used to grow denser plants.
Guide On Germinating Seeds In Rockwool Cubes
Seed germination requirements can sometimes demand careful and precise planning or process. Take for instance moisture requirements: moisture is an essential factor when it comes to seed germination.
Therefore, rockwool cube is a great choice to use in germinating seeds because rockwool cube has the ability to retain moisture. Rockwool cubes will keep your seeds moist at the same time preventing your seeds from sitting in a waterlogged environment. They can retain just the right amount of water to keep your seeds from drying out thereby improving the germination of your seeds.
How To Start Seeds In Rockwool
- The first step to take in germinating seeds in rockwool cubes is the preparation of the rockwool cubes. Rockwool cubes need to be soaked in pH adjusted reverse osmosis water before use. They don’t require too much soaking; however, ensure the cubes are fully absorbed in water.
- Soaking is required because rockwool cubes usually have a high pH of about 7.8 but your seeds will require a pH of 5.5 which is a slightly acidic medium. This way, your seeds will have an optimal chance of germinating and sprouting appropriately.
- Remove the rockwool cubes from the soaked water and give them a gentle shake to take off excess water. However, don’t squeeze them. Now your cubes are ready to be seated in your germinating tray.
- Most rockwool cubes have holes in them, so get your seeds and drop them into the holes of the rockwool seeds. With a toothpick or other identical objects, gently bring down the seeds to the bottom holes.
- Now squeeze the rockwool hole to close them or simply break off a little piece of the rockwool from the sides to cover the top. The seeds need to be placed in a dark environment.
- Now cover with a humidity dome so that they can maintain a humid environment. Keep them at 70 to 80 degrees. Now you can place them in a gentle grow light.
- You can maintain a moist environment by misting with a spray bottle whenever you notice them drying out. Sprouting should be noticed after a couple of days.
- When the plantlets are about 2 to 3 inches, they are ready to be transplanted into your hydroponics or traditional soil.
Benefits Of Start Seeds In Rockwool
- Water Retention: rockwool cubes have an excellent water retention property which is very essential for the germination of your seeds. At the same time, rockwool will not waterlog your system. It has the ability to drain excess water, retaining just the right amount of water needed by your seeds to germinate.
- Air Circulation: rockwool cubes will provide good air circulation and oxygenation of the root system.
- Clean: rockwool is a clean or sterile medium and it doesn’t have any weeds, pathogens, or pests.
- Reusable: rockwool cubes doesn’t decompose over time. Therefore, it can be reused over again.
- Safe: they are made from natural materials. Thus, it is very safe to use for your germinating seeds because it doesn’t contain any toxic substances.
Why won’t my seeds germinate in rockwool?
The problem might be that they are not getting enough light. Rockwool is a great product for growing plants and it has been used for centuries. It’s made from ground-up volcanic rock so it provides nutrients and structure for plants to grow. It also helps maintain moisture in the growing medium. There are many different types of rockwool, each with its own unique properties.
Rockwool is like a sponge and absorbs all moisture from the air. It’s great for storing seeds but if you put them in it they won’t get a chance to germinate. You need to keep your seed starting containers dry and make sure you water the soil before you cover the seeds with the rockwool. I would use vermiculite for a seed starting medium because it’s a bit drier than the rockwool.
Do seedlings need nutrients in rockwool?
As long as they are not sitting in wet rockwool, yes. If you do not want to water them, use a bulb planter and keep them in a shaded area. If you’re going to be transplanting them to larger containers, you can add some nutrients to the potting mix if you want to. If you have the fertilizer that comes with the plant, that will be fine. You could also use a diluted liquid fertilizer such as a 10-10-10.
How often should you water seedlings in rockwool?
Rockwool needs more frequent watering than soil because it does not absorb water as well. It is best to water your Rockwool when the top inch or so of the material is wet, which usually happens within 24 hours. It is important to keep the surface of the material damp, but not wet. If the Rockwool dries out too much, it will crack and become brittle. If you see a rockwool plant that appears to be drying out, cut off the top few inches and let it dry out again before re-watering. Once you have cut back on the water supply, wait for the top inch or so of the material to be wet again, and then re-water. You can also use a spray bottle with a fine mist setting to apply water directly to the Rockwool.
Can I start seeds in rockwool cubes?
It would be fine as long as you keep it moist enough. I think it should be fine to use sand or even peat moss as long as it’s not very wet. As far as keeping it moist, I have a few suggestions:
1. You can put a humidifier in the room where the cubes are.
2. You can put a small fan in the room with the cubes and blow on them.
3. You can use a misting system (like a misting bottle). I’ve heard of people putting plastic bags over their plants and then cutting holes in the bags to spray water into. That way the plant doesn’t dry out.
Rockwool cubes are a great substrate for seed starters. They are safe and will provide your seed with good moisture and oxygen for optimal germination.
They come in various sizes and shapes and they can blend into different growing systems. So, give your seeds an excellent germinating chance with rockwool.
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SSSC’s Cannabis Seeds Germination Guide:
In this article we will try and give you an overview over the different germination methods used for cannabis seeds and then tell you our preferred way of starting SSSC-seeds. In general cannabis seeds germinate best at a warm temperature of around 21-26 degrees Celsius.
We have personally tested all the described methods in a side-by-side germination test already years ago and found them to be almost equally effective, if done correctly.
It strongly depends on the growers personal preferences (e.g. organic-grower, soil-grower, hydro-grower etc.), which method of germination is best suited for him individually. Somebody who wants to grow purely organic, will not want to use rockwool-plugs as starting media out of principle. A hydro-grower on the other hand will not want to start the seeds directly in soil, when he later continues his grow on rockwool-cubes.
The best way to germinate cannabis seeds:
a) directly in soil
b) in rockwool cubes
c) on wet paper towel
a) Germination directly in soil:
In nature a seed will start its germination on or in the ground as soon as it comes in contact with enough water. The water will permeate into the seed-shell and start the germination process inside, the small plant-embryo inside the seed will then start to grow. If it stays wet enough and does not dry out during the germination process, the plant will grow into a small seedling. As growers we want to re-create this natural process.
Therefore, the first thing to do is putting the seeds into a small glass of water and let them soak there for 24 hours.
There is special planting soil for seedlings (which is not fertilized) you can buy at any gardening shop, or you can simply use a light-mix (=soil with perlite) for cannabis (which is only slightly fertilized).
It is very important to put the seed NOT too deep into the soil when you plant it! The deeper you put it into the soil, the more the seedling has to fight to reach the surface.
The maximum depth a seed should be buried is about two times its own diameter, therefore with a cannabis seed that is about 5 mm deep!
The seed is planted with the TIP OF THE SEED POINTING DOWNWARDS into a small hole in the center of the pot and then covered with soil.
The WHITE TAP-ROOT that will grow out of the seed, WILL ALWAYS GROW OUT OF THE TIP OF SEED and then search its way downwards into the soil. Therefore it is important to put the seed into the soil in the correct position, this will ensure that your roots don’t get tangled and your plant can develop quickly.
The small pots are put under a grow-lamp (e.g. T5, LED, HPS, MH) and after 24 to 48 hours in the soil the stem of the seedling should start to show and poke out of the soil. The first set of round leaves, the so called cotyledons, will open up and the seedling can start photosynthesis and start growing.
b) Germination on rockwool cubes:
Rockwool Cubes are especially designed and produced as a sterile growing medium for plants and come in different standard sizes for easy re-planting. The smallest cube-size (25-36 mm side-length) is used for the germination of seeds.
First the rockwool cubes MUST BE soaked in ph-adjusted water (ph=5,5) for 2 hours in order to get their ph-value to 6,0 (they are ALWAYS produced with a ph-value of 7,5, which is much too high for cannabis and MUST be neutralized first!) and then the excess water is drained.
Now the rockwool cubes are ready for use. They can be either be soaked again, this time in a mild nutrient solution (EC=0,3) with a ph of 6,0 or used directly.
The rockwool cubes are put into special trays, which hold 77 to 125 little cubes.
Each cube has a tiny hole on the top, which is made to hold the seed.
The seed is carefully (with a pincet) put into the tiny hole with THE TIP POINTING DOWNWARDS, in order to make sure the root grows downwards.
The tray holding the rockwool-cubes has to stay moist at all times during the germination process.
Now it takes appr. 48-72 hours under a grow-light for the seedling to appear and grow out of the top of the rockwool cube.
c) Germination on wet paper towels:
As alternative to starting your seeds inside the growing-medium (e.g. soil-, peat, rockwool) you can pre-germinate them on wet paper towels and let the white tap-root grow out of the seed-shell before you transplant the seeds into your preferred growing medium.
The seeds are put on one half of a wet paper towel, which is then folded over to cover the seeds from both sides. After all air-bubbles are carefully pressed out of the folded wet paper-towel and insuring that the seeds are completely encased inside, the paper-towel is put inside a zip-loc plastic baggy. The bag is closed and then hung up at a dark place (e.g taped to the inside of a closet-door). By hanging the seeds inside the bag instead of laying them on a plate, you ensure that the white tap-root can grow out of the seed-shell STRAIGHT and does not curl up in a circle. This will later enable you to transplant the germinated seed easier into the growing medium. After 48-72 hours inside the plastic bag and encased by the wet paper-towel the seeds will have opened up and the white tap-root should have grown out of the seedling for about 5-10 mm. Now they can be taken off the wet paper-towel with a pincet (VERY CAREFULLY without hurting the fragile white tap-root!) and planted into the growing medium with the WHITE ROOT POINTING DOWNWARDS! Since the seed has started its germination already, it does not need to be buried deep. The best way is to put it into a small hole in the soil with the seed-shell still peaking out of the soil a little bit. This way it can start to grow upwards right away and you will have a tiny plant already 24 hours after transplanting the pre-germinated seed into the soil and putting it under a grow-lamp.
Here at the SSSC we prefer to grow organic (see Karel’s ‚Bio Grow-Book‘) and therefore we prefer to germinate in soil.
Here is our little step-by-step guide to germination success in soil:
1.) Soak the seeds for 24 hours in a small glass with distilled water
2.) Fill a small pot with potting soil or light-mix
3.) Water the soil well with ph-neutral (ph=6,0) water, but make it not too wet (not soggy!)
4.) Make a small hole in the center of your pot
5.) The hole should be appr. 5 mm deep and wide (e.g. you can use your small finger)
6.) Place the seed in the center of the hole with the TIP OF THE SEED POINTING DOWNWARDS!
7.) Cover the hole with soil and carefully push the soil from all sides inwards to make sure the seed is well enclosed inside the soil
8.) Push down and compress the soil at the outside, directly at the walls of the pot, in order to make a little ring around the whole pot. This ring is for drainage while watering. It makes sure the seed does not get flushed out of the soil at the beginning and later prevents the stem from standing in water and starting to rot.
9.) Water the little pot from time to time, by pouring a little water into the ring on the outside, before the soil gets too dry. The water will be sucked towards the center of the soil right away. Try to avoid pouring water directly into the center of the pot, in order to avoid flushing out the seed per accident!
We hope you find the germination method that fits best to your growing style and wish you lots of success with your SSSC-seeds!
Hydroponic Growing Mediums: How to Plant into Rockwool Cubes
Pros and Cons of Using Rockwool Cubes as a growing medium
Pros of growing in Rockwool cubes
- Sterile medium for cloning
- Good drainage
- Excellent for seed germination
- Can use any nutrient solution
- Easy to transplant
- Easy for roots to penetrate
- Can be added to compost
Cons of Rockwool cubes as a hydroponic growing medium
- Naturally High pH
- Not sustainable
- Not biodegradable
- Potentially dangerous to human health
- Grows surface algae
What is a Rockwool cube made of?
Rockwool cubes are made from chalk and the basalt rock that is formed by volcanoes, heated to a high degree (3000 ℉ ) of heat then spun and cooled. Next, a binder is added and the substrate is flattened to form a sheet. Rockwool is often sold as a hydroponic growing medium in granulate mini blocks, starter plugs, cubes, and slabs. Their dense structure promotes strong root development making it ideal for seed starting in a hydroponic system. Although Rockwool is made from natural materials, the process is not natural making the substrate unsustainable, energy-intensive, and not biodegradable.
The physical properties and harmful chemicals of Rockwool cause skin, eye, and lung irritation and have been linked to long-term health concerns. ( Environmental Protection Agency lists it as a “Group 2B” material)
Are Rockwool cubes good for cloning?
Rockwool is a sterile, manufactured substrate containing no pests, weed seeds, or diseases making it a common choice for cloning plants in a sterile environment. This also means that it contains no beneficial fungi or nutrients. This is ideal for those wanting to retain full control of their nutrient solution and regimen.
When cloning in any soilless media it is important to maintain humidity with a humidity dome throughout the rooting process. We recommend using our 6” tall humidity domes to provide growing space and ideal conditions for new seedlings and stem cuttings.
Hydroponic Gardening with Rockwool
Seed Starting Using Rockwool Cubes
Seeds are easy to plant in the 2 inch Rockwool cubes that fit inside the Bootstrap Farmer 32-cell insert tray. These typically have an indentation in the center for the seed or seeds. For smaller seeds, dip a moistened toothpick into your seeds to pick up one or two. Insert the toothpick into the indentation and twist it against the side of the hole to release the seeds.
Once all of the cells have been planted, ensure that the media is evenly moist and place under a blackout dome until the majority of the seeds have sprouted.
Planting clones in Rockwool cubes
Planting softwood clones in mineral wool work very well because of the moisture-retaining properties. When a new cutting is starting to form root buds, drying out could send the cutting back into survival mode instead of new plant development. To plant clones, use sterile equipment to take a stem cutting from the mother plant. Dip the end of each stem cutting into rooting hormone, honey, or aloe powder to protect it from bacteria. Push the cutting into the cube at least an inch deep but not through to the bottom.
The cubes can then be placed into a tray with holes or one of these mesh tray sets for the rooting period. The mesh tray will allow for easy bottom watering with the 1020 deep tray while the humidity dome will ensure proper moisture levels until roots have formed. Place the entire 1020 on a heat mat with a thermostatand keep between 70℉ and 80℉ until root growth is established.
What can I use instead of Rockwool cubes?
Sterile growing media like ProMix , soil blocks, coco coir , hemp mats , clay pellets, decomposed granite, perlite, vermiculite, peat moss, and potting soil are all viable alternatives to using a Rockwool slab or cube for starting seeds, planting, and cloning.
How often should you water seedlings in Rockwool?
Rockwool is very good at holding moisture. Its superior water retention abilities make it ideal for delicate new growth. Plants growing in Rockwool can handle daily waterings. The material of the Rockwool also allows excellent air circulation, making overwatering difficult. This makes it an ideal substrate for hydroponic techniques. Rock wool cubes are often planted into 32 cell trays and watered in a flood and drain system like this automated grow rack .
What is the pH of Rockwool?
Rockwool tends to be too basic for most plants that prefer acidic soil conditions. With a pH between 7 and 8, you must presoak Rockwool in a slightly acidic solution (pH 5.5 to 6.5) for at least an hour before use. This can be done by adding several drops of lemon juice or pH down to the water, using pH test strips to attain the correct acidity. Once in use, you need to pay attention to the pH as it can quickly shift. This is why many prefer to use coco coir instead of Rockwool.
Can I put Rockwool in my compost?
While Rockwool or any type of mineral wool is not biodegradable it can be added to compost in order to add drainage and eventual mineral content to the resulting soil. If you plan to add your used Rockwool cubes to your compost bin you will want to shred them as much as possible before mixing them in. Left whole they can persist in the soil indefinitely because mineral wools do not contain any organic matter.
Some growers choose to reuse Rockwool although it is not recommended because once the cubes are full of roots they can begin to harbor mold, fungus, and detrimental bacteria. If you do choose to reuse your cubes, allow the roots inside to dry completely and then sterilize them by submerging them in boiling water for at least 10 minutes. For more information on proper equipment, cleaning check out this article on How to Wash and Care for Seedling Trays.
Rockwool can be a great tool for hydroponics, cloning, and seed starting. While it does come with some limitations, it can be the perfect substrate for certain applications. For more information on ways, growers and gardeners alike start their seeds indoors, check out Seed Starting: 101 Starting Seeds Indoors For Your Garden.