How To Plant Weed Seeds In Rockwool Cubes

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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 How To Start From Seed With Rockwool Cubes In this lesson, we will learn how to start your own seed with Rockwool Cubes. Below is a detailed guide to success with Rockwool, and a step-by-step 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.

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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.

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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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How To Start From Seed With Rockwool Cubes

In this lesson, we will learn how to start your own seed with Rockwool Cubes. Below is a detailed guide to success with Rockwool, and a step-by-step video tutorial can be found at the bottom of the page.

Preparation

Items you will need:

Rockwool Cubes have a PH of roughly 7.8. This is pretty alkaline, yet our plants prefer to grow in a slightly more acidic environment (between 5.5 – 6.5). In order to prepare our Rockwell cubes for the seeds, we need to soak them in some PH adjusted water, that way they have everything the seeds need to germinate and sprout; water and a slightly acidic environment.

Step 1: Hydrate And Stabilize The Rockwool Cubes

Get a bowl or some other container that is big enough to fill with water and have room left for your Rockwool cubes. Your average salad bowl will work fine for 3 Rockwool cubes, if you are planning on doing more than you will need a larger container.

Fill the container/bowl with water from your tap. You may also choose to use water filtered through a britta or reverse osmosis (R/O) water, I’ve had success with all 3 of them so whichever you have on hand will work fine.

Using either a PH test kit or a Ph meter, determine the Ph of the water. Water comes out pretty alkaline, usually around 7.4, so you will need to acidify it a little bit to bring that Ph down to the desired level. Aim for as close to a Ph of 5.5-6 as you can get.

To accomplish this, use either Ph down chemicals, or lime juice (as it’s acidic). Add these to the water in small increments (VERY SMALL), and test the water to see where the Ph is. Continue doing this until you have a Ph of 5.5-6.

Important: Do not let the PH of the water go below 5. A Ph this low will damage the fibers of the Rockwool Cube

Now that we have the Ph adjusted water, it’s time to stabilize and hydrate the Rockwool cubes in it. Insert the Rockwool Cubes into your container and let them soak for roughly 1 hour. Once the hour is up, the cubes will be big and fat with water. Take them out of the bowl of water and put them somewhere you don’t mind getting a little wet. Save the remaining water for step 3.

DO NOT SQUEEZE THEM TO DRAIN ANY WATER

Rockwool Cubes are designed to maintain the correct water to air ratio and squeezing them may damage their structure. Some of the marijuana forums advocate doing everything from squeezing them completely dry, all the way to flinging them around like paintbrushes in order to get excess water out. None of that is necessary, leave it as it is. It will stay moist for several days without needing to be watered this way as well.

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On top of that, Rockwool is like asbestos, you don’t want to be squeezing it or breathing it or generally touching it any more than you need to. Here is a good article on some of the health concerns of Rockwool. I use it because it is what works best, but be cognizant to the fact that it is a potentially dangerous substance to be making contact with so don’t do anything more than you need to with it.

Here is what it should look like:

Step 2: Plant Your Seeds

Most Rockwool cubes come with holes in them, if yours did not, than create a hole in one side that is approximately a quarter inch (0.75 cm) deep.

They should look like this:

Take 1-2 seeds and insert them carefully into the holes. Use a toothpick or similar object to push them down to the bottom, as you want them to be at the bottom of that hole. Rip or push a piece of the Rockwool over the hole (you don’t have to fill it completely), so that the seed can germinate in a dark moist environment.

Now, if you can, place them in a tray with a dome on it. This will help create a little humidity in there which seedlings like. This is not mandatory, but it helps. Whichever you choose, take your cubes and put them in a cool dark place, and leave them alone. The temperature should be roughly 68 degrees F, though my house stays at about 72 and they do fine there. I usually place them above my refrigerator and just leave them for a day or two. My lettuce seedlings sprouted with a quickness the last time I tried, and by the 3rdday they had grown so tall that I had to take the plastic dome off of my container because they were bumping up against the ceiling.

Step 3: Leave Them Alone And Let Them Grow

If you put more than one seed in your cube (just in case one didn’t make it), than you probably have several seeds sprouting up in each cube at the end of ~3 days. Once the first true leaves emerge, we want to select for the strongest one (the one that grew the tallest), and cut off the tops of all other seeds that are growing next to it. Do not pluck them out, as you may uproot it’s neighbors. Simply cut it off as close to the hole as you can without messing with the stronger one that you plan on keeping alive.

Depending on how hot it is (and other factors) you may need to water your cubes 1-4 times a day. Use the Ph adjusted water when doing so (that’s why I had you save the leftovers from step 2). If you already threw that water out, go make another batch of Ph adjusted water and keep it in a separate bottle or container for watering. Note: Do not over water, in fact while some say to water 1-4 times a day, I did it only once a day when I got home after work.

Some people claim they use a diluted nutrient solution to water their Rockwool cubes with during germination. Do not do this, as my experience has always been negative. Note the picture below, where I did an experiment by adding a very diluted grow nutrient to the Rockwool cube on the far left. It died within an hour or two, and the others went on to live happy lives. In my opinion, they do not need nutrients until they get into your hydro system.

Do not add any nutrients to your Rockwool Cubes. The one on the left got nutrients, the other two did not.

Step 4: Transplant Into Their Final Destination

About 2-3 weeks after germinating, you are ready to transplant these babies into the hydroponic system of your choice. A good rule of thumb to go by is that you want to transplant them once the first roots begin poking out of the Rockwool cube. Don’t wait too long though, as eventually the roots will begin tangling around the cube since it is their only source of water. You want to catch them right as they pop out, so that when you transfer them into your hydro system the roots will grow down into the system, and not just try to feed off the Rockwool cube alone.

If you have any questions, suggestions, or comments, please leave them below and I will do my best to answer all of them.

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.

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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.

Reusing Rockwool

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.

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