Starting Seeds In Rockwool Cubes – How To Do It Successfully

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Last Updated on January 21, 2023

Starting seeds in Rockwool cubes can be a great process for your plant’s growing experience. Rockwool is a soil-free medium or substrate that can be used for starting seeds. They can as well be used in hydroponics or stem rooting.

Rockwool offers so many benefits to 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.

Let Us Get To Know What Rockwool Cubes Are

Rockwool is made from spinning chalk and basalt rock, which are then 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 rock wool cubes are mainly used to grow denser plants.

Get to Know What Rockwool Cubes Are

A Guide On Germinating Seeds In Rockwool Cubes

It is important to know how to germinate seeds for hydroponics with rockwool. Seed germination (whether for hydroponics or conventional soil planting) requirements can sometimes demand careful and precise planning or processes. Take for instance moisture requirements: moisture is an essential factor when it comes to seed germination

Therefore, a Rockwool cube is a great choice to use in germinating seeds because it can retain moisture. Rockwool cubes will keep your seeds moist while preventing them from sitting in a waterlogged environment. They can retain just the right amount of water to keep your seeds from drying out thereby improving seed germination.

Read more about How to Use Rapid Rooters Plugs

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 the best chance of germinating and sprouting appropriately.

  • Remove the Rockwool cubes from the soaked water and give them a gentle shake to take off the excess water. However, don’t squeeze them. Now your cubes are ready to be placed in your germinating tray.
  • Most rockwool cubes have holes in them, so get your seeds and drop them into the holes of the Rockwool cubes. With a toothpick or other identical objects, gently push 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.
  • Next, cover the rockwool cubes 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 them with a spray bottle whenever you notice them drying out. Sprouting should be noticed after a couple of days, depending on the plant seeds that you are germinating.

Benefits Of Start Seeds In Rockwool

Here are the benefits your seeds stand to enjoy by using rockwool cubes for germinating seeds.

  • Water Retention: Rockwool cubes have excellent water retention capacity which is very essential for the germination of your seeds. At the same time, Rockwool will not waterlog your system. It can 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 don’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.

Sturdy Seed Starter Tray With Humidity Dome Seedling Trays

Common Problems With Germinating Seeds In Rockwool

While rockwool is a very reliable medium to use for germinating seeds, it is not without its fair share of troubles. Here are some of the most common problems and how you can tackle them head-on if you ever experience seeds not sprouting in rock wool.

  • Temperature

One of the most popular problems associated with seeds not sprouting in rockwool is the temperature. It could be that the temperature is too hot or too cold to support germination. If you investigate and find this to be the case with your seeds not sprouting, move your rockwool cubes into a warmer or colder environment as required. If this is not possible, you can purchase a germination chamber such as this one which comes with a heat mat tray that can be used to attain more favorable temperatures, especially in colder climates.

  • Poor seed quality

Sometimes all the conditions may be right in your rockwool cubes, but if the seeds are of poor quality, the rate of germination will be low or completely zero. Poor seed quality is mostly due to the degradation of the seed. This usually happens over time. Seed death or degradation starts to happen as soon as the seed is mature and viable.

Generally, seeds will have high germination rates for the 2-3 years of their lives with an average of 80% success. Once this rate hits about 75%, the seeds start to lose their ability to germinate quickly. As a result, old seeds have poor quality and will not germinate properly. Be sure to use the freshest and newest seeds to germinate in rockwool.

To try and increase the shelf life of your seedlings, carefully store them in airtight glass jars and place them in the refrigerator or freeze them. You can put a little bit of powdered milk or wrap the seeds in a paper towel to absorb excess moisture.

  • pH Imbalance

Rockwool has a naturally high pH, which makes it highly alkaline when wet. A lot of seeds germinate poorly in alkaline conditions and prefer slightly acidic ones. To get the right pH for your plants, irrigate the rockwool cubes with an acidic nutrient solution to re-balance the pH of the growing environment.

  • Mold

Although rockwool has a high moisture content that prevents plant roots from dying out, this moisture can promote the growth of fungi. These fungi will encourage seeds to mold and rot instead of germinating properly. If you notice mold starting to form, irrigate less.

  • Insufficient airflow

Insufficient airflow through the Rockwool cubes can also create problems for seed germination. To prevent this issue from occurring, avoid crushing or compressing your rockwool so that it retains enough air pockets to allow for sufficient airflow.

What Is The Best Time To Transplant Rockwool Seedlings

Knowing when to transplant rockwool seedlings is an important determining factor in getting good survival rates for transplanted seedlings. 

Transplant seedlings when they reach a height of about 2-3 inches in height. Also, check that the roots of the seedlings are beginning to poke out at the bottom of the cubes. By this time, they should also have developed a full set of two to three sets of true leaves.

After your seedlings in the rockwool cubes have developed and are ready to transplant, simply plug the cubes and plant them directly into your grow sites and your plants are ready to go!

How To Care For Seedlings In Rockwool

Taking good care of seedlings post-germination is also important to ensure a successful crop. Here are the measures that you need to take to;

  • Before you need to transplant the seedlings, place the Rockwool cubes in a nursery tray and cover it with a humidity done to ensure that the moisture stays locked in and the Rockwool never runs dry
  • Maintain an environmental temperature of between 70 and 80 degrees Fahrenheit 
  • Keep the rockwool cubes moist by consistently spraying them with water every few days when you notice them starting to dry out.
  • Remove the Rockwool cubes from the humidity cube and place them under the light as soon as the seeds start to sprout

Closing Remarks- Starting Seeds In Rockwool Cubes

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.


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.

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