How to start seedlings using rockwool

The first step to growing plants in indoor hydroponics is to start seedlings! We recommend using rockwool to start seedlings as it is clean and straightforward to use, and works well for indoor hydroponics.

What do I need?

  • Rockwool

  • Seeds

  • Water

  • Container (preferably food grade plastic)

  • Tweezers (optional)

What is rockwool?

Rockwool is the trade name for mineral wool, an inert, insulating material used largely in the construction industry but extensively in hydroponics as well. It is made of thinly spun stone fibers that creates a soft, sponge-like medium that is well suited for sowing seeds, allowing them to germinate, and allowing the plants to continue their growth throughout their entire lifecycle. Rockwool sold for hydroponic use is typically cut into 1-inch cubes that make it easy to use for single-plant use.

Because the way in which the fibers are woven together, rockwool provides for an excellent air-to-water ratio that is ideal for hydroponics, similar to moist soil. Unlike soil, however, rockwool is a stable material that does not degrade, and can simply be disposed of when the plant is ultimately harvested.

Is rockwool safe?

Rockwool is made of thin, mineral fibers that are made of molten rocks. Therefore, the material itself is not harmful. However, because they are so thinly spun, the fibers are very lightweight and can get airborne, potentially causing irritation if inhaled. Therefore it is recommended that they are handled carefully. Moistening the rockwool cubes immediately prior to handling can help prevent fibers from getting airborne.

Additionally, the fibers are thin, but because they are made of hard mineral material, they can create small abrasions on your skin. If you have sensitive skin, wearing gloves while handling the rockwool cubes may be advisable.

Preparing the rockwool cubes for seedlings

The first step is to get the rockwool cubes ready for seed planting. Using a pair of scissors or by simply using your hands, tear the cubes apart into their individual blocks.

Then, soak the rockwool cubes in a tray of water for several minutes. I have found that tofu containers work really well because they are a good size, and I know for a fact that the plastic materials used are food safe. The purpose of soaking the rockwool cubes in water is to ensure that any residue and pH altering materials are removed from the rockwool. The time needed would be dependent on the quality of your rockwool cubes, but typically 5 - 30 minutes should be sufficient.

You may also choose to keep the cubes connected, like I did in the photo below. A 4x5 gride of 20 cubes fits perfectly in a standard tofu container. When you have this many seedlings to keep track of, it might be helpful to take a note of what seeds you planted where, especially if you plan on growing varieties that appear very similar. I used a simple A/B/C/D and 1-5 numbering scheme.

Place the seeds in the rockwool cubes

After soaking the rockwool cubes, remove the water and stand the rockwool cubes upright. Then, using tweezers or another sharp object, poke a small hole on the top of the rockwool cube to allow for enough space to deposit a seed.

Then, using tweezers, grab the seeds and place them in the hole that you just created in the rockwool cube.

Typically, just one seed per rockwool cube is sufficient. In some cases, if you observe a lower than usual germination rate, you may want to place multiple seeds in one rockwool cube. It is usually not a good idea to grow multiple plants from single rockwool cube, however, so you will need to cull some of the excess seedlings after they emerge.

Once the seeds have been placed, place a plastic bag to ensure that the rockwool cubes remain in a high humidity environment. The reason for this is to ensure that the seeds germinate, as moisture is one of the necessary conditions to trigger a seedling's emergence from soil.

Be sure to place the plastic loosely enough, though, because you could end up inadvertently suffocating the seedlings.

What to do before your seedlings emerge

Depending on the plant type, germination time can range widely. Lettuce, for example, will typically emerge within 24-48 hours, while other species can take 2 weeks or more. During this time, lighting is not necessary and as long as the rockwool cubes remain sufficiently moist, conditions should be sufficient for germination. You may want to pour enough water into your tray so that you have a thin layer of water that the rockwool cubes can draw moisture from.

As soon as your seedlings emerge, you will be able to see them sprout from the hole that you placed your seeds in.

Day 2

Day 3

Day 4

Day 5

After your seedlings emerge from the rockwool cubes

As soon as your seedlings emerge, be sure to immediately provide enough light for the seedlings. Otherwise, you will end up with long, spindly stems as the seedlings will try to "reach" for light, and this will result in small, stunted plants at later points in their growth. At this stage, you can keep the rockwool cubes in the same tray, and you may or may not want to introduce nutrient solution.

If you've gotten this far, congratulations! Read on for next steps on...

Transferring rockwool cubes to net pots and deep water culture tubs

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