Rockwool is a commonly seen horticultural substrate in hydroponic stores, garden centers, and even home improvement outlets. It is also widely available online. Unlike other common types of media, rockwool is not made of living or formerly living items. It is not the byproduct of agricultural processing. Rockwool is, as the name implies, made of rocks.
Sometimes known as stone wool, or mineral wool, rockwool is made up of a type of rock known as basalt, and chalk. These materials are super-heated to temperatures as high as 3000 F (1650 C) until it becomes lava. The lava is then put into a spinning chamber which creates the strands and fibres which, when combined with a binding agent, make up rockwool cubes, blocks, mats, etc.
Growers of all sizes and crops favor rockwool for numerous reasons. It is sterile, porous, and non-degradable, unlike some other types of growing media. It absorbs water well, while possessing the ability to drain well too. It can be used for seed starting, rooting cuttings, and transplants. Crops ranging from tomatoes, to strawberries, to cut flowers, to cannabis can all be grown effectively in rockwool.
Benefits of Using Rockwool
There is a reason that rockwool is such a popular substrate choice for many hydroponic growers. By most analytics, it is the most popular. There are numerous advantages to using it. Unlike some other types of media, rockwool is sterile, meaning that it does not pose the potential risk of introducing pathogens, insects or weed seeds into your system. It is also highly customizable and available in a wide range of sizes that can accommodate nearly any type of hydroponic or growing setup. Anywhere from small cubes perfect for individual seeds all the way up to giant slabs are on the market. It is lightweight too, so it is easy to handle and transport.
Rockwool has excellent water retention, while still allowing for oxygen to flow around roots. Once it has become fully irrigated, the rockwool becomes heavier and then provides the structure and stability needed for root development. One of the main benefits to rockwool though, is its ability to be reused multiple times. It can retain its structure for several years if handled properly. It does need to be properly sterilized between uses though to prevent any potential transfer of pathogens between crops.
How to use rockwool for propagation
As mentioned above, rockwool can be used for just about all phases and types of growing, but it is primarily chosen by growers for seed starting, and the propagation of cuttings. When germinating seed, small rockwool cubes, such as a 1.5” size is typically chosen. These have been proven as a very viable choice for seed starting since they keep the seed moist, while not keeping them in too much water which can cause rot.
Growers typically select larger cubes such as 4” or 6” when they use rockwool for cuttings. Rockwool is excellent for cuttings, for the same reasons why it is great for seed starting. It keeps the cutting constantly moist while it is developing it won root system, yet not so wet as to rot the cutting. They are also valued because cuttings need a sterile environment to thrive and rockwool can help to provide that.
Steps for using rockwool for the first time, as well as reusing it can be found below, but here are some specific steps when seeding and cultivating cuttings using rockwool.
For seed germination in rockwool:
Step 1. Depending on the size of the seed, place two seeds in each cube. For larger seeds like corn, beans, peas, or cucurbits, use one seed per cube. A good rule of thumb is, if the seed is smaller than the eraser on the end of a pencil, do two per cube. For best results, push the seed(s) down further into the cube with a toothpick, clean pencil, plant tag, or anything handy that is clean and will not damage the seed.
Step 2. Once the seeds have been pushed down, pinch the top of the cube closed. And place them onto a tray that can be covered with a humidity dome.
Step 3. Keep the seeds moist and humid during their development by maintaining a humidity level between 70and 80 percent. The cubes can be sprayed with a misting or spray bottle as needed to keep them moist and the surrounding air under the dome humid. Once the seeds begin to sprout, remove them from the humidity dome.
Step 4. For cubes with more than one seed, cut the weaker or smaller one-off. Don’t pull it out as its roots are likely intertwined with the sprout that you want to keep, and you will disturb its development.
Step 5. Transplant the seedling into a larger cube or another medium when they have developed their first set of true leaves (not the ones they emerge from their seed with, but the first set to develop afterward).
For propagating cuttings in rockwool:
Step 1. After properly taking the desired cuttings from the mother plant and using a growth hormone, place the bottom stem of the cutting into the rockwool, taking care to avoid poking it all the way through to the bottom of the cube.
Step 2. Place each of the cubes with cuttings onto a nursery tray, which has a bottom layer of a medium such as perlite or vermiculite placed on it ahead of time.
Step 3. Like with germinating seeds, then cover this tray with a humidity dome and attempt to keep the humidity level between 70 and 80 percent. Once roots begin to appear poking out of the bottom of the cube, it is time to transplant the cutting into its next container or home.
For propagating cuttings in rockwool:
Unlike several other types of substrates and media, rockwool cannot be used right out of the package. There is some prep work involved. It will need to have its naturally high pH lowered, and to be pre-soaked before use. Rockwool will typically have a pH of around 8.0 which is not conducive for plant growth. This is due to the presence of lime on the rockwool fibres.
At the initial stage of this preparation process, rockwool is most likely to shed off fibres that can irritate the skin, and lungs, so gloves and a mask should be worn. A pH meter will also be needed for this process.
Step 1. Place the rockwool in a solution of acidic water. This will help to dissolve the lime and lower the pH. This can be done with a commercially available pH adjuster, or even by using distilled water initially. If nothing else is available, water from the tap can be used for this first part.
Step 2. Monitor the pH levels and adjust accordingly. Plan on this phase taking up to a few days to reach the desired pH level. If the water source being used does not drop the pH of the rockwool below 6.0, then other products, such as “pH down” type solutions will need to be used. The goal is to achieve and maintain a pH of about 5.5. If the pH falls much below this, remove the rockwool from the solution, flush it with clean water and start again. Too high of an acid level will damage the rockwool fibres.
Step 3. Once the rockwool has achieved and stabilized at the desired pH (about 5.5), It should be moved into the hydro system. The system should then be run as normal but without any plants yet. If after 24 hours the pH of the hydro system is at the desired level, then the rockwool is stable and ready to be used. If the rockwool is raising the pH of the system’s water, it will need to be removed, flushed, and treated again with an acidic solution until it is stable and does not influence the system’s pH.
Steps for reusing
As mentioned above, one of the many benefits of rockwool is its ability to be used for successive crops numerous times since it does not decompose. It does need to be sterilized in between uses though to achieve the full benefit of it. There are a few ways this can be achieved. At a minimum, rockwool should be thoroughly leached with clean, pH neutral water to flush it out.
Chemical disinfectants can then be used, but it is important that the rockwool then be thoroughly flushed out again to avoid the possibility of any chemical residue being transferred to the next crop. Hydrogen peroxide is often the formulation of choice. Many growers choose not to use chemical formulations and instead sterilize rockwool with steam or boiling hot water. Best practices suggest that when reusing rockwool, crops in different families are used in succession to further avoid the possibility of transferring disease between crops.
The process for reusing rockwool is fairly simple and can be accomplished in just a few steps.
Step 1. Remove all plant material from the previous crop. This includes any root particles, leaves, etc. from the plants that were growing in the rockwool before.
Step 2. Sanitize the rockwool. As mentioned above this can be done through chemical means, or by using steam or boiling water. Enzyme solutions can also be used to sterilize your rockwool. The end result needs to be a sterile substrate, free of any residual chemicals.
Step 3. Stabilize the rockwool. Make sure that the pH of the rockwool is at or near 5.5. This may be achieved by soaking the rockwool in a pre-made commercial solution, or a DIY one. Some growers use lime or wood ash to raise the pH where needed and use sulfur or phosphoric acid to lower it.
Note that gloves and a mask should be worn for all steps of this task to avoid potential contact with rockwool fibers on your skin or worse, in your lungs. It is also important to try to retain the integrity of whatever foil or plastic is on the edges of the rockwool. This helps to retain the shape, but more importantly prevents light from encouraging the development of algae.
When rockwool has finally come to the end of its useful life in a hydroponic system, it does not need to be tossed out completely. It can still be shred and incorporated into other grow media or even added to outdoor gardens where it can still offer some of its benefits as a soil conditioner.
Considerations When Using Rockwool
Rockwool has more than proven its usefulness and utility to the horticulture industry. There are however some considerations to make for its use and to keep in mind when deciding if it is the media that you should use for your operations. First, it is potentially a health hazard if care is not taken when using it. When it is dry and fibrous, especially when unpacking it for its initial use, gloves should be worn while handling it. Its microfibers can become airborne and will irritate the skin of most people. Long sleeved shirts, pants and closed-toed shoes should also be worn while using it when dry. For others with sensitive respiratory systems, the dust can be a lung and airway irritant. Masks are a good idea for handling dry rockwool as well, again, especially when first preparing the dry media and then again when disposing of old material.
There is a learning curve to using rockwool products. Growers need to learn how often and what volume of water is needed to sufficiently saturate rockwool, while avoiding dry patches from developing. The properties of rockwool require different irrigating practices than do other media like peat moss or coir. Rockwool may seem dry on the surface but could be fully saturated at the bottom. Care must be taken to not overwater. Typically, it should go form being wet, to just barely moist in between waterings.
The pH of rockwool is too high for most plants and needs to be adjusted before using. Either continual adjustment of the pH will be needed or preconditioning of the material in a low pH solution is necessary before implementing into your growing regimen. Also, while not necessarily a “bad” thing, rockwool contains no naturally-occurring nutrients. All nutrients needed for optimum plant growth and development need to be supplied by the grower supplementally. This would likely be done anyway in a hydroponic setup, but there are some alternative media choices that do offer plants some amount of nutrition.
There are also environmental concerns. While the raw material used to make rockwool is fairly abundant, it is not renewable, at least not in our lifetime. Renewable resources, such as coconut fibers (coir) are becoming a viable alternative. It can be difficult to dispose of too. Because it does not decompose, or breakdown overtime, it will permanently occupy space in landfills if disposed of in the normal waste stream. There are, however, some recycling programs out there that can utilize rockwool products to reincorporate them into other industrial products.
Other Uses for Rockwool
Rockwool was not initially created for the horticulture industry. It started “life” as an insulating material and has expanded from there. Its lightweight and highly aerated characteristics made it a favorite of contractors to use for both interior and exterior insulation applications. It was and is still used to insulate exterior walls, HVAC ducts, and roofs. For interiors it is used in fire-rated doors, interior walls, floors insulation, ceiling insulation, and even as acoustic tiles for sound dampening. It is also used as insulating material in dozens of electrical applications, appliances, and other devices. It was not until the 1960s that Danish growers and researchers began to experiment with rockwool as a substrate or media for plant propagation. Nearly 60 years later, it is highly apparent that rockwool for horticultural use is here to stay.
