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Rockwool is a horticultural growing media made from the natural ingredients Basalt rock and Chalk.
These are then melted at 1600° C into a lava which is blown into a large spinning chamber, which pulls the lava into fibers like “cotton candy.” If you have ever visited a volcano you have probably seen these fibers flying around in the air surrounding the volcano. Once the fibers are spun they are then compressed into a mat which is then cut into slabs and cubes. The rockwool granulates are just bales of uncompressed fibers. The process is very efficient, producing 37 cubic foot of wool from 1 cubic foot of rocks. Since rockwool is born in fire it renders the product chemically and biologically inert and creates the ideal growing medium for hydroponics. Since its development in Denmark in the early 1970’s, rockwool has become the major vegetable and flower production medium throughout Europe and North America.
Horticultural Rockwool growing media is primarily available in two general formats. First, as rigid slabs, blocks, and cubes known as “bonded” products because the fibers are held together with a “gluing” or binding agent which renders them stiff and brittle. This is the primary format for the vegetable and cut flower industries. Secondly, rockwool is available as a highly refined and consistent hydrophilic or hydrophobic granulate which is basically water absorbent or water repellent. This format can be used as a component in various peat moss based soilless media or for ground bed incorporation to improve the tilth of heavy clay or light sandy soils.
What are the use of Rockwool?
Rockwool cubes hold a tremendous amount of water for their size, which provides a buffer against power outages that make shut down pumps or timers. On average they also hold at least 18% oxygen between the fibers providing a tremendous amount of oxygen to the root zone, as well as making it incredibly difficult to overwater the plants.
Growers primarily use rockwool cubes as growing media for two main purposes in their hydroponics systems: germinating seeds and propagating new cuttings. Typically the 1.5” cubes are used for starting seeds or propagating cuttings. Some of the larger cubes (up to 4”) are used as a growing medium for compact plants as well, but on a much smaller scale.
Starting seeds can sometimes be a stressful adventure. It’s a constant balance between keeping them wet enough to promote germination without having them so wet they dampen off and die. Rockwool cubes are popular for germinating seeds in because of their excellent moisture retention — they are great at helping to keep seeds or seedlings from drying out but don’t let them sit in a waterlogged environment.
Moisture is the critical factor in seed germination. Water enters the seed through the seed coat or tiny opening called a micropyle. The presence of water will activate the enzymatic reactions within the seed that begin germination.
Cloning New Plants
Propagating new plants is the process of taking an existing plant part (usually a leaf or stem cutting) and growing it into a whole new plant. This method results in an identical clone of the original and is cheaper than purchasing seeds and starting from scratch.
Maintaining a high humidity around the cutting is critical for successful propagation. Plants need to keep from drying out; without enough moisture the plant will go into self-defense mode and will stop trying to develop new roots.
Growing new plants through propagation techniques needs to happen in a sterile environment, free of bacteria or fungus. Rockwool cubes are completely inert because of the heating process they undergo during manufacturing. This ensures they are sterile and free of any harmful microorganisms that could hinder propagation.
Do's and Don'ts of Using Rockwool Cubes
One of the drawbacks to using rockwool cubes is the special care and consideration that needs to be taken when working with them. While they are a beneficial resource for hydroponics growing system, they do have some important drawbacks that need to be addressed.
- DO take the time to properly prepare rockwool cubes before starting seeds or trying to root cuttings. Their natural pH is too high for optimum plant growth and will create problems and instability within the hydroponics system if left at that alkalinity.
- DO wear protective gear to keep yourself safe when handling rockwool material. The fibers are irritating to skin, lungs, and eyes so it’s imperative to protect yourself. At the minimum, wear a dust mask to prevent inhaling fibers into your lungs. It’s also recommended to wear eye protection and long sleeves if there is a chance of prolonged contact with your skin.
- DO sterilize the feeding solution before running it back through the rockwool cubes. Even though it is a manmade, inert material, it can still be prone to algae and bacterial growth that can contaminate the hydroponics system if the feed solution is recirculated without being treated properly.
- DO dispose of the materials properly after you are done using the cubes. Unlike other growing mediums, rockwool is not composed of natural materials so it will not break down over time. If it reaches the landfill it will be there indefinitely. Instead of throwing them away, break up the pieces and work them into your garden or potting soil to help increase their water retention.
- DO heat treat the rockwool cubes if you are going to reuse them for a successive growing season, or even numerous seasons. This can be done by steaming them, or pouring boiling hot water through them to kill off any bacteria or fungus that may be residing with the cube’s fibers. Some websites recommend using chemical treatments to sterilize rockwool cubes to use again but this can be dangerous unless you are absolutely sure you have rinsed all of the sanitizing chemicals out of the fibers.
- DON’T squeeze the cubes when they are wet. If you need to remove some of the water from them after preparing them in a pH controlled solution it’s best to shake them gently. Rockwool cubes are known for their internal structure that gives them such great water holding capacity and oxygen movement; squeezing them compacts the structure and hinders the benefits they are so well known for.
- DON’T forget rockwool is completely inert and can not provide any nutritional value to the plants growing in the cubes. Everything the plant needs has to come from the nutrient solution supplied to them.
Step by Step Using Rockwool for Hydroponics
- Prepare the rockwool cubes for use, making sure to soak them in pH adjusted water to bring their pH down between 5.5 and 6.5.
- To Plant Seeds:
- Insert 2 seeds in the hole on the top of the rockwool cube. Use a toothpick or other similar object to press the seeds down to the bottom of the hole.
- Pinch the hole closed.
- Place the cubes in a nursery tray and cover with a humidity dome to lock in moisture.
- Maintain at 70 – 80°.
- Keep rockwool cubes moist by watering sparingly every couple of days or misting with a spray bottle when they start to dry out.
- Remove from humidity dome and place under lights as soon as seeds sprout.
- Cut the tops off the seed in each cube (if you planted 2 per cube) that isn’t the strongest or tallest. Do not pull them out as it may dislodge the healthier plantlet at the same time.
- Transplant when plantlets reach 2-3” in height.
- To Propagate Cuttings
- Water the stock plant well the night before beginning the propagation process.
- Remove a 3-4” leaf stem cutting from the main stem of the plant, cutting it off as close to the main stem as possible without damaging the node.
- Dip cut end in rooting hormone.
- Plant the cutting in the rockwool cube making sure it doesn’t poke out the bottom of the cube.
- Fill a nursery tray part way full with perlite or vermiculite.
- Set rockwool cubes on top of growing media.
- Cover nursery tray with a humidity dome to lock in moisture.
- Maintain close to 80°.
- Crack humidity dome when roots begin to emerge, gradually increasing the day after.
- Remove the humidity dome a couple of days after roots first appear.
- Transplant when roots begin to poke out the bottom of the cubes.
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