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How to Use Rockwool Cubes for Plants

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In a process similar to that used to make cotton candy, basalt and chalk are spun into Rockwool, which is turned into cubes. Rockwool is a brand name; the generic term is rock wool. Rockwool cubes, 1 or 2 inches wide and 1 1/2 inch deep, are used for germinating seeds for seedlings that are then transplanted into soil or hydroponic gardens. Cubes 3 and 4 inches wide are used as a primary growing medium for small plants. Rockwool has a high pH, so you have to adjust your water or growing solution to fit the needs of your plants.

Germinating Seeds

Test your water’s pH with a test kit. After you drop 2 or 3 drops of solution from your kit into 1 quart of water, it will normally turn green, meaning it has Rockwool’s usual pH of 7 to 8. Add drops of lemon juice to the water until your kit indicates it has a pH between 5.5 and 6.5, the range that plants usually like. Water below pH 5 will damage the Rockwool fibers. If the pH goes below 5, add more water.

Soak the Rockwool cubes in the water for 1 hour to stabilize them. Rockwool contains fibers that are dangerous to your lungs if you inhale them. Wear a dust mask when handling the cubes.

Insert seeds in the hole on top of the cube. Insert two seeds for basil, broccoli, cabbage, cucumbers, eggplants, peppers and tomatoes and six to eight seeds for herbs.

Place the cubes containing the seeds in a nursery tray at 70 to 80 degrees Fahrenheit.

Water every one or two days or use a formulated nutrient solution used for hydroponic growing. The seeds should begin to sprout in a day or two. The roots will show through the bottom of the cube.

Transplant to a soil or hydroponic garden when the plant is 2 to 3 inches tall and has three or four leaves. Most plants take one to three weeks; tomatoes and peppers may need a week longer.

Transplanting

Make a hole in your planting soil or mix that is large enough to accommodate the cube containing the seedling.

Water your seedling with tepid water.

Place the cube with the seedling into the hole in the planting soil or mix. The cube should stick out a bit; the surface of the cube should not be lower than the surface. This prevents puddles from collecting the surface, a cause of root rot.

Planting Cuttings

Water your plant stock well with tepid water the night before you take a cutting.

Take a 4-inch cutting from a firm, green stem. Cut the leaf stem close to the main stem of the cutting, leaving a stub so you don’t damage the node. Remove some of the larger leaves from the cutting. Do this early in the morning so the plant has had an opportunity to build up a reserve of water.

Dip the stem of the cutting into rooting hormone. You can use either a powder or a gel.

Plant the stem in the Rockwool cube, making sure the end doesn’t poke out of the bottom.

Fill a growing tray until level with perlite, vermiculite or potting soil and tap the tray gently to settle the growing mix.

Place the cube on the growing mix and water the planting mix around it to settle it. Add more growing mix if necessary.

Place a clear tray lids over the plants and place the tray on a heating pad set to 80 degrees Fahrenheit.

Crack the tray lid 1/4 inch the day the roots appear. Open the lid 1/2 inch the next day. Remove the lid after that and they are ready to transplant. Roots usually start to appear after 14 days.

How to Use Rockwool Cubes for Plants. In a process similar to that used to make cotton candy, basalt and chalk are spun into Rockwool, which is turned into cubes. Rockwool is a brand name; the generic term is rock wool. Rockwool cubes, 1 or 2 inches wide and 1 1/2 inch deep, are used for germinating seeds for …

Growing In Rockwool Cubes – Is Rockwool Safe For Plants

If you’re looking for a soilless substrate for seed starting, stem rooting or hydroponics, consider using rockwool growing medium. This wool-like material is made by melting basaltic rock and spinning it into fine fibers. Rockwool for plants is then formed into easy-to-use cubes and blocks. But is rockwool safe to use for the production of food?

Advantages and Disadvantages of Growing in Rockwool

Safety: Formed from natural materials, rockwool contains no harmful chemicals. It’s safe to use as a rooting medium and substrate material for plants. On the other hand, human exposure to rockwool represents a health issue. Due to its physical properties, rockwool growing medium can cause irritation to skin, eyes and lungs.

Sterile: Since rockwool for plants is a manufactured product, it’s contains no weed seeds, disease pathogens or pests. This also means it contains no nutrients, organic compounds or microbes. Plants growing in rockwool require a balanced and complete hydroponic solution to meet their nutritional needs.

Water Retention: Due its physical structure, rockwool drains excess water quickly. Yet, it retains small amounts of water near the bottom of the cube. This unique property allows plants to attain adequate hydration while allowing more air to circulate and oxygenate the roots. This difference in moisture levels from the top to the bottom of the cube makes rockwool ideal for hydroponics, but it can also make it difficult to determine when to irrigate the plants. This can result in over-watering.

Reusable: As a rock derivative, rockwool doesn’t break down or erode over time, thus, it can be reused many times. Boiling or steaming between uses is recommended to kill pathogens. Being non-biodegradable also means it will last forever in a landfill, making rockwool for plants a not-so environmentally friendly product.

How to Plant in Rockwool

Follow these easy instructions when using rockwool growing medium cubes or blocks:

  • Preparation: Rockwool has a naturally high pH of 7 to 8. Prepare a solution of slightly acidic water (pH 5.5 to 6.5) by adding several drops of lemon juice using pH test strips to attain the correct acidity. Soak the rockwool cubes in this solution for about an hour.
  • Sowing Seed: Place two or three seeds in the hole at the top of the rockwool growing medium. Water using a hydroponic nutrient solution. When the plants are 2 to 3 inches (5 to 7.6 cm.) tall, they can be transplanted into soil or placed in a hydroponic garden.
  • Stem Cuttings: The night before taking the stem cutting, water the mother plant thoroughly. In the morning, remove a 4 inch (10 cm.) cutting from the mother plant. Dip the cut end of the stem in honey or rooting hormone. Place the cutting in the rockwool. Water using hydroponic nutrient solution.

Rockwool is the substrate of choice for many large hydroponic farms. But this clean, pathogen-free product is also readily available in smaller-sized packages specifically marketed for home gardeners. Whether you’re dabbling with cultivating lettuce in a hydroponic jar or you’re setting up a larger system, growing in rockwool gives your plants the advantage of superior root zone technology.

If you're looking for a soilless substrate for seed starting, stem rooting or hydroponics, consider using rockwool growing medium. A wool-like material, rockwool for plants is easy to use and available in cubes and blocks. Learn about rockwool in this article.