seeds in rockwool

How to Grow Lettuce from Seed in Rockwool

In this tutorial we are going to learn how to grow lettuce (type: May Queen) from seed in the rockwool as growing medium. The process is easy and straight forward.

Equipment need

Before we start we have to make sure that we are having all necessary parts for growing lettuce. The parts that we need are:
– A pack of May Queen Lettuce seeds
– A sheet of rockwool blocks
– A water pH Meter
– A pH Minus solution
– A thermometer that can measure the temperature of the water and the temperature of the air
– A enough large transparent food container with cover

All the things needed in this tutorial you can buy at local garden or hydroponics store. Or if there is none of them around, you can find them on this Amazon list here:

Prepare rockwool for planting lettuce

  1. First we take a bowl and that is large enough to have all of our rockwool soaked in water.
  2. Fill 3/4 of bowl with tap water
  3. Take a pH meter and remove the black cap from it.
  4. Place pH meter tip (the point that cap was placed before we removed it) in water and turn the pH meter ON.
  5. Wait few minutes for value to stabilize
  6. If the pH level is higher than 4.0 then add few drops of the pH Down (or pH Minus) liquid to bowl and stir it well and go to 3. point of this step and repeat points 3,4,5 and 6 until you get pH level of the water in bowl to be 4.0
  7. Now you can put cap back to pH meter and turn pH meter off and place it somewhere else
  8. Take the thermometer and check the temperature of the water
  9. If temperature is not in range of 15 to 20 ВєC (59 – 68 ВєF) then heat up or cool down water so it’s temperature is in our desired temperature range
  10. Now we can place our rockwool in the bowl with water
  11. Put it aside for 30 min and then place it on the steep surface so the rockwool can release all unnecessary water.
  12. Leave the rockwool on that surface for 15 min before proceeding to next step.

Make holes in the rockwool

In case that your rockwool is combined with many pieces in one big piece now is the time to split it in those smaller blocks.
1. Take a small stick, wooden or plastic it doesn’t matter, just make sure that it is not more than 2.5mm wide. For example you can use stick for placing small meat pieces on the grill or toothpick.
2. From bottom of the stick measure 6 mm and with the marker or pen write a tin line exactly on 6mm point from the bottom of the stick
3. Take each block of the rockwool and push the stick in it so that you reach that 6mm mark that you’ve previously made. Please take special caution to make sure you don’t push the stick more than 6mm in the rockwool.

Place the seeds in rockwool holes

  1. Takeout the seeds from packaging
  2. Place a single seed in each hole that we’ve previously made in the rockwool.

To make it easier, you can use the stick that we used before and carefuly with the stick push the seed inside the hole. Also make sure that you don’t push the seeds more than 6mm in the rockwool.

You can also put a ice cream stick on the edge of the rockwool and write the date so later when you plant more lettuce you can easily see when are the seeds planted

Place the seeds container near the window

  1. In the transparent food container place all the rockwool blocks so the holes are facing up and close the lid
  2. With thermometer check the temperature in the room, it should be in range of 15 to 20 ВєC (59 – 68 ВєF). In case your room temperature is warmer or colder than accordingly adjust the temperature so it is in between 15 and 20 ВєC (59 – 68 ВєF)
  3. Place the food container near the sunny window

In next 2-4 days we should see small sprouts coming out. During this time please maintain the room temperature in between 15 and 20 ВєC (59 – 68 ВєF), also check the rockwool and see if it’s drying out, and if necessary spray water on it so it get moisture but make sure to not over water the rockwool so when you pull the block of the rockwool up, water should not leak from it.

How to Grow Lettuce from Seed in Rockwool – In this tutorial we are going to learn how to grow lettuce (type: May Queen) from seed in the rockwool as growing medium. T…

Starting Seeds In Rockwool – Rockwool Grow Guide

What is Rockwool, and why are home gardeners turning to it for starting seeds?

I knew very little about Rockwool other than it’s a substrate common in commercial hydroponic growing systems. But after trying Rockwell out, I now know the benefits of using this unusual growing medium for starting seeds at home and want to share this information with you.

In this article, I explain the origins of Rockwool and the advantages of Rockwool versus soil. I also discuss how to care for and transplant Rockwool seedlings, and tips for cloning plants in Rockwool.

Keep reading to understand why with a little practice, many gardeners prefer using Rockwool for starting seeds.

  • What Is Rockwool
  • Rockwool VS Soil
  • How To Start Seeds In Rockwool
  • How To Care For Seedlings In Rockwool
  • When To Transplant Rockwool Seedlings
  • Rockwool Cloning Tips
    • Preparation
    • Gather clone cuttings
    • Condensation and venting
  • In Summary

What Is Rockwool

Rockwool ingredients consist of molten basalt rock with a limestone additive. The liquid rock and limestone mixture goes into a machine that spins the material into superfine fibers that are both sterile and inert. One cubic foot of rock makes 37 cubic-feet of Rockwool.

Rockwool, also known as mineral wool or stone wool, came into use in the 1930s as an insulating material during the construction of homes and businesses throughout Europe.

1.5″ Rockwool Starter Plugs | Editor Recommended

Top Features

    Made from molten rock Includes Twin Canaries Chart Absorbs nutrient solution while retaining oxygen for rapid plant growth

Rockwool is now readily available in convenient shapes likes cubes, blocks, and slabs ideal for any gardener to grow plants from seeds.

Back in the late 1960s, Denmark scientists thought that Rockwool’s stable moisture and aeration levels could work for seedlings in hydroponic growing systems. They began tests to see how plants they grew in Rockwool faired.

The success of these tests has led to further improvements in a Rockwool formulation explicitly made for starting plants in hydroponic growing systems.

Rockwool VS Soil

For centuries, soil has been the tried-and-true medium for starting seeds. While soil has its benefits, Rockwool offers properties that can improve seedling success rates. Let’s take a look and the pros and cons of Rockwool versus soil.

Rockwool pros:

  • Stable retention of air/water
  • Reusable through several crops
  • Sterile and compostable material
  • Convenient to use
  • Ideal for hydroponic growing systems
  • Easy to monitor/control nutrient levels

Soil pros:

  • Easy to use/safe to handle
  • Affordable
  • Can help buffer excessive use of fertilizer
  • Adjustable for texture/moisture/nutrients

Rockwool cons:

  • Fibers could irritate eyes, lungs, and skin
  • Takes practice to perfect seed germination technique
  • Naturally high pH level needs adjustment
  • Initial investment costly

Soil cons:

  • Hard to regulate nutrient/moisture/aeration levels
  • Pests/Diseases/mold thrive in soil
  • Messy to handle

The most significant benefit to Rockwool over potting soil is the ability to grow more in less space. Plants in soil have to expend energy to grow roots long enough to reach the nutrients they need to survive.

Plant roots in Rockwool have instant access to water and nutrients. Therefore, the plant expends its energy on growing taller and stronger, instead of root development.

How To Start Seeds In Rockwool

I use Rockwool for growing vegetable and flower seeds before transplanting them into my garden, but it’s a staple in hydroponic gardening systems.

Here are the steps to start seeds using Rockwool.

  1. Fill a container with water set at a pH level of 5.5.
  2. If your Rockwool has a plastic wrap, poke small holes at the base of the cube so excess water can drain.
  3. Submerge the Rockwool in the water for at least one to two hours so it can fully absorb the liquid.
  4. Remove and shake the Rockwool to release excess moisture. Don’t squeeze the material, as this compresses the fibers and destroys its natural aeration properties.
  5. Most Rockwool comes with holes on the top. Set one seed in each opening and gently push it down.
  6. Place the Rockwool in a tray with good drainage. Investing in Rockwool cube and tray sets takes the guesswork out of this step. Place the tray in a room between 70 to 80-degree Fahrenheit. Use grow lights or set the tray in natural daylight.
  7. Always keep the Rockwool moist by misting it and placing a plastic dome over the tray.

How To Care For Seedlings In Rockwool

Rockwool has no natural nutrients for your seedlings. This feature is great for allowing gardeners to precisely dose their plants with the specific nutrients they need for optimal plant growth.

I add fertilizer to my misting water when caring for my seedlings in Rockwool. Since the plants uptake the nutrients easily, I find I don’t need to use as much as I would when I plant my seedlings in soil.

Watching the moisture level in the Rockwool seems daunting at first since you can’t let the material dry out and kill off your seedlings.

The plastic tray dome helps regulate the humidity and temperature levels of the Rockwool. Removing the lid during watering or for a short time during the day allows for beneficial air circulation.

Over time, you learn how the material reacts in your home’s environment. I find my trays need misting every other day, while other people may need to water twice a day.

I keep the dome on until my seedlings are tall enough to push against the lid.

When To Transplant Rockwool Seedlings

When you plant seeds in Rockwool, cut them apart after the first set of leaves fully open. You can transplant these seedlings, while still inside the Rockwool medium, into individual pots you fill with standard potting soil.

You also know it’s time to transplant Rockwool seedlings when the root system begins to bust out of the starter block.

You can transplant these blocks directly into your garden if outdoor weather conditions permit or place them in containers with soil until the season is ready.

You also can place the seedling into a larger block of Rockwool to continue growing the plant hydroponically. Cut a hole the size of your seedling block into a four-inch Rockwool block and place the seedling inside. It’s that simple!

I find my seedlings fair best when I grow them one more time through a four-inch Rockwool block. I then transplant them directly into my garden.

Rockwool Cloning Tips

Cloning plants works fantastic in Rockwool. Here are some tips to help make the cloning process more successful.


Disinfect your tray and prepare a room where the temperature can stay around 75-degrees with about 50-percent humidity. Make sure you have a barrier, like a sheet of cardboard between the tray and table surface to keep any cold surface from harming the clone rooting process.

For best results, set a T5 24-watt grow light an inch or two above your tray (with the dome on).

Soak your Rockwool cubes as you would for normal seed germination, remove excess water, and set them in the tray.

Gather clone cuttings

Cut clones from new growth. These can be suckers growing from the plant’s base or stems near the top of the plant. Cut the clone at the stalk snipping it at a 45° angle. Remove all leaves but the top two, which promotes new root growth.

Dip the cut end into a rooting hormone powder.

Condensation and venting

After you push the clone stems into the Rockwool put the dome on the tray. Keep the vents closed to create condensation inside the tray by pulling the moisture slowly from the Rockwool. This process triggers the clone to start growing roots as the plant tries to locate water.

Remove the lid daily to shake off excess moisture and allow air circulation. If you don’t see condensation, your tray is either too cold or set in direct sunlight which can evaporate the liquid. Move the tray to a better location before the clones wither.

Once you see roots growing through the bottom of the Rockwool, open the vents on the dome, making them larger each day. After the vents are fully open, remove the cover a couple of hours each day. Extend the removal period until the cover is always off.

At this point, you can transplant the clones as they now should be healthy and have active root development.

In Summary

Once you get comfortable using Rockwool as a growing medium, you’ll find it to be a pretty forgiving substrate. I like how the material allows plants to grow strong and fast by delivering water and nutrients directly to plant roots without much fuss.

While great for full-hydroponic home gardening systems, Rockwool is beneficial for growers like me who use it to start seeds for transplant into outdoor gardens.

I hope you find this guide to starting seeds using Rockwool to be helpful. Consider experimenting with Rockwool versus soil in your next seedling crop and see if you too become a Rockwool convert!

Learn what Rockwool is, the advantages of Rockwool vs Soil, and how to start seeds. Use these hydroponics tips for care, transplanting, and cloning your plants.