Rockwool Cubes | Informational Guide to Rockwool Seed Starting Soil | Bootstrap Farmer

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Did you know rockwool is the most widely used substrate for hydroponic tomatoes?  It's also massively popular in the use of greenhouse strawberry production. For good reason too, as it's an excellent substrate for cloning, germinating and hydroponic growing.   


Growing vegetable starts in rockwool


Most of us who try starting a small garden see very little success the first year.   There's all sort of issues that pop up that we didn't think would be a problem.  Most of these issues start at the germination table.  It takes awhile to get the hang of knowing when to keep on the humidity dome, how much to water and how often. 

We've all come home to find dried out, dead seedlings.  We've all killed transplants because we weren't gentle enough.  Over the years as we try new things, the solutions become second nature.   

Rockwool is one of those growing media, that once you find it and use it a few times, it's hard to switch back to anything else.

 I wrote this post just to circle back on what I've learned about rockwool, and to relay the benefits to some of you who haven't tried it out yet.  

 What is Rockwool?

Rockwool is an extremely popular growing medium in the hydroponics community.  It's composed of two natural ingredients, basalt rock and chalk, that are heated to 3000 degrees Fahrenheit and “spun” and cooled, transforming it into a material that is perfect for a plant’s root zone. 

Rockwool is great a balancing moisture and oxygen, which provides a perfect middle ground to prevent over-watering or under-watering.  Its fibers don't restrict the accessibility of nutrients for the plant as it provides an ideal medium for nutrient uptake.  Rockwool alone has no naturally occurring nutrition, so the plant depends totally on the nutrient solution you give it.

There are rockwool products for all sorts of uses, coming in varying sizes.  It can be found in blocks, plugs, slabs, chunks, and cubes.  For seed germination, the 2” rockwool cube is a great “one size fits all” choice.



Growing with Rockwool

Rockwool should be thoroughly soaked in pH balanced water before use.  Most rockwool cubes have a pH of around 7.8, which is higher than the plant’s preferred levels of 5.5 – 6.5.  Before soaking your cubes, test your water’s pH balance and adjust accordingly.  Use pH balancing chemicals to get your water within range.  Be careful to add in small amounts, checking the pH levels after each addition.  Once your water is pH balanced, add your cubes and let them soak for about an hour, or until thoroughly wet.

Once your rockwool cubes are soaked, you’re ready to plant.  A smart tip for seed placement is to wet the tip of a toothpick and “pick up” one seed with the wet end.  Place the seed at the bottom of the hole while gently pressing it to one side.  If you are planting two seeds, press the second seed to the opposite side of the bottom of the hole.  Place your seeded rockwool cubes in a tray, such as our 1020 tray or 32-cell tray, and cover with the humidity dome to lock in moisture.  You won’t have to mess with your cubes over the next 2-3 days.

What’s Next?

Now it’s time to live, and let grow!  In about 3 days, you will see your seeds starting to sprout and a couple of more days you will see first true leaves.  If you wanted to increase your germination rate, and decided to  plant two seeds per cube, now is the time you need to cut the tops off of the seed that isn’t the strongest (or tallest) in each cube.  DO NOT pull them out as you may disturb the strong plant that is sharing the cube.  Keep an eye on your plants and cubes.  Feel the cubes every now and then and if they feel they are becoming dry, spray pH balanced water onto the cubes themselves.

A few weeks after germination, you will be ready to transplant your crop to your hydroponic system.  Pay close attention to your cubes around this time, as the ideal time for transplant is just as the roots begin to come out of the bottom of the rockwool cube.  This will help ensure the plant’s roots grow downward into your system.  If you wait too long to transplant, the roots will begin to wrap around the bottom of the cube until it is given room to expand.

Handling Rockwool

It is recommended you wear a dust mask, gloves and goggles when handling rockwool.  Rockwool’s physical makeup might cause skin or eye irritation.  While some growers choose to throw away their used rockwool, it is possible to shred and reuse it in other growing mixes.  You can even incorporate it into outdoor soils and gardens as a soil conditioner.  Sometimes it’s better to be safe than sorry when it comes to contamination, so it’s not recommended you reuse rockwool for most applications.  


Sources & Helpful links:


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