Rockwool Preparation for Hydroponic Growing

April 11, 2008 at 12:10 am 2 comments



Preparing your rockwool before adding precious seeds or freshly cut clones is a vital step to healthy plants and a bountiful harvest yet many hydroponic gardeners skip this step (usually because they don’t know any better…I sure didn’t my first time). Today we are going to properly prepare 2x2x2” rockwool cubes for seeds or clones in 5 easy steps.  Before we start gather all the materials and tools so they are in reach:


2x2x2” rockwool cubes (as many as you need)

2” netted pots

Plastic garden tray

Flower Liquid Fertilzer

5 Gallon bucket

5 gallons filtered water such as reverse osmosis or bottled water

1 mL increment measuring tool (such as syringe or shot glass measurer)

pH Up

pH Down

Dish soap


Wash Cloth or Sponge

Terry Cloth


pH Tester

toothpick or small screwdriver

EC Tester (optional)

PPM Tester (optional)

Step 1 – Test the Water:

Believe it or not you need to test the water before you begin to ensure it is actually clean.  Start by checking the pH, it should be in the range of 6.9 – 7.1, if it is outside of this range change the filters in your water filtering system, find another source of water or obtain a water filter system such as a 5 step reverse osmosis machine.  I would highly recommend checking the EC (electrical conductivity) as it should be below 1.0 (usually it will be around 0.2 if your pH is in the range above).  You should also check the PPM (parts per million), this should be under 200 PPM and most likely will be closer to 100-110.  The water temperature should be 68-72 degrees.  If all these check out we are ready to move on to the next step. 

Step 2 – Wash the plastic:

Using warm water (do not use filtered water on this step), soap and bleach wash the netted pots, 5 gallon bucket and plastic garden tray with a clean wash cloth or sponge.  Rinse with warm water and dry with clean terry towel (or any towel that does not leave lint behind).

Step 3 – Rinse the rockwool:

Add 1 gallon of water to the 5 gallon bucket.  Submerge the rockwool cubes for 30 seconds so they are completely soaked.  Pull out of the water, lightly squeeze for 3 seconds, submerge again for 3 seconds; repeat 5 times.  Set the rockwool in the plastic tray.

Step 4 – Treat the rockwool:

Next we are going to treat the rockwool with a 5% fertilizer solution so there is a source for the seed or clone to draw nutrients.  First you want to empty your 5 gallon bucket & rinse it out with some of the filtered water.  Add 1 gallon of filtered water that you tested in Step 1 to the bucket.  Depending on the plant you are growing will determine the fertilizer that is best but I am going to use BC Bloom as it is great for all varieties of tomatoes I’ve grown in rockwool.  Most nutrients have instructions for a 5% solution (may also be labeled seeds/cuttings mixture).  If there are only instructions for 1 Gallon then the easiest way is to divide by ten.    So if it says 10 mL then only put in 1 mL, this actually results in a 10% solution but that is just as good and the math is a lot easier (I actually like this better for some varieties). Once you’ve added the nutrients test the pH; the aim is to get the water pH to 6.2 (6.1 – 6.3 is acceptable).  If your pH is below 6.1 (most likely it will be) then use pH Up.  If your pH is above 6.3 then use pH down.  I recommend using a dropper to add the pH Up/Down since the water volume is very small and a little bit goes a long way; I usually use 1 drop for every 0.2 pH points that need adjusted but never add more than 3 drops at once.  If you had too much pH Up you can use pH Down to adjust and vice versa (I did this many times at first…mainly because I wasn’t using a dropper).  The goal is to utilize as little pH Up/Down as possible to achieve the optimal 6.2 pH and you will get better with practice. Submerge the rockwool cubes for 30 seconds again.  Pull out of the water, lightly squeeze for 3 seconds, submerge again for 3 seconds; repeat 5 times.  Place the rockwool in the netted pot and set pot with rockwool in plastic tray.

Step 5 – Add Seeds or clones:

Now use the toothpick or screwdriver to make a small hole that is a little larger than your seed or clone stem.  If you are using seeds I recommend a ¼” deep hole; if making a clone from a cutting make the hole a ½” deep.  Place the seeds/clones under 18 hours of fluorescent lights and watch your young plants grow.  Water with a 5% solution for the first 14 days as needed to keep the rockwool moist.

Congratulations! We did it.  If you have any questions, comments or suggestions post them below.  Thanks for reading everyone!


Rockwool Preparation for Hydroponic Growing


Entry filed under: Gardening, Hydroponics. Tags: , , , , .

2 Comments Add your own

  • 1. Mr WordPress  |  April 11, 2008 at 12:10 am

    Hi, this is a comment.
    To delete a comment, just log in, and view the posts’ comments, there you will have the option to edit or delete them.

  • 2. Barry Brown  |  May 9, 2008 at 7:26 am

    Mark, my wife and I are starting (planning?) to possibly start an organic farming website…we may want to affiliate when we’re both up and running.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

Trackback this post  |  Subscribe to the comments via RSS Feed


April 2008

Most Recent Posts

%d bloggers like this: