Starting Plants in Rockwool – Seed & Clone Cuttings Instructions

  • Posted on
  • By David Ittel

Before you start, below is a list of items needed to follow these instructions.

-Tray 10 in x 20 in (Nursery flat) with holes

-Tray 10 in x 20 in (Nursery flat) without holes

- Compartment tray insert 72 cell

- Nutrients of your choosing

- Starter cubes or plugs

- Humidity dome

- Light source = LED or T5 lighting recommended


1)  Soak Rockwool Starter Cubes or Plugs

a) The first step is to soak your rockwool and a simple three tray setup makes it easy to top or bottom-feed. I start with a no-hole bottom nursery flat tray, next is a middle nursery flat tray with holes, and then nested on top is a 72-compartmentalized tray. If you bottom-feed, watering is as simple as dunking the top two trays into the watering tray, and then lift the top two trays back out letting the solution drain back into the watering tray.  Then put the top two trays back into an emptied out bottom tray to catch any leftover drips. For top-feeding, put the top two trays over a sink, and water with a watering can. After it drains, put the two trays back in the bottom tray.


b) Now that you have your three trays, fill the bottom tray with a one-third strength nutrient solution of approximately 5.5 pH, and then soak the rockwool cubes or round plugs you plan to use. When I say one third, look at the instructions on your chosen fertilizer and find what dosage it recommends for vegetative stage and dial it back to one third of that.  So if it says 15 mL per gallon do 5 mL per gallon.


2) Insert Seeds or Cuttings into Rockwool and Rooting

a) SEEDS - Place the seeds directly into soaked rockwool cubes in a tray setup with a dome. Watering won’t be needed the first week. At most you might need to lightly mist with a hand sprayer. Place in a warm location. Light is unnecessary until the seeds germinate. Depending on what I am starting, germination can begin in as little as two days or can take more than two weeks. I check for signs of germination by looking down in the cubes hole twice a day. When I see a seed germinate, I place that cube in another tray setup without a dome. The top of the seedling should be approximately 4 inches under a LED or T5 light. Once under the lights, I check the need for watering daily. Initially, I use a one-third strength solution at approximately a 6 pH.


b) CUTTINGS - The growing area must be completely sterile. Disinfect all tools, benches, etc with a solution of one-part household bleach to nine-parts water. Soak the number of round plugs needed for the cuttings to be taken (as instructed above). Take cuttings from clean disease-free stock. Cut off the leaves from the base of the cutting so that you have at least one node free of vegetation. Large leaves on the upper part of the cutting can be trimmed to smaller size if necessary. Dip’N Grow or a rooting gel can be used as the initial root-inducing substance. Gently open the round plug and lay the stem inside so that it contacts as much rockwool as possible. It is essential that there is good contact between the cuttings and the rockwool. The round plugs with the cuttings can then be inserted, in a staggered arrangement, into a tray setup. Cover with a clear dome. At least once daily, remove the dome and mist the cuttings with lukewarm water. Watering the cubes should not be necessary until roots have formed. If it does become necessary, water the cubes with one-third strength nutrient solution at 6 pH. Recent studies have shown that cuttings benefit from more light intensity than previously thought. If you are using cool LEDs or T5 cool bulbs, keep the light source about 8 inches above the propagation dome. Begin checking for rooting one week after the cuttings have been taken. This can be accomplished by giving the cutting’s stem a gentle tug. If there is resistance from within the cube, this indicates rooting has occurred. Once the cuttings have rooted and start actively growing, remove the dome, and over the next few days, begin moving them closer to the light. The tops of the cuttings should end up 4 inches from the LED or T5 bulb.


3) Transplanting – Cuttings and Seeds

a) When 4 to 7 healthy, white roots have penetrated the base of a single rockwool cube or plug, it is ready for transplanting into a plastic-wrapped growing block. First, the cubes or plugs should be soaked in their current strength nutrient solution, and the blocks, prior to use, should be soaked in plain water adjusted to pH 5.5 for 1/2hour. Thoroughly flush the blocks with the same nutrient solution used for the seedlings or cuttings.


b) Optional. Dose the block's hole with a product that has various strains of mycorrhizal fungus with growth-promoting bacteria and organic biostimulants. These helps promote a healthy strong root zone that is better at resisting stress and disease.


c) Place the small cube or plug into the hole of the larger block. To discourage the growth of algae, use a cap or cover on top of the block. When roots have penetrated the bottom of the plastic-wrapped block, it's time to place the block on the larger 6” rockwool block or on a plastic wrapped slab. If you use the slabs, space them to achieve your desired plant density. I like using the “Dutch Leach Trays” to hold the slabs. There is a top and a bottom to each slab; the printing on the side of the slab should not be upside down. To prepare the slab for growth, cut a flap or an X in the plastic where each plant is to be placed. The space should be large enough to provide ample room for your growing block to make full contact with the rockwool of the slab. Fill the slab with water adjusted to pH 5.5 until the plastic sleeve of the slab bulges with excess solution. Allow the slab to soak for 1/2hour or more. The slab should then be sloped two degrees towards the drainage area. Cut a 2" drainage slit in the plastic at the base of the slab at the lowest end to drain the slab. Slits can also be made along the length of the slab base if desired. Flush the slab with full strength nutrient solution with a 6 pH.  Place the growing blocks on top of the slab. If using drip emitters, keep them on top of the blocks making sure that the dripping solution does not touch the plant's stem. If you use the 6” blocks, follow the same presoak and flush procedure. I still use a 6” cubecap or block cover cut to accept a growing block on top. The Grodan 6” blocks have a small starter cube hole on top that I stuff with a round plug before putting my growing block on top.


d) With automatically-timed waterings of rockwool, it is advisable to do your first light watering one hour before the lights come on to charge the slab. Water frequently until the plants’ roots have established themselves from the block securely into the slab or 6” block. Once the plants have secured themselves, you can begin watering when the rockwool dries to approximately 60% of its saturated weight. If you gently squeeze the rockwool and no solution comes out, it is time to water. Hand-watering also gives good results with only one watering a day; just be sure that the slab is large enough so that it doesn't go below 60% saturated weight between waterings. We suggest flushing the slabs with 100% excess nutrient solution at least once every 2 weeks.