Grow year-round with ebb and flow Rockwool hydroponics (from seed)

Ebb and flow Rockwool hydroponics is a great way to grow herbs and leafy vegetables year round.

I write leafy vegetables as I this past winter chose to focus on leafy vegetables and herbs and put fun projects like strawberries, raspberries, tomatoes, and chilies on the back burner – at least for now.

This article will focus on a step-by-step guide to growing two leafy vegetables and 2 aromatic herbs. 

I have chosen lettuce and arugula for my leafy vegetables and red ruby basil and coriander (cilantro) for the herbs I am growing.

The method we will follow is to use Rockwool cubes and a homemade ebb and flow hydroponic system with grow lights.

Read this article for a DIY guide on how to build an ebb and flow hydroponic grow system at home. 

4 steps from seed to plant with Rockwool hydroponics

We will plant all herbs and vegetables from seed to germinate in Rockwool cubes. The coriander seeds will however be gently crushed before planting to speed up germination time.

1. Prepare Rockwool cubes

Draining rockwool grow cubes of excess water before planting seeds
Grow cubes placed on a tray to drain excess water

Before we plant our seeds we need to soak the Rockwool cubes.

Place the Rockwool cubes in a container with 3 cm (1 inch) of water and watch the cubes absorb the water.

The cubes will change color from light yellow to almost dark brown as they absorb water.

Next place the cubes on a flat surface to let the excess water drain off.

You are looking for the level of moisture of a damp sponge that has been soaked in water and then squeezed to remove all the excess water.

It can take 5-10 minutes for the Rockwool cubes to drain properly.

It is important you let the cubes drain to remove excess water. If the rockwool cubes are soaking and dripping wet, your seeds will rot before they germinate.

2. Place 2-4 seeds per Rockwool grow cube

Next, you plant, or more accurately place, 4 seeds per cube. Rockwool cubes have a hole stenciled in the middle of the cube.

Imagine that this hole is a square and then place 1 seed per side for a  total of 4 seeds for each hole.

Smaller seeds are best handled using a match or toothpick. Wet the tip with water and pick up each seed individually and place it in the hole. 

Easy to follow technique to plant even small seeds in Rockwool grow cubes

When all seeds are placed, you need to cover the hole. The fancy version covers the hole using vermiculite sprinkled on top of the hole and gently misted with a spray bottle.

But it works equally well to cut off a small bit of Rockwool from one of the corners and then use this small bit to cover the hole.

Using the corner of a grow cube to cover seeds planted in grow cube
I prefer using a piece of rockwool to cover the hole as it makes it easier to clean and reuse rockwool for hydroponics.

3. Place Rockwool cubes in a warm spot under a cover

To germinate seeds quickly, place Rockwool cubes under a cover in a warm spot.

You can place the Rockwool cubes in a mini greenhouse or simply put them in a container covered with plastic. It is up to you if you use what you have or spend money on something purpose-built.

Place the Rockwool cubes on a heat mat or other warm place like the top of a kitchen appliance like a refrigerator to speed up germination.

Check on the progress daily and mist with a spray bottle as needed. Add water to your container to rehydrate the cubes if they dry out.

As already mentioned, you want the growing environment to be moist as a damp sponge and not soaking wet.

4. Place seedlings under grow lights

The Rockwool cubes need to be moved when the seeds germinate and you see the first leaves.

If you are growing indoors and there are less than 6-8 hours of natural sunlight per day you need to use artificial lights also called grow lights.

As we are using ebb and flow hydroponics we will place the Rockwool cubes into our bed of LECA pellets (Hydroton).

Use your (clean) hands to make a hole in the LECA bed and insert the Rockwool cube. As the root system is protected by the Rockwool cubes we do not have to worry about hurting the delicate roots.

Hydroponics for growing year-round

Hydroponics allows us to grow herbs and vegetables year-round. And the lettuce, arugula, spinach, etc. that we harvest in wintertime packs a flavor and texture that store-bought produce fails to meet. 

And the same goes for herbs where both flavor and cost savings are drivers to grow our own herbs at home year-round.

But when you hear that hydroponics allows you to grow year-round, you may assume that we grow hydroponically during wintertime only.

That is, however, not correct. Hydroponics is not only for wintertime when we are unable to grow outdoors. Hydroponics is a very effective way to grow herbs and vegetables regardless of the time of the year.

And come spring and summer hydroponics work at least equally well with the added bonus that no extra grow lights are needed. 

Best yield using ebb and flow hydroponics

If you prefer you could of course also place the Rockwool cubes in net pots in a Kraken or Deep Water Culture (DWC) system.

But ebb and flow will yield better compared to Kraken or DWC hydroponics with or without aeration. 

Ebb and flow systems are also more effective from a space allocation perspective. Rockwool cubes with seedlings and plants at different stages of development can be grown next to each other for continuous harvest.

Just make sure to keep up with adding liquid fertilizers as you plant and grow more plants.

You can summarize each effectiveness and level of difficulty as follows.

MethodLevelYield
Kraken/DWC (no air)Easy+
Kraken/DWC (air)Medium++
Ebb & flow (Rockwool)Medium+++

Can you reuse grow cubes with Rockwool hydroponics?

You can absolutely reuse Rockwool cubes as long as you clean them thoroughly after each use.

Make sure to rinse and clean away any traces of seeds, roots, vermiculite, or any other organic materials. Rinse using a garden hose. Start with low water pressure and increase it as needed while ensuring that the cube does not lose its shape and structure.

When the cubes are clean, place them in boiling hot water for 2-3 minutes.

Next, let the cubes cool off and then shake gently to help remove excess water.

Finally, we let the cube dry completely before giving it a final inspection. 

Do you feel like it would be easier to squeeze the cube to get rid of excess water? 

Well you really should not. 

When you squeeze the rockwool cube you will destroy the shape and structure of the cube. Maybe not the first time around but it will happen. Be a bit patient and let the grow cube drain on a board. This patience will pay off and you will be able to reuse your rockwool grow cubes many times over.

I reuse all our Rockwool cubes and for this very reason, I never use vermiculite.

Why?

Vermiculite is a lot harder to get rid of when we clean it as it gets caught in the Rockwool fibers. 

I prefer to cover the hole in the Rockwool cube with Rockwool and have found that it works equally well – and it is a lot easier to clean.

Ebb and flow Rockwool hydroponics: How many seeds can I plant?

We recommend planting 2-4 seeds per Rockwool cube for most types of seeds. But there are of course exceptions to the rule.

Rockwool grow cubes have one central hole per cube. In this hole we can place up to 4 seeds, imagining the hole being square and following the principle of 1 seed per side. 

For larger type seeds we may settle for 2 seeds but what about smaller seeds like parsley and basil?

Here you may want to plant more seeds per Rockwool cube. And the way to do this is to stick to a maximum of 4 seeds in the premade central hole. You can however make new additional holes using a knife or other sterile, sharp instrument.

Do not use your nail or pinkie finger to make a hole in the grow cube. It works but you will also unnecessarily compress the fibres of cube. Instead, use a sterile sharp object like a knife for a more precise and better end result.
Using rockwool hydroponics to grow herbs and leafy vegetables from seed
Lettuce and arugula in Rockwool hydroponic ebb and flow system
Helpful resources:

University of Illinois Urbana-Campaign

Meet the author: Mattias is an experienced gardener spending most of his free time on his knees among herbs, plants, and garden vegetables. For the past two years, he has been sharing gardening projects and how-to tutorials on the NordicLavender website and YouTube channel.