Hydroponic Growers Rely On Rockwool Cubes For Successful Seed Starting And So Should You.

As I said in my last post, who needs soil to grow plants? Rockwool is the perfect soil substitute for plants. In this post, I discuss the beneficial properties of Rockwool and learn how to use Rockwool cubes for seeds starting in hydroponic systems. You really should try these Rockwool cubes for seed starting. They are inexpensive and reusable too.

But What Exactly is Rockwool?

Rockwool is produced by combining basalt rock and chalk then melted at a very high temperature. At around 3000°F, the mixture forms fluid lava. The lava then enters into a spinning chamber to create the fibers as it cools down. It is kind of like a process similar to making cotton candy.

Why Is Rockwool Used by Hydroponic Growers

Rockwool has a beneficial structure for plants because it retains water and holds more oxygen when compared to other soil mediums. It is evident to most readers that plants need water, but it is just as crucial for plant roots to have access to plenty of oxygen. This increased capacity to hold water, along with the added benefit of oxygenation within the plant’s root zone, is very helpful when starting seeds and cutting propagation. These characteristics of Rockwool make it the ideal growing medium.

Additionally, Rockwool is chemically and biologically inert.  In short, that means that it does not interfere with or alter plant growth in any way or harbor bacteria or fungi, which could infect and damage young seedlings. These overall benefits contribute to its popularity amongst growers, accommodating almost any plant they like to grow.

A) Lettuce seeds, two per cube are added to presoaked cubes B) Growing conditions allow for quick germination C) a suitable root structure forms easily D) Transfer to a net cup and it is ready for a hydroponics growing system.

Seed Starting Using Rockwool

Rockwool is popular with the hydroponics growing community, and it is commercially sold (Amazon) as cubes of various shapes and sizes. There is a bit of a learning curve when it comes to Rockwool, but the benefits are worth it, and at the same time, you are picking up a new gardening skill. In this post, I am using organic lettuce seeds that are easily inserted into holes in 1.5-inch presoaked cubes. Eventually, as the plants mature, they are transferred to a deepwater culture hydroponics system. If you are just starting out in hydroponics, I recommend growing lettuce because it’s easy and gives quick results. This happens to be Black Seeded Simpson, a variety purchased from Park Seed, a company known for its quality products.

Some Do’s And Don’t When Using Rockwool

  • Do wear protective gear to keep yourself safe when handling Rockwool material. These fibers can be irritating to skin, eyes, and lungs.
  • Do not squeeze the cubes when they are wet because it can damage the internal structure.
  • Do take the time to properly prepare Rockwool cubes before starting seeds. Rockwool has a naturally high pH, typically around 8.0, which is too high for many plant types. You need to follow the procedure for adjusting its pH down.

Propagation of Basil Plants Is Easy

This spring, I planted my basil in containers. I like Genovese, which is the classic Italian basil.  It has extra-large leaves with a strong aromatic flavor. Basil grows quickly in containers, its easy to water and to pick a few leaves to add to a delicious recipe. As the summer ends, basil begins to flower, pinch them off, and the energy goes into vegetative growth instead.   That means more leaves for your favorite pesto.  But eventually, basil plants get tired, so I propagate basil by using cuttings. The propagation of basil plants is easy, and now let’s see how it’s done.

First, you need to begin by selecting the upper parts of the plant for cuttings.  You must choose new shoots, the younger, the better.  Use a clean pair of scissors to make a sharp cut.  I emphasize clean because there is the potential to infect the plant cutting.

A) Select cuttings B) Add rooting hormone C) Use Roolwool as support, D) Cuttings need water for a few weeks E) Wait for roots to appear

After you select your best cuttings, wet the tip and dip in rooting hormone.  The use of rooting hormone is not necessary but almost assures root growth, and that’s a good thing.  Next, I place them into a Rockwool cube, which serves as inert support while the roots develop.  All that is left to do is add them to a tray of water and wait for roots to appear.

This time of year means its time to bring your basil inside.  If you have a sunny window that should do or you may have a greenhouse for protection from the change in season.  At any rate, propagation of basil is easy, but it’s not just about making more plants.  It’s more about extending your growing season so that you will have plenty of basil brimming with an aromatic aroma to keep flavoring your most delectable dishes.

I intend to grow my basil hydroponically using a deep water culture system along with a Mars Hydro SP 150 LED grow light.  Like I have been saying, basil grows excellent in containers, and that means hydroponic containers too.  After all, who needs soil to grow plants?

What is Deep Water Culture (DWC) Hydroponics?

Hydroponics is a method of growing plants without soil where plants are fed using a nutrient solution. The plants are supported in various substrates such as rock wool, expanded clay aggregate, gravel, sand, or coir peat. Since most hydroponic methods employ some type of growing support these methods are often referred to as “soilless culture”, while water culture alone is true hydroponics.  In this post, we will explore what deep water culture hydroponics is all about and take a look at how the process works.

Deep water culture is both an easy and effective method of hydroponic gardening.

In deep water culture, plants are grown in containers full of nutrient solution.  These containers can be small 5 gallon buckets or larger tubs and tanks for commercial systems. The nutrient solution in which the plant roots are suspended is usually aerated with an electric pump, tubing, and airstone which help to diffuse the air into solution. Generally speaking, aerated solutions are required to prevent roots from drowning.  More exactly, roots require oxygen in air because they perform a metabolic process called aerobic cell respiration. Just remember, DWC is the practice of growing plants in aerated water. It’s considered by many to be the simplest form of hydroponics. If you are a beginner in the field of growing plants then a DWC system is for you. These hydroponic systems are cheap and simple for DIYers to setup.

Plants are grown in slotted net pots suspended in holes cut in the lid of the reservoir.  Larger systems use a flotation raft instead of a simple lid. Reservoir size can be increased as plant size increases.  A single reservoir can be dedicated to an individual plant or many plants.  A large scale “raft” deep water culture system is shown below.

hydroponics deep water culture
A large commercial deep water culture system. In DWC, growth rates and yields can be astounding.

Which water level is best?

A well-hydrated plant typically grows incredibly fast and growers can manipulate water and nutrients levels in the root zone to decrease vegetative times by 15 to 25%. This decrease can trigger plant responses such as essential oil production, flowering, and fruiting. For instance, a dryer root zone can cause basil plants to increase their essential oil production. Whereas, a wetter root zone can cause plants to increase their photosynthetic rates by focusing on larger vegetative leaf production.

Choosing the Best Crops for Deep Water Culture

Are there any specific plants that DWC suits best? Here’s a list of potential crops for first-time growers.

Crop Growth Rates
Basil 8–10 weeks from seed
Lettuce 5–6 weeks from seed
Okra 7–9 weeks from seed
Kale 5–6 weeks from seed
Collard Greens 7–8 weeks from seed
Sorrel 4–6 weeks from seed
Chard 4–5 weeks from seed
Bok Choy 8–11 weeks from seed
Tomatoes 8 -10 weeks

Pros of Deep Water Culture:

  • Great for fast-growing plants
  • Flexible plant container sizes
  • Allows for larger root mass
  • Efficient use of water
  • Fewer plants with larger yield
  • Cost-effective to build and requires few parts

Cons of Deep Water Culture:

  • A chiller will likely be needed to cool the reservoir
  • Plants can be prone to root diseases
  • pH fluctuation may occur and requires periodic monitoring


Hydroponics Ramps Up

Hydroponics is a method of growing plants without soil where plants are fed using a nutrient solution. The plants are supported in various substrates such as rock wool, expanded clay aggregate, gravel, sand, or coir peat. Since most hydroponic methods employ some type of growing support these methods are often referred to as “soilless culture”, while water culture alone is true hydroponics.


Hydroponics as an alternative to traditional soil-based farming has ramped up.  For instance, as of 2017, Canada had hundreds of acres of large-scale commercial hydroponic greenhouses, producing peppers, tomatoes, and cucumbers.  Greenhouse production is becoming more economically feasible in the United States with production focusing on artesian and romaine lettuce,  Beef Steak, and cherry tomatoes. Vegetables grown in a greenhouse are often of higher quality and have been grown with reduced or no pesticides.  Additionally, yields can be 5x as much lettuce per acre than in a field. The United States hydroponic revenue has been estimated to reach $607 million with an annual growth rate of 3.6% over the last five years.

Commercial growers are utilizing two hydroponic methods, continuous-flow solution culture and static solution culture.  Nutrient Film Techniques (NFT) is a variation of the continuous-flow method. It uses a circulating hydroponic system that utilizes plastic channels to grow plants.  This works best for leafy greens such as lettuce or herbs. NTF provides a clean growing environment. The main advantage of the NFT system over other forms of hydroponics is that the plant roots are exposed to adequate supplies of water, oxygen, and nutrients. NFT can be suspended vertically as in the picture below.  This setup is at the heart of vertical farming.


In static solution culture, plants are grown in containers of nutrient solution, such as plastic buckets, tubs, or tanks. The solution in which the plant roots are suspended is usually aerated. A hole is cut in the lid of the reservoir for each plant. A single reservoir can be dedicated to a single plant, or to various plants. Reservoir size can be increased as plant size increase.  A large scale “raft” static solution system is shown below.


Despite the technological developments that have improved our agriculture productivity, the main issues of land availability, seasonality, high water demands, and carbon emissions seem like insurmountable facts. These long-standing obstacles continue to prevent us from meeting sustainability and food security goals. Therefore, in order to increase agricultural output to feed an ever-increasing human population requires the implementation of new farming practices, with a focus on hydroponics.  The science and technology is not that complicated. Nutrients are constantly monitored by computer systems leading to better yields using fewer pesticides, herbicides, fungicides.

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