Food for thought: Growing Medium

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To maximise the performance of Vertical Farming systems based on hydroponics, it is important to make efficient use of space and resources, such as water, nutrients and growing media. 
For example, the growing medium needs to offer space for the roots to grow and can affect the water-economy of the plant and its ability to utilize nutrients. 

It’s therefore important to consider which growing medium offers the best performance technically and economically for your system and the crops you plan to grow. 

Questions to consider includes: 
• Which growing medium provides the best results for various crops? 
• What are the options available on the market with reasonable delivery costs? 
• What are the growing medium costs per kilogram of produced crop? 
• Can the growing medium be recycled? 
• How much does the fertilizing need to be adjusted in different growing medium? 

Tech Tyfu have been working with two Vertical Farming systems since early 2020 – a “flood and drain” system and a “nutrient film technique” (NFT) system. 

Considering the texture of the growing medium as well as their ability to retain water and nutrients has been a key part of managing both systems. Here is some first-hand information on how different media performed, which will hopefully be a starting point for growers when it comes to choosing the best growing medium. 

The “flood and drain” systems were used to grow various microgreens such as pea shoots, wheatgrass, micro leek and radish. We learned that microgreens that are typically harvested between day 7 and day 16 do not need any nutrients added to the water. Nature has already taken care of the nutrient requirements of young seedlings by provisioning it in the seed store. There is also doubt whether young seedlings can absorb any nutrients from water in their infancy. 

The best growing medium results we obtained in 2020 for microgreens was the Growfelt Wool matting by a company called Growfelt (Figure 1). Growers found this to have good water absorption and retention, with a good germination rate and lower bacterial growth.

 Figure 1: Growfelt Wool with Pea shoots

 A cheaper alternative, Growfelt Purple (Figure 2) also performed well, but was outperformed by the Growefelt Wool when it came to water absorption and retention. Growfelt provide a 1m x 10m sample for £25.00. (Based on 2020 figures.) 

(Figure 2: Growfelt Purple)

Despite being made from 100% hemp and being free of any additives and fully compostable, the German Hemp matting by Schneiderfilz did not perform as well when it came to germinating. It seemed to accommodate bacterial growth more than other materials we used. We also found that individual fibres were prone to separating from the matting causing clogging issues in the systems draining pipes. This could create further issues for the pump later on if we’d left it for longer.

 Figure 3: Schneiderfilz Hemp matting

A European Innovation Partnership scoping study, titled ‘Consideration for the application of Vertical Farming’ by Dr Williams Stiles (2020) of IBERS, Aberystwyth University (see ), highlighted coconut coir as an excellent growing medium, as well as rock wool. 

“In an experiment undertaken by Khander & Kotzen (2018), chicory (Cichorium intybus) and basil (Ocimum basilicum) were used as model organisms to test the relative differences between a range of substrate types. For both the basil and chicory plants, coconut fibre was shown to perform best, and rock wool was shown to be second best. However, some additional challenges were reported for both substrates, including the blocking of irrigation systems by the coconut fibre material and the lack of recyclability of rock wool, reducing sustainability.” 

As with hemp, coconut fibre is considered a viable choice for minimising waste from a growing system. It cannot be produced in Wales nor even in Europe as far as we’re aware, which does unfortunately raise questions about the impact of transporting it long distances. Coconut Coir has excellent water retention and possesses good aeration properties, which leads to stronger roots and good overall health because of its neutral pH level. This sterile grow medium is bacteria free which will prevent plants from developing diseases, pest infestations, or fungus growth. 

In 2020, a Tech Tyfu pioneer grower successfully cultivated multiple harvests of watercress in a “flood and drain” system. The crop needed more depth to accommodate greater root space and better drainage and was grown in shallow baskets of clay balls. James Hooton at Hooton’s Homegrown successfully grew ‘Anglesey Watercress’ in his vertical farm using this growing medium. 

Crops that will need a number of weeks or months to grow generally perform better in such medium, with matting suitable for short term crops that are ready in days or 2-3 weeks.   

Nutrient Film Technique System (NFT) – Salad leaves and strawberries 

In the NFT units, Tech Tyfu pioneer growers have used rockwool grow cubes or plugs. Rockwool holds water and air which encourages fast rooting and growth. Rockwool is finely spun and can be provided with a hole in the top of the cubes, which is ideal for starting off seeds or rooting cuttings. 

Strawberries and salad leaves were grown successfully in the NFT system using rockwool plugs, available from suppliers such as Hydrogarden ( and GroWell ( 

Grodan is the leading brand for rockwool based growing medium. See 

 One of the highlights in 2020 for Tech Tyfu was the trial conducted by Glynllifon College using raw wool plugs in their NFT system. 

(Figure 4: Strawberries growing in wool at Coleg Glynllifon).

The team at Glynllifon cleaned and carded the fleece before inserting the grow plugs in small holding baskets in the NFT growing channels. The initial results were very promising, with the wool providing good water retention and aeration for plant growth. Figure 4 shows a young strawberry rootstock growing in a NFT system and supported by the wool plug. Could this be an innovative and much needed new application for local wool? 

Khandaker, M., & Kotzen, B. (2018). The potential for combining living wall and vertical farming systems with aquaponics with special emphasis on substrates. Aquaculture research, 49(4), 1454-1468. 

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