Refining the processes involved in recycling coir growing media

Title: Second Life: Development of sustainable recycled growing media
Funders: Overland Ltd, Growing Kent and Medway and The East Malling Trust
Term: April 2023 to March 2025

There has been a major shift in soft fruit production in the UK out of field soils and into soilless substrates, and with government policies aiming to reduce reliance upon peat, the majority of production is now in coconut coir. However, the increasing price of coir, limited availability and volatile shipping expenses have resulted in increased costs for growers. The carbon footprint associated with shipping substrate from Asia is also a concern, whilst additional labour costs are incurred in replacing and disposing of waste coir.

There has been much interest in re-using, composting or recycling coir although growers have so far been reluctant to use at scale due to concerns over pest, disease and weed build-up reducing both yields and the quality of the fruit produced. An early EU funded research project carried out by Niab identified that Junebearer strawberry can be replanted in used coir bags with little effect on yield as long as no disease was observed in the first year. Everbearers on the other hand suffer around 7% year on year yield decline when grown in reused coir bags.  

Overland has developed an automated process to recycle coir from strawberry bags which includes automated, low labour removal of bags from the tunnels, followed by the removal of plastic, plant leaves, roots and crowns, to leave clean coir. The coir is then treated to reduce the risk of pests and diseases, before making it available for growers.

Overland partnered with Niab to do further work to assess how the cycles of both growing and recycling change coir properties over time. We found that the water holding capacity increases while the air filled porosity (AFP) decreases in recycled compared to virgin coir. The extent of this change varied with different coir manufacturers. Recycled material also has slightly lower pH, higher electrical conductivity and higher nutrient content compared to virgin coir.  Levels of crown rot (Phytophthora cactorum) tend to increase in directly re-used and composted coir compared to virgin, but this has not been evident in the fully recycled coir that Overland is producing.

The project

Overland and Niab secured further funds from Growing Kent and Medway to accelerate this research and bring sustainable recycled coir media into commercial strawberry production. The aims of the project were to 1) develop energy efficient and robust procedures to eliminate pest, pathogen and weed risks in recycled material; 2) to demonstrate the use of recycled media on a commercial scale and develop wrap around agronomy advice; 3) compare lifecycle analysis of the virgin and recycled coir to measure any economic and environmental gains from using recycled media.  

Results

In work to eliminate pest, pathogen and weed risks from recycled coir, a heating process was developed and refined which successfully inactivated pest, pathogen and weed seeds from the substrate. Further work was done to study and compare the microbiome (community of microorganisms) within both virgin and recycled coir. We investigated fungal, bacterial and oomycete organisms in each coir type collected from a commercial scale trial during peak harvest.

A greater biodiversity of fungi was found in virgin coir compared to recycled and within these, we recorded more potential fungal pathogens on the roots of strawberry plants grown in virgin coir compared to recycled. These pathogens included species of Ilyonectria, Neopestalotiopsis, Verticillium, Mucor, Macrophomina and Fusarium. We also found more Colletotrichum and Penicillium species in virgin coir but not all of these are considered pathogenic. In assessing beneficial fungi, we found more Trichoderma (both commercial biocontrol species and others), Metharizhium and Serendipita in virgin coir. In contrast we found more Rhizophagus species (beneficial arbuscular mycorrhizal fungi) in recycled coir.

Bacteria were more diverse in recycled coir, but some were more prevalent in recycled and others in virgin coir. Potentially pathogenic groups were again found to be more abundant in virgin coir. 

Of the oomycetes (pathogens such as Pythium and Phytopthora species), very similar levels were recorded in virgin and recycled coir. There was a very slight increase in Phytophthora cactorum in recycled coir.

It is worth noting that no disease symptoms were observed on plants grown in virgin or recycled coir.

In work to demonstrate the use of recycled media on a commercial scale, the everbearer variety Katrina was planted in virgin Legro bags, directly reused Legro bags and Overland’s recycled Legro bags at a commercial site (Kelsey Farms) in 2023. Each coir type was used in nine commercial tunnels (over 3,000 bags per coir type) with an independent irrigation schedule. Sadly, during harvest the virgin and re-used coir were mistakenly picked and recorded together, so the recycled coir was compared to both virgin and directly replanted coir together.

The yields were similar (around 1.3 kg per plant) and no differences were found in pests (thrips, aphids, weevils), weeds or root rot pressure between coir types, but there were visual differences in plant growth. In the recycled coir, plants appeared to be stronger and cropped 7-10 days earlier than the virgin coir bags. The plants grown in recycled media also used 12 % less fertigation over the season. This reduction in water and fertiliser use in recycled material was especially prominent during hot days.

The grower was very happy with the performance of the recycled media, and has since expanded its use to over 200,000 pots of raspberry. A similar trial was repeated in 2024 at a Summer Berry Company site in Chichester, where 1.45 kg of fruit per plant (variety Favori) was produced in both recycled and virgin coir with 8% water and fertiliser saved in recycled media. The trial was done in troughs where roots could be inspected fortnightly. More uniform and higher density roots were observed in recycled material.

At Niab’s East Malling site in 2023, the everbearer variety Malling Supreme was planted in a small trial with both virgin and recycled Legro and Cocogreen coir in troughs rather than bags. Separate irrigation rigs were used for recycled and virgin media but not for each coir brand. The two recycled coir samples used 4% less water than virgin. Reduced need for wetting up and maintaining moisture in recycled material at the start of the season was the primary reason although reduced water use on the hot days due to the higher water holding capacity of the recycled coir also contributed.

The total yield from recycled coir was slightly lower comparing Legro recycled and virgin coir. This was due to the fact that virgin materials of both brands were fairly comparable in terms of water demand, but recycled materials with different previous growing histories were not. Namely, recycled Legro material had much higher water holding capacity compared to recycled Cocogreen. This meant that recycled Legro coir was over irrigated and recycled Cocogreen underirrigated resulting in a slight yield reduction. This highlights the need for the of use different irrigation/fertigation regimes with different coir types, or at least to use separate valves to manage coir moisture adequately.

In 2024, low grade, single crown, bare rooted Malling Centenary plants were planted in virgin and recycled material as a worst case scenario experiment. Yields were very low in both coir types (75-95 g per plant), but the plants yielded significantly more fruit in recycled material.

In an exercise to compare the carbon footprint of using recycled versus virgin coir, Niab and Overland calculated that removal-recycling-delivery of recycled coir emitted around 40% less CO2 than sourcing virgin coir shipped from Sri Lanka.

In summary, to date we have demonstrated that recycling coir offers much better potential than either re-using or composting coir. Recycled material can achieve strawberry yields and quality that is comparable to virgin material. Recycled coir poses no greater pest, disease or weed threat than virgin coir and there were fewer pathogenic fungi recorded in recycled coir compared to virgin, although there was a very slight increase in the oomycete P. cactorum in recycled coir. The rate and level of physical and chemical degradation does vary depending on the coir type, manufacturer and growing history but we believe that cost effective coir recycling is possible with little yield reduction.

However, it is important that the irrigation and fertigation of crops grown in recycled coir are managed separately from virgin coir, to adjust for the lower AFP in the recycled product, otherwise over-watering can occur leading to root death and reduced yield and quality.