The challenge of defining “sustainable growing media”: why the term is used incorrectly and what needs to be done about it

1 Introduction

The concept of sustainability is widely assumed to be self-evident and inherently supportive of the greater good, yet it has become increasingly detached from its original meaning of maintaining a specific system continually (Grober and Cunningham, 2012; Kidd, 1992; von Carlowitz, 1713). In the growing media literature, the term has been discussed critically for more than a decade, with several influential reviews attempting to clarify its meaning and guide material development and assessment (Barrett et al., 2016; Bilderback et al., 2013; Carlile and Coules, 2013; Carlile et al., 2015; Caron and Rochefort, 2013). Nevertheless, the term remains inconsistently applied and is frequently used to describe materials based on assumed environmental benefit, often without measurement, and before their ability to sustain viable crop production has been demonstrated. This is not unique to growing media but is particularly evident even in comprehensive overviews of the field (e.g. Gruda et al., 2024), where sustainability serves as a guiding aspiration rather than a measurable standard for the materials being assessed.

Growing media (commonly known as potting soils or substrates) are defined as materials other than soil in situ, the function of which is to support the growth of plants or mushrooms (Caron et al., 2023; European Parliament and Council, 2019). Their purpose is to regulate water, air, and nutrient dynamics within a confined root/mycelium volume. In addition, they must meet practical constraints related to weight, structure, handling, consistency, and availability at scale. In practice, relatively few materials have consistently met these requirements across crops and production systems while also being available at scale (Gruda et al., 2023; Himanshu et al., 2026; Hirschler and Thrän, 2023; Koseoglu and Roberts, 2025; Schmilewski, 2008). Historically, horticultural peat and, more recently, coconut coir have occupied this role in organic growing media, while mineral wool has become widely established in soilless vegetable and cut-flower production. However, all are subject to environmental and/or social scrutiny, whether related to peatland degradation and carbon emissions for peat, long-distance transport and supply chain conditions for coir, or non-biodegradability and end-of-life disposal for mineral wool.

In practice, materials such as compost, wood fibre, bark, coir or biochar are often described as sustainable based on characteristics such as being non-peat, recycled, renewable or associated with favourable environmental expectations (Gruda, 2019; Hirschler and Thrän, 2023). However, none of these characterisations, on their own, convey whether the material can sustain horticultural production in terms of quality, economic viability, and supply. Nor do they even necessarily demonstrate reduced environmental impact at a system level (Barrett et al., 2016; Goglio et al., 2025). Sustainability is therefore often framed narrowly in selected environmental concerns that are weakly specified or insufficiently quantified, while production performance and economic feasibility (e.g. at a commercial greenhouse scale) are treated as secondary considerations, left untested or not stated at all.

A newly formulated growing medium with adequate properties is expected to perform more or less as reliably as the growing medium it will replace, particularly in a sector characterised by low margins and limited tolerance for production risk (Brumfield et al., 2015; Wei et al., 2020). A growing medium that substantially increases production risk or reduces yield can undermine the very system it is meant to support, regardless of its environmental intent. Therefore, sustainability claims that are not grounded in demonstrated production performance therefore provide little practical guidance and may obscure, rather than reduce, environmental, economic, and social risk.

This tension between functional reliability and environmental concern forms the backdrop against which sustainability has emerged as a central concept in growing media research. However, the term “sustainable growing media” is widely used without a consistent or operational meaning, creating ambiguity for researchers, manufacturers, policymakers and growers who must evaluate and apply such claims in practice. In this paper we examine why the term “sustainable growing media” is persistently challenging to define, describe, and apply in a useful and science-based manner. Rather than proposing a fixed definition, we analyse what the term can reasonably be taken to mean when applied to growing media and offer guidance on how it should and should not be used in research, policy, and manufacturing.

2 Definitions of sustainability

The concept underlying sustainability was first formally articulated by Hans Carl von Carlowitz (1713) in the context of forestry, where it referred to the careful management of an indispensable resource so that it remained continuously available. In this original sense, sustainability was explicitly practical and system specific. It concerned continuity, reliability, and the long-term maintenance of essential functions.

Nearly three centuries later the Brundtland Commission (1987) reframed sustainability, defining it as meeting present needs without compromising the ability of future generations to meet their own. The reframing included broadening the scope from long-term, economically feasible production to a generalised higher level that further includes ecological and societal goals. While this has been foundational for international policy, its meaning becomes diffuse and less well defined when transferred from societal systems to individual materials or products.

The subsequent consolidation of sustainability into three pillars (environmental, economic, and social) was intended to make the concept more operational (UN World Summit 2005). However, this framework provides little guidance on how these dimensions should be prioritised or balanced when trade-offs arise (Purvis et al., 2019). In technically constrained sectors such as horticulture, where materials must satisfy strict functional requirements and supply realities, these trade-offs between pillars of sustainability are unavoidable.

2.1 Past attempts to define “sustainable growing media”

Efforts to define sustainable growing media have remained unresolved due to these broader conceptual challenges. One of the most ambitious efforts took place in the United Kingdom between 2011 and 2013, when a multi-stakeholder task force undertook the development of a shared definition (Knight, 2012). Despite an unprecedented collaboration across industry, retailers, NGOs, and government (Dawson, 2013), no agreed definition emerged. The process attempted to address aspects of all three sustainability pillars but recognised the complexity of measurement and how incomplete the assessment really was, with the narrative changing from ‘sustainable’ to ‘responsible’ (Alexander and Bragg, 2014).

Thus, focus shifted away from defining sustainability toward concepts that could be operationalised, such as the ‘Responsible Sourcing Scheme in the UK’¹, “Renewable raw materials” in the Dutch ‘Covenant on the Environmental Impact of Potting Soil and Substrates’², ‘Responsibly Produced Peat’³, and, more recently, ‘Responsibly Produced Coir’⁴. Criteria for responsible sourcing could be specified, audited, and enforced, whereas sustainability remained too difficult to codify. As a result, “sustainable growing media” persisted as an aspirational label without clear operational content, while governance moved toward narrower, verifiable proxies. National initiatives targeting reductions in horticultural peat use, such as the German ‘Peat Use Reduction Strategy’⁵ and the Swiss ‘Peat exit plan’⁶, are also examples of easily verifiable proxies (peat volumes/percentages) but largely lack measurements of sustainability at growing media or horticultural levels. The German label ‘HORTICERT’⁷ has attempted to address this by certifying that peat substitute materials meet certain ecological, social and economic standards across their production chain, including a standardised values for greenhouse gas emissions derived from the Growing Media Environmental Footprint Guideline (Growing Media Europe, 2021; Growing Media Europe, 2024). However, these metrics are somewhat arbitrary, peat is excluded from certification, and there is no required demonstration of horticultural performance.

3 The three pillars of sustainable growing media

The difficulty of defining sustainable growing media is closely tied to the three-pillar framework itself (Figure 1). Sustainability claims depend on which pillar is emphasised (Carlile and Coules, 2013). Environmental objectives have often prioritised peat reduction and the promotion of non-peat materials (often without impact measurement), social considerations have often focused on the labour conditions of coir production, while economic considerations have emphasised quality and consistency, cost control, and continuity of supply. For growing media, this has allowed materials to be characterised as sustainable or unsustainable depending on perspective or opinion, even when the evidence base is limited or even contradictory. The result is that the term often fails to convey clear, common, or even correct criteria.

3.1 Environmental sustainability

For most stakeholders, sustainability in growing media is primarily associated with environmental outcomes and treated as an inherent material property. However, even within this limited scope, terminology is used inconsistently and claims often rely on assumptions rather than quantified impacts. Peat is frequently cited as environmentally unsustainable due to its association with carbon emissions, peatland degradation and biodiversity loss (Gruda, 2019; Gruda et al., 2024, Gruda et al., 2023). While environmental impacts of peat extraction are well documented (Chapman et al., 2003), justification is often based on overly broad qualitative statements or global peatland statistics without adequate contextualisation to extraction practices, use intensity, or substitution effects (Hirschler and Osterburg, 2022; Koseoglu and Roberts, 2025; Prasad et al., 2024). Oversimplifications are also observed for other materials including coconut coir (Hirschler et al., 2022), where environmental assessments can overlook saline wastewater generation during processing (Woznicki et al., 2024); wood-based materials, where renewability alone does not reflect processing emissions or nitrogen immobilisation during use (Atzori et al., 2021); mineral materials (e.g., mineral wool, expanded perlite), where questions of non-biodegradability, recyclability, and end-of-life treatment are frequently overlooked; and societal side-stream materials (e.g., green compost, composts, solid digestates), where impacts related to transportation, nutrient losses and emissions during processing are inconsistently addressed (Raviv, 2013). Moreover, the use of materials that fail to meet functional requirements can itself increase environmental impacts by reducing production efficiency and increasing resource use at the crop system level. In practice, all materials have environmental impacts (Quantis, 2012; Stichnothe, 2022), and it is necessary to make comparisons that objectively consider footprints of both the growing media and overall crop production.

Life cycle assessment (LCA) provides the most established framework for comparing environmental impacts of growing media, enabling assessment across multiple categories, including climate change, acidification, eutrophication, ecotoxicity, land use and water consumption (Pryshlakivsky and Searcy, 2013). However, comparability between studies remains limited due to variation in assumptions, system boundaries, functional units, reporting formats and data quality (Weidema et al., 2018). Recent European initiatives have aimed to address these limitations through standardised Product Environmental Footprint (PEF) methodologies, including sector specific category rules (CR) for horticultural products (FloriPEFCR for cut flowers and potted plants, Broekema et al., 2024; and FreshProducePEFCR for fruits and vegetables, Weststrate et al., 2026) including growing media (Growing Media Environmental Footprint Guidelines; Growing Media Europe, 2021; Growing Media Europe, 2024). These frameworks standardise assessment across sixteen environmental impact categories by defining system boundaries, functional units and data quality requirements, and include an aggregated single-score indicator based on European Commission weightings, reducing the risk of cherry-picking individual metrics such as carbon footprint while ignoring others.

Environmental sustainability factors that are not comprehensively or directly captured within standard LCA frameworks, but are nonetheless important, are biodiversity effects, nature conservation outcomes, renewability of materials, and end-of-life considerations such as reuse or recycling (circularity). While these aspects should be better considered, they only add clarity when their boundaries and limitations are explicitly defined. Evidence of reuse potential exists for some growing media under controlled conditions (Vandecasteele et al., 2024; Woznicki et al., 2024), but environmental performance alone is insufficient to justify broader sustainability claims without concurrent consideration of functionality, economic feasibility and social implications.

3.2 Economic sustainability

Economic sustainability is necessary for the adoption of novel growing media in horticultural production. It determines whether materials and practices can move beyond experimental or niche applications and be maintained at commercial scale. In this context, economic sustainability refers not only to product affordability, but to the capacity of the system to generate stable value across the entire life cycle and value chain (Auer et al., 2017), while supporting reliable horticultural production.

Conceptually, the economic dimension operates at three interlinked levels: (i) manufacturing, (ii) use, and (iii) reuse or refurbishment, as well as for diverse market segments (e.g. hobby, professional). At the manufacturing level, economic sustainability depends on the cost, availability and consistency of raw materials, processing requirements, energy and water inputs, logistics and quality assurance. These factors directly affect production costs, scalability and supply continuity, which are critical for both conventional and alternative growing media constituents. At the use level, economic performance is inseparable from horticultural function. Growing media influence yield stability, crop duration, water and nutrient use efficiency, disease pressure and management complexity. Any increase in variability, risk or compensatory inputs translates into economic penalties for growers and reduces adoption likelihood. At the reuse or refurbishment level, economic sustainability is linked to the feasibility of nutrient recycling, processing costs, quality retention and risk mitigation when spent media are reintroduced into production systems. While reuse of spent growing media has been demonstrated under controlled research conditions for materials such as peat and coir (Vandecasteele et al., 2024; Woznicki et al., 2024), commercial-scale implementation remains limited. The costs of sterilisation, quality testing and logistical handling may exceed those of virgin material, particularly as substrate prices decrease with scale. Economic viability of reuse therefore depends strongly on local conditions, material type and available infrastructure, and cannot be assumed from research-scale results alone.

Assessing economic sustainability remains challenging. Economic indicators are often fragmented, context-specific and weakly integrated into sustainability assessments. While life cycle approaches increasingly address environmental impacts, the economic pillar is frequently reduced to product price or omitted altogether (Ruett et al., 2024). Robust, peer-reviewed frameworks that systematically connect manufacturing costs, crop performance (Vinci and Rapa, 2019) and end-of-life options are scarce, despite repeated calls for integrated evaluation. As a result, economic claims around alternative growing media are often based on short-term trials, optimistic assumptions of scale-up, or non-transparent industry data, rather than reproducible scientific evidence.

A pragmatic way forward is to operationalise economic sustainability through a limited but structured set of indicators spanning the three life-cycle phases, including manufacturing costs, raw material availability, affordability, scalability and quality consistency, crop performance and input efficiency during use, and the costs and benefits of reuse or refurbishment (see Figure 1). Strengthening the scientific basis of these indicators is essential, as economic viability remains a primary driver of adoption and a prerequisite for achieving environmental and social sustainability in horticultural soilless systems.

3.3 Social sustainability

Without explicit attention to health, safety, social impact, and functionality risk distribution, environmentally motivated raw material substitution in growing media may shift social burdens and risks (such as impacts on worker safety, labour conditions, or local communities) rather than reducing them. Yet social sustainability remains the least specified pillar for growing media assessments and the most underdeveloped. It encompasses labour conditions and community impacts in constituent production, occupational health and safety during manufacturing and use, and transparency and traceability across often complex global supply chains (Figure 1).

Public and occupational health have increasingly become prominent as more recycled and side-stream organic materials are incorporated into growing media. Under certain conditions, for example where poorly composted materials are used, these inputs may introduce pathogenic microorganisms and other microbiological hazards or exposure risks (Alsanius et al., 2025). While such risks do not undermine the value of circularity, they highlight the need for explicit risk analysis, control measures, and good processing techniques. Clear communication of risks and material quality from producers and quality assurance bodies to growers, regulators and other relevant stakeholders is also essential. Environmental indicators alone cannot capture these dimensions. Social sustainability therefore involves not only aggregate benefits but also how risks and burdens are distributed across workers, growers, consumers and communities.

Social impacts in growing media can be measured using social certification standards or social life cycle assessment (S-LCA) frameworks. S-LCA combines global databases, sector/country indicators, and company-level data. These approaches follow international guidelines developed by UNEP and SETAC and consider multiple stakeholder groups (workers, local communities, consumers, and value chain actors) across impact categories such as human rights, labour rights, occupational health and safety, governance, community infrastructure, and economic development. While past growing media LCAs included a human health impact category (Paoli et al., 2022; Quantis, 2012; Vinci and Rapa, 2019), these are often insufficient and few provided a social assessment (Toboso-Chavero et al., 2021), underscoring the need to integrate S-LCA with environmental and economic assessments.

Within S-LCA, social risks are often quantified using indicators such as weighted labour “risk hours” derived from commercial databases (e.g., the Social Hotspots Database (Norris et al., 2013), aggregated across country, sector and company levels. Company-level assessments rely on primary data (e.g., questionnaires) covering health and safety, gender equality, child labour, wages, working hours, and community engagement, but data availability and report quality remain limiting factors.

Overall, social sustainability in growing media requires moving beyond absent or partial indicators towards integrated, life-cycle-based assessments that address social conditions, health risks and governance throughout the life cycle. Evidence-based evaluation depends not only on cumulative performance metrics but also on understanding who benefits, who bears risk, and how social impacts are managed and communicated across the supply chain.

3.4 Trade-offs and system dependency

The three-pillar framework presents sustainability as a balanced outcome but offers little guidance for resolving trade-offs when balance cannot be achieved. For growing media, these trade-offs are unavoidable and strongly system dependent. Materials are selected not on abstract sustainability attributes, but on their ability to meet crop-specific functional requirements under practical, economic and risk constraints. As a result, environmental, economic and social performance cannot be assessed independently of production context (Barrett et al., 2016; Carlile et al., 2015; Gruda, 2019).

Evidence from both research and practice shows that some materials promoted as sustainable often face limitations related to feedstock variability, processing requirements, logistics, cost and biosecurity risk (Litterick et al., 2025). When these factors are included, environmental rankings may change direction, and improvements in one sustainability dimension can coincide with increased burdens in others (Toboso-Chavero et al., 2021). We therefore advocate that sustainability in growing media should rather be understood as a conditional, system-level outcome than a fixed material property, and as such require explicit acknowledgement of trade-offs, uncertainty and context-specific constraints rather than binary classifications.

4 Discussion

The difficulty of defining “sustainable growing media” lies less in the absence of suitable materials than in the tendency to treat sustainability as an intrinsic attribute rather than a conditional outcome. We have shown sustainability claims in growing media routinely collapse complex, system-dependent considerations into simplified labels that are weakly specified and rarely supported by evidence relevant to production practice. This has allowed the term to persist as an aspirational descriptor while masking unresolved trade-offs, and shifting, rather than reducing, environmental, economic, or social burdens.

Rather than proposing a new definition, or abandoning the concept altogether, we recommend a set of minimum conditions for the responsible use of the term (Box 1). These conditions are intentionally modest. They do not require comprehensive assessment in all cases, but they do require transparency, explicit scope, and proportional evidence. Sustainability claims should specify the production context to which they apply, identify which dimensions are being addressed, and avoid implying benefits beyond the evidence presented. Framing sustainability as comparative and conditional (e.g. “more environmentally sustainable based on measured climate impact”), rather than absolute (i.e. “sustainable”, “not sustainable”), offers a more scientifically defensible and practically useful basis for decision-making.

Recommendations for the use of the term “sustainable growing media”. Based on the analysis in our paper, we make the following recommendations to define minimum conditions for its meaningful and responsible use in research, policy, and industry contexts.

1.Consider all three pillars of sustainability explicitly

Sustainability claims should address environmental, economic, and social dimensions. Improvement in one pillar must not be assumed to imply improvement in others. Where one or more pillars are not assessed, this limitation should be stated clearly.

2.Specify which sustainability pillar(s) are being addressed

When full three-pillar assessment is not feasible, claims should be explicitly scoped (e.g. “reduced climate impact under stated assumptions” or “verified responsible sourcing”). Broad, unqualified sustainability claims should be avoided.

3.Frame sustainability at the system level, not the material level

Growing media sustainability should be evaluated within a defined production system, including crop type, production scale, management practices, and supply chains. Sustainability is a context-dependent outcome, not an intrinsic material property. This is especially true for growing media, which are only one part of horticultural production.

4.Measure impacts rather than infer them from material categories

Assumptions based on labels such as “peat-free”, “renewable”, “recycled”, or “biogenic” are insufficient and misleading. Sustainability claims should, wherever possible, be supported by measured indicators (e.g. environmental Life Cycle Assessment (LCA), Life Cycle Cost (LCC), Social Life Cycle Assessment (S-LCA), performance trials) with transparent methods and stated limitations.

5.Demonstrate functional and economic performance for the intended use

Growing media described as sustainable should demonstrate reliable horticultural performance, economic feasibility, and supply continuity for the system in which they are promoted. Materials that disproportionately increase production risk or require compensatory inputs cannot be assumed to reduce overall sustainability.

6.Make trade-offs and uncertainties explicit

Trade-offs within and between sustainability pillars are unavoidable and should be acknowledged rather than obscured. Where results depend strongly on assumptions, data quality, or system boundaries, this dependency must be made transparent.

7.Avoid absolute or binary sustainability claims

Terms such as “sustainable”, “unsustainable”, or “fully sustainable” should not be used without qualification. Sustainability should be framed as comparative, directional, and conditional, not absolute.

8.Use precise, verifiable descriptors where evidence is limited

Where comprehensive sustainability assessment is not available, it is preferable to use specific and verifiable descriptors (e.g. “responsibly sourced”, “quantified carbon footprint under stated assumptions”, “demonstrated reuse potential”) rather than the blanket term “sustainable”.

Our recommendations have practical implications across research, industry, and policy. For research, they reinforce the need to link environmental assessments to functional performance and feasibility, and to report assumptions, boundaries, and uncertainty with sufficient clarity to enable interpretation and comparison. For industry, they clarify that material substitution alone does not justify sustainability claims unless reliable performance and supply can also be demonstrated for the intended system. In such cases, narrower descriptors (such as responsible sourcing, quantified environmental indicators, or demonstrated reuse potential) provide more accurate and verifiable communication. For policy, the findings help explain why proxy-based targets can support transition objectives but cannot be conflated with sustainability outcomes at the growing media or production system level.

The three-pillar framework specifically for growing media (Figure 1) which we have presented, also has clear limits. Sustainability trade-offs cannot be eliminated, and no single model can resolve how competing objectives should be weighted across all crops and systems. Life-cycle-based tools remain sensitive to methodological choices, while social and economic indicators are unevenly developed. While we did not try to resolve these challenges here, we have hopefully made them more visible and discourage claims that imply a level of certainty that current evidence cannot support.

Future progress will depend less on redefining sustainability and more on improving how evidence is generated and communicated. At present, no simple or cost-effective set of criteria exists that can reliably support sustainability claims across the diversity of growing media and production systems. Addressing this gap requires further methodological and empirical work. Priority areas include integrating functional performance with environmental and economic assessment, improving transparency around processing and supply-chain impacts, and developing approaches that better capture risk, uncertainty, and distributional effects.

We present this contribution as a call for more systematic and coordinated research, rather than as a finalised assessment framework. Sustainability in growing media should be treated as an evolving, system-level outcome that must be demonstrated within a defined production system rather than a fixed label. By applying the minimum conditions outlined here and using precise terminology where evidence is limited, the growing media literature and sector can move toward clearer, unbiased, more credible, and more useful sustainability claims.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Footnotes

Edited by: Chris Blok, Wageningen University and Research, Netherlands

Reviewed by: Ep Heuvelink, Wageningen University and Research, Netherlands

William (BILL) Fonteno, North Carolina State University, United States

Footnotes

https://responsiblesourcing.org.uk/key-documents/

https://convenantpotgrond.nl/over-het-convenant/

https://www.responsiblyproducedpeat.org/en/what-we-do

https://www.rhp.nl/en/responsibly-produced-coir

https://www.bmleh.de/SharedDocs/Downloads/EN/Publications/peat-use-reduction-strategy.html

https://www.bafu.admin.ch/en/peat

https://www.horticert.org/