Sustainable growing media: look beyond carbon footprints

Many horticultural crops are grown in substrates, or growing media, such as peat, coir, wood fibre and mineral wool. Worldwide, around 116 million cubic metres of growing media were used in 2022. Demand could be 2.5 to 2.9 times higher by 2050. This makes the choice of raw materials increasingly important for reducing horticulture’s environmental impact.

Peat is under growing pressure because of its high carbon footprint and the damage to natural ecosystems caused by peat extraction. Renewable materials such as coir and compost are often proposed as alternatives. But how sustainable are they in practice? “It is easy to fall into the trap of one-to-one substitution,” says researcher Jeroen Weststrate. “A material with a poor reputation is replaced by one that appears greener. But our research shows that substitution does not automatically result in greater sustainability.”

Coir is one example. At first glance, its climate impact is considerably lower than that of peat. However, the environmental impact of coir is not determined by the material alone. Coconut cultivation, processing and transport to Europe also contribute to impacts such as acidification, nutrient pollution and air pollution.

The same pattern can be seen in complete growing media mixes. In four of the seven application groups studied, the ranking of mixes changed when environmental impacts other than climate change were included. For fruit vegetable production, for example, one mix containing 50% peat had the highest climate impact within its group, but the lowest overall environmental score. Some mixes with a high proportion of coir had a lower climate impact, but a higher total environmental burden.

In the study, Weststrate and Elizabeth Migoni Alejandre of Wageningen University & Research, working in collaboration with Growing Media Europe, brought together up-to-date industry data, scientific literature and existing databases for the very first time. Using Life Cycle Assessment, or LCA, they calculated the environmental impacts of twelve frequently used raw materials and thirty growing media mixes used for among others fruit vegetables, soft fruit, pot plants and ornamentals, tree nurseries and the hobby market. “Until now, data on the environmental impacts of growing media have often been scattered across different studies and based on different methods,” says Weststrate. “That made it difficult to compare raw materials and mixes on an equal basis. Our dataset changes that.”

The new dataset can help companies take the environmental impacts of raw materials and mixes into account at an early stage of product development. For policymakers, the analysis provides a basis for looking beyond simple categories such as ‘fossil’ and ‘renewable’. “This could become a shared knowledge base for further innovation,” Weststrate says.

Ultimately, how a mix performs in the greenhouse also matters. “A mix that scores well on paper is not necessarily the best option in practice,” says Weststrate. “If it requires more water or fertiliser, for example, or has a shorter lifespan, the overall environmental benefits may be smaller.” This will be the next step in the research. “Only then can we determine which solution actually delivers the best overall environmental performance.”