A closer look at growing media use and requirements

Plants with red and pink flowers and green leaves in a greenhouse.

Published June 20, 2023

Growing media consists of components that provide water, air, nutrients and support to plants in all stages of production. In turn, growing media plays a key role in the health and marketability of greenhouse crops from roots to tips. Choosing the best growing media mix for your crops requires research and trials, as well as communication with both your growing team and suppliers.

Editor’s note: We surveyed North American greenhouse growers in May 2023 and received 153 qualified responses. Not all answers add up to 100% due to rounding.

Potential risks of recycling growing media

Recycled growing media can have some detrimental properties that should be considered and monitored. First is the potential loss of physical properties of reused media. Over time, organic particles naturally break down (decompose), thereby decreasing structure and air porosity. If materials are ground up or otherwise handled during sterilization processes, the particle size will continue to reduce, therefore changing the physical properties when repotted. Blending in some percentage of new growing media with the old/reused would likely help with this problem.

Secondly, reused growing media can house potentially harmful pathogens, pests and other unwanted goodies if not properly sterilized or treated. The infection or exposure that improperly recycled materials can have on a grower operation can be very detrimental to future crops and cropping cycles. Recycled materials left outdoors can easily be contaminated with weed seeds and/or pathogens. Recycled materials not properly sterilized could contain Pythium, Fusarium, Phytophthora and other plant-sensitive diseases. The recent increase in legalized cannabis production has also exposed new biological concerns (many may be unfounded) including the presence of E. coli and Salmonella in growing media that are now being monitored and, in some cases, regulated for.

Lastly, there can be some toxicity effects from reused growing media. Mostly in the form of residual fertilizer salts that can create high EC in mixes when re-blended or reused in future crop productions. This can alter fertility practices and management as well as crop growth response and control. Desalinization via washing or flushing with hot water is needed to remove potential salts from materials like coco coir.

Source: Brian Jackson, director of the Horticultural Substrates Laboratory at North Carolina State University

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