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Introduction to Growing Media

Growing medium is a material in which plants are grown
Gerald Schmilewski
Coordinator of the Growing Media Sector Group

A short introduction

Often also referred to as “substrate” or “potting soil”, a growing medium is a material, other than soil on the spot, in which plants are grown.

As an important supplier to the modern horticultural industry, the peat and growing media sector is an important contributor to its sustainability. It represents an industry with a €1.3 billion turnover accounting for 11,000 jobs across Europe and is essential to the horticulture industry which is estimated to have a turnover of approx. €60 billion and provides for over 750,000 jobs.

Growing media are used by the horticulture industry as well as consumers to support the development of plants. The growing medium ensures that the plant can healthily grow by providing it with a range of essential elements:

  • an optimum rooting environment for physical stability
  • storage of air for the roots
  • water absorption and retention --> availability to the plant when needed
  • supply of nutrients for the roots

Growing media are used to grow a wide variety of plants including vegetables, fruits, floriculture ornamentals, tree and shrub ornamentals and speciality plants.

The range of growing media constituents used includes peat, coir pith, woodfibers, bark, composted materials i.e. green waste, and bark. Mineral constituents like perlite, pumice, clay and vermiculite are also used. Growing media are often formulated from a blend of such raw materials, usually enriched with fertilizers, lime and sometimes biological additives in order to achieve the correct balance of physical, chemical and biological properties for the plants to be grown. Having the right growing media mix is as important for an optimal plant growth as water and fertilisers.

Plant species differ considerably in their need for water and nutrients, and therefore need different kinds of growing media to provide the best growing conditions. Due to this, a wide range of different kinds of growing media are available on the market. The horticulture industry uses thousands of different growing media mixes.

In this context, the growing media industry chooses different materials based on their performance, the plants’ needs, availability and sustainable development considerations.

For example:

  • for young trees and shrubs a substrate with wood fibres or bark is often used as it provides more physical stability
  • clay is frequently used for plants with high water needs as it can store the water longer
  • perlite is a material used to enhance the drainage of water

In sum, the right growing medium provides growing conditions that are predictable and reliable for the grower, contributing to higher yields and more efficient growing.

Growing Media constituents

The products supplied by EPAGMA members contain the complete range of available growing media constituents. Some of the most important ones include:

Bark: The tough protective outer sheath of the trunk, branches, and twigs of a tree or woody shrub. Bark is used as the sole constituent in orchid cultivation or as a constituent in potting mixes for tree nurseries and floriculture. Only certain barks are suited as growing media constituents. Bark is also used as a mulching material.

Clay: This material is often added in the form of dried granules or as a powder. Clay has a high ability to bind water as well some nutrients. It therefore influences the water characteristics of the growing medium. It can also partly act as a nutrient buffer, making it possible to add more fertilizer without reaching to high salinity levels.

Coir pith: Coir is obtained by mechanical processing of the husk of coconuts. It is primarily imported from the Far East (Sri-Lanka, India, Philippines). This material has good wettability characteristics and is often mixed with other constituents in mixes for sowing, propagating and potting. Sometimes also used as the sole constituent of grow bag mixes in vegetable and flower cultures.

Green Compost: A material produced from organic waste materials such as tree branches, leaves, grass clippings and plant residues. These residual materials are decomposed by microorganisms under controlled conditions. Plants do not grow in 100 % compost, and the material must be diluted with e.g. peat. Good quality composts for the growing media industry are becoming rare in some EU member states due to the increasing use of woody input material for energy production instead of composting.

Black Peat: Peat is formed when plants such as peatmosses are submerged in water and only partly decomposed due to a lack of oxygen. In some areas peat accumulated over the years in small lakes, growing from the bottom to the top. The lower layers of peat are called “black peat”. They are the oldest and most decomposed, characterised by hardly to non-recognizable plant structures and a dark brown to almost black colour. This peat is used in all horticultural segments and is the second most important constituent of growing media throughout Europe.

White peat: This peat is weakly to moderately decomposed and taken from the upper and younger layers of a peatland. It has visible plant structure and a yellowish brown to dark brown colour. White peat is used in all horticultural segments and is the main constituent of growing media throughout Europe.

Perlite: A material that is manufactured from naturally occurring hydrated volcanic rock (perlite), expanded by heat to form a cellular structure. Usually mixed into growing media in order to improve the flow-ability of a growing media mix, increase the air content and improve the water uptake.

Rice hulls: Are the hard protecting coverings of grains of rice and are obtained in the rice manufacturing industry. Rice hulls can be added to mixes to improve air capacity. It is a constituent of lower importance.

Sand and Grit: Are used in growing media to improve the flowability of the mix as well as to add weight where needed. These materials can also improve the water movement in the growing medium to some extent.

Wood fibres: Fibres that have been obtained by mechanically or mechanically-thermally fraying of un-treated wood and/or wood wastes. Wood fibres are used in mixes for pot plants, trees, shrubs, etc. and used in combination with peat and other constituents.

The role of peat in growing media

Today, peat remains the main constituent for many growing media mixes as no other material combines as many favourable characteristics as this material. It is favoured due to its high water-holding ability and good aeration. As the pH and the nutrient content of peat are low, almost any kind of growing medium can be produced with the addition of liming material and fertilisers. This allows the production of a growing media with the exact pH needed for the relevant plant. In addition, peat is free of human and plant pathogens. Peat is widely available and can fulfil consumer demand. Due to these reasons, peat is recognised as an essential constituent by a wide range of national environmental and organic labels as well as under the EU’s organic farming framework.

The use of peatlands for horticultural peat production represents only a small fraction of the human use of peatlands. In this regard, the International Peat Society estimates that around 14% of peatlands are affected by human use. Only 0.05% of peatlands are used for horticultural peat production.

The industry acknowledges that the concerns around peat stem from allegations against its extraction and after-use of peatlands. However, EPAGMA members are aware of the fact that site selection, peat extraction and after-use can be undertaken in a responsible manner. Such responsible sourcing will help to ensure the protection of peatlands designated for conservation as well as provide for rehabilitation and restoration of peatlands that were degraded well before peat extraction by the industry.

EPAGMA member companies have committed themselves to responsible peatland management in the EPAGMA Code of Practice and EPAGMA is supporting the current development of a certification system for responsibly produced peat. Please read more about EPAGMA’s work on sustainability here.  

EPAGMA members agree that, when alternative materials that are economically viable, available and present the same beneficial growing conditions; these can be used to complement the essential role of peat in GM. For more than 30 years the industry has invested in and driven a wide range of research into alternatives on peat and will continue to do so.