Bark: How it Contributes to Substrate’s Characteristics

Bark as an Alternative in Peat-Reduced Substrates 

Growing media are increasingly incorporating organic components other than peat, as explained in our article exploring why professional plant production is moving away from peat. 

Among these alternatives, bark has become one of the most widely used materials. In this article, we examine its technical characteristics, explain the key evaluation criteria, and highlight the main considerations associated with its use. 

What Is Bark for Ornamental Horticulture and What Are Its Characteristics?

Bark used in growing media for ornamental plant production is a by-product of the timber industry. As a natural material, it exhibits a degree of variability that growers must consider when selecting or formulating a substrate. 

Several factors influence bark quality and performance. These are discussed in detail in ICL’s technical guide to bark properties, stabilization, and agronomic use. Key factors include: 

• Botanical origin: the tree species and the maturity of the plant determine the bark’s chemical profile and the ratio of outer corky bark (rhytidome) to inner living phloem. Mature trees generally produce safer, thicker outer bark with a higher suberin content, lower phytotoxicity and greater structural stability. Bark ageing and/or composting help break down hydrophobic compounds and soluble sugars, reducing the risk of nitrogen drawdown caused by microbial immobilization while ensuring stable air-filled porosity and water-holding capacity within the substrate. 
• Industrial processing: the final characteristics of bark are strongly influenced by post-debarking processing operations, including: 
  • size reduction; 
  • preliminary screening; 
  • composting; 
  • final particle-size grading. 
  • These processes generate fine (0 to 5 mm), medium (8 to 15 mm), and coarse (10 to 20 mm) fractions. Particle size has a significant influence on properties such as water retention and air content. 
• Degree of biological stabilization: biological stabilization, achieved through composting or controlled stockpile ageing, is essential for reducing tannins, terpenes, and the potential phytotoxicity associated with manganese naturally present in raw bark. These compounds exhibit dose-dependent phytotoxicity: at high concentrations they can inhibit root elongation, seed germination, and beneficial rhizosphere microflora. Stabilization reduces their concentration below phytotoxic thresholds, helping ensure predictable nutrient uptake and healthy root development. 
• Physico-chemical properties: key parameters include: 
  • total porosity; 
  • air-filled porosity (AFP); 
  • pH; 
  • electrical conductivity (EC); 
  • cation exchange capacity (CEC). 
  • Further information on the importance of EC and salt measurements can be found in this article. 

Composted Bark vs Fresh Bark 

Unless otherwise specified, the term bark generally refers to composted or adequately matured bark. Fresh bark is rarely used in professional growing media because of its high biological activity and limited stability. Composting or controlled maturation reduces microbial activity, stabilizes organic matter, decreases the concentration of readily degradable compounds, and produces a more uniform material suitable for horticultural applications. For a more in-depth discussion of pH stability in peat-free and peat-reduced substrates, read our related article. 

Softwood Bark vs Hardwood Bark 

The terms softwood and hardwood do not refer to the hardness of the wood itself, but rather to the botanical classification of the tree species from which the bark is derived. 

Most bark used in growing media originates from coniferous species (softwoods). Softwood bark is generally preferred because it offers: 

• high lignin content; 
• excellent structural stability; 
• good porosity; 
• low soluble salt levels; 
• predictable performance over time. 

Commonly used species include Pinus taeda, Pinus radiata, Pinus sylvestris, Pinus pinaster, Pinus nigra, Pseudotsuga menziesii, Picea abies, and Larix decidua. For an overview of softwood bark species and their ageing methods click here.

Hardwood bark, obtained from genera such as Quercus, Castanea, Fagus, Eucalyptus, and Populus, can also be used in growing media following appropriate processing. However, it generally: 

• decomposes more rapidly; 
• exhibits greater variability in quality; 
• may contain relatively high concentrations of potentially phytotoxic compounds, including tannins and phenolic substances. 

How Does Bark Contribute to the Substrate?

Within growing media, bark primarily provides structural support. Its incorporation:

• increases substrate porosity,
• improves oxygen diffusion throughout the root zone,
• and contributes to maintaining the balance between macro- and micro-porosity over time (mechanical stability).

This helps preserve the structural integrity of the growing medium and reduces the risk of substrate collapse during crop production. 

The overall effect largely depends on particle size. Coarser particles improve aeration, while finer particles increase water-holding capacity. 

From a chemical perspective, composted bark generally contains a high proportion of stable organic matter and has a lower carbon-to-nitrogen (C:N) ratio than fresh bark. Its cation exchange capacity (CEC) is typically lower than that of peat and, consequently, its buffering capacity is also more limited. 

These properties may vary according to tree species, degree of composting, and processing technology. 

Which Aspects Ornamental Growers Need to Consider When Using Bark 

The quality of bark depends not only on tree species but also on the entire production process. Two products marketed simply as pine bark may display substantially different characteristics depending on

• the origin of the raw material,
• the processing method,
• and the particle-size distribution. 

For professional growers and growing media manufacturers, understanding these technical characteristics is the first step towards selecting the most suitable raw material. The quality of the final growing medium depends not only on the substrate formulation, but also on the consistency and quality of each individual component. 

For a deeper understanding of this topic, we have prepared atechnical guide to bark properties, stabilization processes and agronomic considerations. 

Conclusions

Bark has become an important component of many peat-reduced growing media, offering valuable structural and physical benefits when correctly selected and processed. Understanding its origin, degree of stabilization, particle-size distribution, and physio-chemical characteristics can help growers make informed substrate decisions and optimize crop performance. 

For personalized guidance, contact your regional technical support team through your country’s website or by completing the form accessible via the banner at the bottom of this page.