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Copy link Overview of Softwood Bark Species and Stabilization Pre-Treatments
A useful overview of the global distribution of softwood bark species and the processing requirements that make them suitable for growing media formulation.
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Besides the guide to bark’s properties, stabilization and agronomic use, and the explanation of how bark contributes to substrate’s characteristics, this article focuses specifically on the stabilization processes required before bark can be used in growing media and provides an overview of the most commonly used softwood bark species.
Bark: Why Composting or Aging is Required
Fresh bark presents several challenges that limit its direct use in growing media. In its raw state, bark:
- contains phytotoxic organic compounds (such as monoterpenes, tannins, and phenolic compounds)
- exhibits high hydrophobicity (due to natural waxes)
- has a very high Carbon-to-Nitrogen ratio (C:N> 300:1).
If fresh bark is used without appropriate pre-treatment and nutrition management, the following issues may occur:
- phytotoxicity: volatile terpenes and soluble phenolic compounds can inhibit root development, particularly in young plants.
- nitrogen immobilization: microorganisms rapidly decompose cellulose and wood residues, temporarily tying up nitrogen and reducing its availability to the crop.
- hydrophobicity: raw bark tends to repel water, making initial wetting and re-wetting more difficult.
Bark Aging: Pre-treatment Methods
To be suitable for ornamental plant production, bark should generally undergo stabilization before being incorporated into growing media. Stabilization initiates a range of biological and chemical transformations that improve the material’s physical and agronomic characteristics.
- Aging (stacking): Bark is stored in windrows for periods ranging from 3 to 12 months. During this time, rainfall and natural weathering help leach phytotoxins and break down surface waxes.
- Controlled composting: Bark is hammer-milled, moistened, and combined with nitrogen sources such as urea or ammonium nitrate to undergo thermophilic fermentation at 50–65°C for 1 to 6 months. This process stabilizes the C:N ratio, destroys weed seeds and pathogens, balances pH, and improves water-holding capacity.
Different Kinds of Bark and their Composting / Aging Requirements
Region Europe
| Tree Scientific Name | Tree Common Name | Prefered Local Sub-regions - Zones | Composting / Aging Requirements |
|---|---|---|---|
| Pinus pinaster | Maritime Pine | Southern and South-Western Europe (Iberian Peninsula, Atlantic France, Italy) | Aged or Composted - 3–6 months for substrates; - screened and raw/aged for coarse orchid fractions. |
| Pinus sylvestris | Scots Pine | Central, Northern and Eastern Europe (Germany, Poland, Scandinavia) | Mandatory Composting (high cellulose requires stabilization against N tie-down). |
| Pinus nigra | Austrian / Black Pine | Southern and Central Europe | Aged or Composted prior to blending with peat or wood fibre. |
| Picea abies | Norway Sprouce | Northern Europe and Alpine regions | Strict Composting Required (decomposes rapidly if raw). |
Region North America
| Tree Scientific Name | Tree Common Name | Prefered Local Sub-regions - Zones | Composting / Aging Requirements |
|---|---|---|---|
| Pinus taeda & Pinus elliottii | Loblolly Pine & Slash Pine (Southern Pines) | Southeastern USA (NC, GA, FL, AL, MS, TX) | Aged or Composted - 3–6 months with N addition; - Standard for container nurseries. |
| Pseudotsuga menziesii | Douglas Fir | Pacific Northwest (OR, WA, Northern CA, BC Canada) | Fresh or Aged (6–12 months) Lower phytotoxicity than pine; widely used both fresh-screened and aged. |
| Pinus strobus | Eastern White Pine | Northeastern USA, Great Lakes, Eastern Canada | Composted / Aged before inclusion in container mixes. |
Region South America
| Tree Scientific Name | Tree Common Name | Prefered Local Sub-regions - Zones | Composting / Aging Requirements |
|---|---|---|---|
| Pinus taeda & Pinus elliottii | Loblolly Pine & Slash Pine | Southern Brazil (PR, SC, RS) Northeast Argentina (Misiones, Corrientes) | Thoroughly Composted (4–6 months with N supplementation) to break down resinous compounds. |
| Pinus radiata | Radiata Pine / Monterey Pine | Central & Southern Chile (Biobío, Araucanía | Aged or composted for general substrates; Screened and heat-treated for export orchid media. |
Region China – East Asia
| Tree Scientific Name | Tree Common Name | Prefered Local Sub-regions - Zones | Composting / Aging Requirements |
|---|---|---|---|
| Pinus massoniana | Masson's Pine | Central, Southern and South-Western China (Guangdong, Guangxi, Fujian, Sichuan, Eastern Yunnan) | Mandatory Composting (3–6 months) due to high initial resin and tannin contents. |
| Pinus tabuliformis | Chinese Red Pine | Northern China (Hebei, Shandong, Shanxi, Shaanxi) | Aged or Composted prior to hammermilling. |
| Pinus elliottii | Slash Pine | Southern China plantations | Composted / Aged following standard Southern Pine protocols. |
Region Australia – New Zealand
| Tree Scientific Name | Tree Common Name | Prefered Local Sub-regions - Zones | Composting / Aging Requirements |
|---|---|---|---|
| Pinus radiata | Radiata Pine / Monterey Pine | New Zealand (North & South Island) South Australia (VIC, SA, NSW, TAS) | Aged or Thermophilically composted Premium fractions undergo specialized aging and dolomite treatment. |
Conclusion
Considered the wide number of variables, personalized guidance might be needed. Our regional technical support is available: visit your country’s website to find who’s the closest ICL’s expert to you or fill in the form available by clicking on the banner at the bottom of this page
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