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Viable alternatives to stone wool and perlite for tomatoes and cucumbers

In recent years, the search for sustainable alternatives to traditional growing media such as peat, coconut fiber, and stone wool has led to the testing of wood fiber as a growing medium or part of a growing medium. Peat, coconut fiber, and stone wool have environmental challenges, and it is desirable to reduce their use in the horticultural sector. This mix has proven to be a beneficial and versatile growing medium compared to peat and coconut fiber. In more controlled greenhouse conditions, where stone wool is traditionally used as a growing medium, it is feasible to use 100% wood fiber, such as Fibergrow® wood fiber from Hunton, which was recently tested in trials of a Norwegion insitute. This fiber is manufactured by a process that opens the fibers and gives it hydrophilic properties and is based specifically on the confiner species Norway spruce or European spruce (Picea abies). Using this fiber significantly promotes the use of renewable resources, as it is based on residual raw materials from the forest industry in Norway. Unlike stone wool , which can generate a lot of waste when used as a growing medium for annual crops in greenhouses, wood fiber material can be burned or more easily recycled after use.

Practical advantages of Fibergrow as a growing medium in hydroponic systems
The practical advantage of Fibergrow as a growing medium in hydroponic systems is its excellent water retention capacity compared to stone wool. "Our trials have shown that if the irrigation strategy is adjusted for wood fiber, it can achieve equally good and often better plant growth and yield. The irrigation frequency should be slightly reduced since wood fiber retains water better and thus requires fewer waterings to reach the runoff point. Furthermore, it is necessary to stop watering, for example, 30 minutes earlier at the end of the day to prevent oversaturation and ensure that the roots get enough oxygen at night. This helps maintain an optimal moisture balance in the growing medium. Adjusting the irrigation strategy based on weight instead of moisture sensors has proven more effective for wood fiber, ensuring more even moisture distribution and better adaptation to the plants' needs."

Use of fine and coarse wood fiber for tomatoes
Two types of Fibergrow – coarse (133 grams/liter) and fine (83 grams/liter) – were tested in tomatoes with stone wool as a control at NIBIO Særheim in 2022. The growing media were manually filled into bags similar to commercial horticultural stone wool slabs. The trials followed the same treatment as for stone wool, including 24 hours of full water saturation for drainage before planting. Root growth was observed 14 days after planting, where stone wool showed better root distribution throughout the growing medium compared to Fibergrow (Figure 1). However, yield results showed that Fibergrow, both fine and coarse, produced slightly higher yields (~5%) compared to stone wool (Figure 2). The conclusion from this trial is that wood fiber is at least as suitable as stone wool for cultivation, despite poorer root distribution, especially if irrigation is adjusted.

Figure 1. Root distribution in fine wood fiber (a), coarse wood fiber (b), and stone wool (c). Photo: Henk Maessen.

Figure 2. Yield in kg per m² of first-class (class 1) tomatoes grown in fine and coarse Fibergrow wood fiber and stone wool at NIBIO Særheim.

Commercial trials with Fibergrow
Trials with tomatoes grown in Fibergrow (coarse type) were also conducted with a commercial grower in 2022 and compared to cultivation in perlite. A challenge with Fibergrow was that the plants were poorly anchored in the wood fiber mass at the start, causing them to topple over before the roots were established. This required securing the plants with tape over the plants and the trough (Figure 3). After 4-5 days, the roots anchored, and there was no difference in plant care thereafter. Stone wool has the advantage of good support properties that do not require additional support, so a small challenge with loose Fibergrow is finding a simple solution to support the plants during the first week after planting. Otherwise, Fibergrow showed good results in the trial. Growth parameters indicated more even and slightly faster growth, with more tomatoes per square meter after the first five weeks in wood fiber (Figure 4). The 2022 season was characterized by challenges for the plants in absorbing magnesium due to low light and heavy fruit load, but little to no magnesium deficiency was observed in plants grown in wood fiber compared to perlite. This is related to better absorption of magnesium and other divalent ions when there is consistently good moisture, attributable to the better water retention capacity of Fibergrow.

Figure 3. Tape was used to secure the plants and prevent toppling at the start. Photo: Ove Sunde.

Figure 4. Accumulated number of tomatoes per square meter (m²) from plants grown in stone wool (blue line) and Fibergrow (orange line) with a commercial tomato grower in 2022

Introduction of Fibergrow slabs
In 2023, Hunton introduced a wood fiber-based Fibergrow slab with a density of 50 grams/liter. This slab was tested both at NIBIO Særheim and with several growers. The trials aimed to adjust the irrigation strategy for the best possible yield by adjusting based on the weight of the slabs. Weight was used because moisture sensors did not work well in the wood fiber, probably due to very high moisture levels that did not register. Good root development was observed in this slab compared to stone wool (Figure 5). Yield results from the trials with Fibergrow slabs at NIBIO Særheim showed an average yield increase of 6.9% compared to stone wool after 17 weeks of harvesting (Figure 6). There were differences in response among various tomato varieties, with three out of four varieties showing higher yields in Fibergrow slabs. No difference in fruit quality, such as blossom-end rot, was observed, indicating good conditions for roots and nutrient uptake.

Figure 5. Root network in Fibergrow slab (a) and stone wool slab (b) after tomato cultivation. Photo: Henk Maessen.

Figure 6. Increase in yield (%) for four tomato varieties grown in Fibergrow wood fiber slabs compared to stone wool .

In Fibergrow slabs, the plants had slightly better access to water, which led to a slightly increased overall growth and yield, especially in response to increased sunlight and LED lighting. This indicates that Fibergrow slabs are more effective than traditional growing media like stone wool in utilizing water and light, leading to higher yields under good lighting conditions.

Final recommendations
With these results and experiences, we conclude that wood fiber is a viable alternative to stone wool and perlite as a growing medium for tomatoes and cucumbers. The use of wood fiber can reduce the environmental impact by reducing the energy-intensive production and waste associated with the use of stone wool and perlite, while the beneficial physical properties of Fibergrow can provide slightly better plant growth under good lighting conditions in hydroponic systems. We thus recommend that tomato and cucumber growers can phase out stone wool slabs and adopt Fibergrow, which our study indicates at least as good a yield.

Specific irrigation strategy recommendations for Fibergrow slabs:

  • Saturate the substrate with a nutrient solution for 24 hours before planting.
  • Provide short irrigation intervals 1-2 days after planting to ensure the roots grow into the wood fiber slab.
  • After root anchoring, follow the same irrigation strategy as for stone wool with 25-30% runoff.
  • With wood fiber, you reach the saturation point faster. In practice, this means one less irrigation from the start of irrigation to the saturation point. Therefore, irrigate one time per less day than with stone wool to reach the runoff point.
  • To prevent excessive runoff and oversaturation, stop irrigation e.g. 30 minutes earlier at the end of the day with the use of Fibergrow compared to stone wool.
  • Wood fiber becomes oversaturated with water more easily on days with low radiation. On days with little light (without supplemental lighting), limit irrigation further to ensure sufficiently low moisture levels and enough oxygen availability to the roots at night.
  • Ensure that the slabs do not dry out too quickly, as wood fiber reaches excessively low moisture levels faster than stone wool.


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