Hemp hurd fiber: a sustainable substitute for sphagnum peat moss in greenhouse and nursery plant production

2023

Greenhouse Study 3

Preliminary results: The hurd product from Biocomposites had a higher pH of 7.0 then was expected. The Biocomposites product required washing prior to mixing treatment medium to remove debris. The product was also contaminated with hemp seed that sprouted in the first 1 to 4 days after potting. Hemp seedlings were easily removed by hand. Based on visual observations of the first petunia experiment it was determined that the particle sizes of the Biocomposites hurd (8 to 10 mm) and the medium grade vermiculite were too large, making the treatment medium too porous. For the second petunia experiment we used fine grade vermiculite (with smaller particle size) and a different hurd product by Hemp Traders with particle size of 2 mm, that more closely resembled the particle size and texture of sphagnum peat moss. The Hemp Traders product had a lower pH of 5.0 and was free of debris and seed. Petunias in all hurd treatment mediums grew better in the second experiment than the first experiment based on visual observations. Future experiments will utilize the Hemp Traders hurd product at 2mm particle size.

Figure 1. Peat and hurd substrates.

The tables below provide the physical properties of the four treatment mediums in experiment 1 (Table 1) that used ~10 mm sized hurd and experiment 2 (Table 2) that used ~2 mm sized hurd.

Table 1.

The figures below shows petunia plants grown in 1:1 peat: vermiculite; 0.66:0.33:1 hurd: peat: vermiculite; 0.33:0.66:1 hurd; and 1:1 hurd: vermiculite (left to right) for experiment 1 using ~10 mm hurd (top) and experiment 2 using ~2 mm hurd (bottom).

2024

Nursery Study 1

Preliminary results: The woody shrubs evaluated all grew very well in the hurd amended media and were visually indistinguishable from plants in the control medium.

study 1 woody shrubs

Greenhouse Study 3

Preliminary results: Petunia and Geranium were able to grow in substrates containing hurd as a replacement for peat. Petunia plants in media containing the low rate of hurd performed similarly and were visually indistinguishable from the control. Plants in the medium rate of hurd treatment had achieved similar plant width to the control. Geranium plants in the hurd treatments were larger than the control. Geranium plants in the low rate treatment were the most visually appealing. It is possible that geranium plants in the control medium did not grow well because the pH was too low or the medium remained too wet.

study 3 geranium

2025

Nursery Study 1

The experimental hurd replacement media had similar AFP, CC, TP, and BD as the control medium, and all values were within recommended ranges for nursery production container media (Table 1). Control medium had slightly lower pH than the experimental hurd media, however all media pH values were as expected for softwood bark-based media and suitable for container production of the plant species evaluated. Media EC and nutrient concentrations were within ranges considered acceptable to optimum for container media of landscape plants.

Plants of bayberry, forsythia and Siberian cypress grown in 50% and 100% hurd replacement medium were similar to control plants for all measured parameters and visually indistinguishable from control plants (Table 2). The only exception was forsythia plants grown in 50% hurd replacement medium, which had slightly lower shoot fresh weight than control plants for reasons that are unclear beyond random effects (Table 2). Bayberry plants in all media grew vigorously, filled out containers quickly, and were of marketable size and quality by the end of the growing season. Siberian cypress plants that were overwintered and allowed to grow out the following spring developed very well and tripled their shoot weight over the eight months from Sep 2024 data collection to the end of May 2025 (Table 2). After potting in experimental media, study plants were not pruned because we did not want to impact their natural grow out. If forsythia plants had been pruned in late spring or early summer, then they would have achieved a fuller habit, and developed the market size and quality typical of forsythia in #1 containers. During the 2024 growing season, EC was similar for all media and pH was slightly lower for control medium compared to the 50% and 100% hurd replacement media.

Table 1. Physical properties and saturated media extract analysis of substrates and 4 bark:2 peat:1 sand medium (control) and two experimental media with 50% or 100% of the peat portion of control media replaced with hurd.

ⁱ Mean separation within columns indicated by different letters, according to Tukey’s honestly significant difference (HSD) test at P ≤ 0.05 and n=4 for AFP, CC, TP and BD and n=3 for SME results.

Table 2. Performance of northern bayberry (Morella pensylvanica ‘Bobzam’ Bobbee^™), forsythia (Forsythia x intermedia), and Siberian cypress (Microbiota decussata) grown in 4 bark:2 peat:1 sand medium (control) and two experimental media with 50% or 100% of the peat portion of control media replaced with hurd.

ⁱ Mean separation within columns, within species, indicated by different letters, according to Tukey’s honestly significant difference (HSD) test at P ≤ 0.05 and n = 10. For Siberian cypress n = 5.

Nursery Study 2

Bee balm plants grown in control medium had higher shoot fresh weight than plants grown in hurd containing media, and greater width and number of inflorescences than plants grown in 50% hurd replacement medium (Table 1). These differences, however, were not evident visually. At time of harvest, bee balm plants from all media treatments had similar amounts of fully and partially open inflorescences and inflorescences in bud (Table 1). Switchgrass plants grown in all media had similar height, width and number of tillers and were visually indistinguishable (Table 1). Control switchgrass plants had greater shoot fresh weight than plants grown in 100% hurd replacement medium; however, root area percent was lower for control plants than plants grown in hurd replacement media (Table 1). Similarly, for bee balm, root area percent was lowest for control plants (Table 1). It is possible that hurd retains less water and nutrients to sustain shoot growth beyond a certain threshold at which point plants may have redirected the allocation of photosynthates to the roots. For both herbaceous perennial species, EC and pH of all three media were similar, except pH was slightly higher for switchgrass grown in 100% hurd replacement medium during the first ~65 d of the study.

Table 1. Performance of Bee Balm (Monarda didyma Sugar Buzz^® Grape Gumball) and Switchgrass (Panicum virgatum Ruby Ribbons™) grown in 4 bark:2 peat:1 sand medium (control) and two experimental media with 50% or 100% of the peat portion of control media replaced with hurd.

ⁱ Mean separation within columns, within species, indicated by different letters, according to Tukey’s honestly significant difference (HSD) test at P ≤ 0.05 and n=10.

Greenhouse Study 3 

Hurd possessed 87% TP, 0.085 g·cm^-3 BD, pH 5.7 and EC of 0.75 mS·cm^-1 (Table 1). Compared to peat moss, hurd had greater AFP, TP, pH, EC and macronutrient content. The blending of hurd with vermiculite at 1:1, and then with peat and vermiculite, generally reduced TP, pH, EC and nutrient content. Media composed of hurd, peat and vermiculite were similar to each other and to the control media (peat:vermiculite) for most of these properties. 

Petunia grown in 0.33:0.66:1 (hurd:peat:vermiculite) medium produced similar shoot fresh weight, flowers and plant width (Table 2) and were visually indistinguishable from plants grown in 0:1:1 (control) medium. Generally, as the amount of hurd substituted for peat increased to 1:0:1, plant performance declined (Table 2). While smaller than control plants, petunia grown in 0.66:0.33:1 were still visually appealing plants and were of excellent market quality. Similar trends in petunia growth were found for experiment 2, with slightly more equivalent performance among hurd containing media.

In experiment 1, the fertilizer was changed to a basifying (190 kg·tonne^-1) formulation when at 7 days after transplanting (DAT) control and 0.33:0.66:0 media were at or below pH 4.0, which is well outside the recommended pH range of 5.4 to 6.2 for petunia. This change resulted in the gradual increase of pH for all media over the course of experiment 1; however, only 0.33:0.66:1 and 0.66:0.33:1 media reached and maintained pH levels within the recommended range. Petunia grown in 1:0:1 medium accumulated less foliar P, Fe and Mn than plants did in the control medium in experiment 1 (Table 2). Petunia can become Fe deficient when grown at high pH levels, which may be one reason why plants in 1:0:1 medium did not grow as well as control plants in experiment 1. Some petunia plants in 1:0:1 medium displayed slightly chlorotic foliage, which is symptomatic of Fe deficiency. 

Petunia plants in the control medium were among the best performers in experiment 1 (Table 2), despite the low pH. Therefore, in experiment 2 a less basifying (71 kg·tonne^-1) fertilizer formulation was used, which prevented the 1:0:1 medium pH from rising to above 6.2. In experiment 2, all hurd containing media had pH within optimal range or below it, however foliar Fe content was less than the control (Table 2).

Geranium grown in 0.33:0.66:1 and 0.66:0.33:1 media had greater shoot fresh weight and plant width, but lower foliar N, P and K compared to control plants (Table 2). Control plants may have accumulated greater foliar N, P and K since they had fewer shoots among which to allocate nutrients. Similar compensatory relationships between available nutrient sources and plant organ sinks have been reported for other crops. Geranium in 1:0:1 medium was smaller than those in 0.33:0.66:1 medium as far as shoot fresh weight and plant width, but similar in size to plants in the other two media (Table 2).

Geranium plants were leached with water on two occasions when EC reached >3.3 mS·cm^-1 on 16 and 28 DAT. The fertility rate was reduced from 250 ppm N to 125 ppm N following the second leaching, and as a result no further leaching was necessary for the duration of experiment 3. Media pH for the control and 0.33:0.66:1 media was well below the recommended range of 6 to 6.6 for geranium for the duration of experiment 3. Geranium grown at low pH levels can develop Mn toxicity, which we suspect had occurred for plants from control and 0.33:0.66:1 media since they had greater foliar Mn content compared to the other hurd media (Table 2). Other indications of Mn toxicity for control and 0.33:0.66:1 plants were chlorotic and necrotic foliage, which likely contributed to the loss of four control plants (Table 2). Although media moisture content was not quantified, the control medium may have been moister than ideal for geranium because irrigation was applied to all as needed for the driest plants in the study, which were those in 1:0:1 medium.

Table 1. Air filled porosity (AFP), container capacity (CC), total porosity (TP) and bulk density (BD) and saturated media extract (SME) analysis of pH, electrical conductivity (EC) and nutrient content of substrates, hurd, peat, vermiculite and wood fiber, and six container media formulations.

ⁱ Mean separation within columns indicated by different letters, according to Tukey’s honestly significant difference (HSD) test at P ≤ 0.05 and n=4 for AFP, CC, TP and BD and n=3 for SME results.

Table 2. Shoot weight, number of flowers, plant width, plant height, number of dead plants and foliar nutrient content of Petunia × hybrida Shock Wave^® Coral Crush and Pelargonium × hortorum Maverick^™ Red Hybrid grown in four different container medium formulations with varying proportions of hurd, peat and vermiculite from experiments 1, 2 and 3.

ⁱ Individual flowers were counted for petunia and inflorescences for geranium.

ⁱⁱ Plant width was measured twice at right angles to each measurement and averaged.

ⁱⁱⁱ Mean separation within columns, within experiment, indicated by different letters, according to Tukey’s honestly significant difference (HSD) test at P ≤ 0.05 and n=10. Foliar nutrient content was n=5.

Greenhouse Study 4 

Media containing hurd in full or partial substitution of peat produced tomato plants of similar height and width as the control peat:vermiculite medium (Table 1). Tomato plants grown in hurd containing media were large, vigorous and visually indistinguishable from control plants (Table 1). While shoot weight was slightly greater for the control plants, this is not necessarily an advantageous outcome since heavier shoots may lead to shoot breakage. Plants from all media had similar foliar nutrient content, except for the nutrients N, Mn and Zn (Table 2). Tomato plants grown in full hurd medium (1:0:1) may have absorbed more N because this medium had a higher pH over the course of the study. Control plants may have accumulated more Mn due to the low pH of the medium, which was at or slightly above 5.0 over the course the study. Content of Zn did not follow any logical trends and differences are likely attributable to inherent variability of the data or sensitivity of the measuring technique (Table 2). EC gradually decreased for all media over the course of the study likely due to increasing plant vegetative growth and uptake of nutrients.

Plants grown in the medium with full replacement of peat with hurd (1:0:1) had less fruits per plant and total fruit yield by weight (Table 1). For plants grown in medium with partial replacement of peat with hurd (0.66:0.33:1 and 0.33:0.66:1) fruit production was the same as plants grown in control medium. There was greater N in the leaves and fruits of plants from the full hurd medium compared to the control medium, which may have been partially responsible for full hurd grown plants producing more vegetative than reproductive growth (Table 2).

Table 1. Shoot weight, plant height and width, number of fruits, and fruit yield per plant of Solanum lycopersicum Little Bing™ grown in four different container medium formulations with varying proportions of hurd, peat and vermiculite.

ⁱ Plant width was measured twice at right angles to each measurement and averaged.

ⁱⁱ Mean separation within columns indicated by different letters, according to Tukey’s honestly significant difference (HSD) test at P ≤ 0.05 and n=9.

Table 2. Nutrient content of leaves and fruits for Solanum lycopersicum Little Bing™ grown in four different container medium formulations with varying proportions of hurd, peat and vermiculite.

ⁱ Mean separation within columns, within leaves or fruits, indicated by different letters, according to Tukey’s honestly significant difference (HSD) test at P ≤ 0.05 and n=5.