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How Rockwool Choice and Irrigation Affect Cannabis Yield and Potency
Controlled trial results show how rockwool block configuration and irrigation shot size affect cannabis yield and THC. Comparing simple versus dual blocks and low versus high irrigation across two F1 varieties reveals that plant structure, not inputs alone, drives performance.
Simple vs dual rockwool blocks × low vs high shot sizes: impacts on growth, dry trimmed yield, and THC across two F1 varieties (Orion and Medusa)
Contents:
Results and key learnings
- Yield: The highest average yields were observed in Orion F1 grown in dual block with low shot size and Medusa F1 grown in single block with high shot size (see Tables 2 and 3).
- THC: Across both varieties, the highest THC rankings were consistently associated with single block + low shot size treatments.
- Block system trend: Dual block systems increased root-zone buffering (the substrate’s ability to hold water and nutrients more steadily) but did not produce a statistically robust yield advantage as a standalone factor.
- Shot size trend: Low shot size tended to support slightly higher yields and THC, though differences were not statistically significant as main effects.
- Decision rule: To maximise yield, prioritise plant structure (height and node count) over substrate or irrigation category; to bias towards THC, favour more steerable root-zone conditions with controlled dry-back.
Practical insights
In practice, these results shift attention away from “which block or shot size is best” and towards how substrate and irrigation choices influence plant structure. Standardising substrate format and irrigation logic improves consistency by narrowing root-zone moisture and EC variability. To reproduce outcomes in controlled-environment agriculture (CEA), climate, fertigation composition, irrigation uniformity, and monitoring protocols must remain stable.
.jpg "CRIC cannabis research and cultivation trials facility")
Introduction
Rockwool is a manufactured, inert growing medium produced by spinning molten mineral fibres into a porous structure. In controlled environments, this structure enables precise management of water, oxygen, and nutrient availability in the root zone, making rockwool for plants particularly suitable for repeatable cultivation trials.
This collaborative trial, led by Royal Queen Seeds and conducted at the CRIC (Cannabis Research and Innovation Centre) facilities in Montréal, Canada, examined how substrate configuration and irrigation shot size affect cannabis growth, yield, and potency. The study compared single-block versus dual-block rockwool systems under low and high irrigation shot sizes across two F1 hybrids: Orion F1 and Medusa F1.
Seeds were planted on 19th February 2025 in rockwool plugs, transplanted on 21st February, transitioned to the generative stage on 3rd March, and harvested on 5th May, corresponding to an approximately 11-week cultivation cycle from seed to harvest.
.jpg "Trial 6 introduction: single block vs dual block irrigation systems")
Materials & Methods
Plants were cultivated in a purpose-built controlled-environment facility using rockwool substrate as the primary rooting medium. Two block configurations were evaluated:
- Single block: one finishing block per plant
- Dual block: increased total root-zone volume via an additional block stage, providing greater buffering against rapid changes in moisture and nutrient concentration
The irrigation strategy was tested at two levels:
- Low shot size: smaller pulses per irrigation event, allowing tighter control of moisture and nutrient delivery
- High shot size: larger pulses per event, resulting in faster substrate saturation
Each variety was grown under a 2 × 2 factorial design (block system × shot size), with six biological replicates per treatment combination.
Measured parameters included final plant height, dry trimmed flower yield per plant, and relative THC ranking. Node count and stem diameter were recorded to explore relationships between plant structure and yield.
Results
Results varied by variety and treatment and are summarised below.
Table 1. Orion F1 : mean values per treatment (n = 6)
| Treatment | Height (cm) | Nodes | Yield (g/plant) | THC level ranking |
| Single block + Low | 87.3 | 22.7 | 286.7 | 2nd position |
| Single block + High | 81.8 | 20.3 | 266.5 | 5th position |
| Dual block + Low | 87.0 | 24.5 | 317.7 | 6th position |
| Dual block + High | 83.2 | 22.2 | 264.2 | 8th position |
| Single block + Low | |
|---|---|
| Height (cm) | 87.3 |
| Nodes | 22.7 |
| Yield (g/plant) | 286.7 |
| THC ranking | 2nd position |
| Single block + High | |
| Height (cm) | 81.8 |
| Nodes | 20.3 |
| Yield (g/plant) | 266.5 |
| THC ranking | 5th position |
| Dual block + Low | |
| Height (cm) | 87.0 |
| Nodes | 24.5 |
| Yield (g/plant) | 317.7 |
| THC ranking | 6th position |
| Dual block + High | |
| Height (cm) | 83.2 |
| Nodes | 22.2 |
| Yield (g/plant) | 264.2 |
| THC ranking | 8th position |
Table 2. Medusa F1 : mean values per treatment (n = 6)
| Treatment | Height (cm) | Nodes | Yield (g/plant) | THC level ranking |
| Single block + Low | 86.5 | 22.2 | 293.8 | 1st position |
| Single block + High | 85.3 | 24.0 | 339.8 | 3rd position |
| Dual block + Low | 82.3 | 22.3 | 323.0 | 4th position |
| Dual block + High | 80.8 | 22.3 | 292.0 | 7th position |
| Single block + Low | |
|---|---|
| Height (cm) | 86.5 |
| Nodes | 22.2 |
| Yield (g/plant) | 293.8 |
| THC ranking | 1st position |
| Single block + High | |
| Height (cm) | 85.3 |
| Nodes | 24.0 |
| Yield (g/plant) | 339.8 |
| THC ranking | 3rd position |
| Dual block + Low | |
| Height (cm) | 82.3 |
| Nodes | 22.3 |
| Yield (g/plant) | 323.0 |
| THC ranking | 4th position |
| Dual block + High | |
| Height (cm) | 80.8 |
| Nodes | 22.3 |
| Yield (g/plant) | 292.0 |
| THC ranking | 7th position |
Overall, treatment-level yield differences were modest relative to plant-to-plant variability, while THC showed clearer directional shifts linked to irrigation strategy.
Key findings & practical takeaways
Across both varieties, plant morphology explained yield far more effectively than substrate or irrigation category alone. A linear model including plant height, node count, and variety explained approximately 72% of yield variability (R² ≈ 0.72).
Both height and node count were independently and strongly associated with yield. Each additional centimetre of plant height corresponded to roughly +4 g of dry flower, while each additional node contributed approximately +10 g, holding other factors constant. These relationships were statistically robust across treatments.
Varietal effects remained significant after accounting for structure. At equivalent height and node count, Medusa F1 yielded approximately 30 g more per plant than Orion F1, indicating a genetic efficiency advantage rather than a difference in plant size.
By contrast, irrigation level and block system showed no statistically significant main effects on yield when analysed as categorical factors. Dual block systems improved root-zone buffering, and low shot size trended towards slightly higher yields and THC, but these effects were subtle compared with biological variability.
THC followed a different pattern from yield. Higher THC rankings were consistently associated with single block + low shot size, suggesting that tighter control of dry-back (how much the substrate is allowed to dry between irrigations) and root-zone steerability (the grower’s ability to actively guide water and nutrient conditions at the roots) can favour cannabinoid concentration even when biomass gains are modest.
Practical recommendations
For growers prioritising yield, the most reliable lever is plant structure, not substrate or irrigation category alone. In this trial, Orion F1 achieved its highest yields under Dual block with low shot size, while Medusa F1 performed best under single block with high shot size. These outcomes reflect cultivar-specific responses rather than universal rules.
For growers prioritising THC, both varieties showed their highest potency rankings under single block + low shot size, consistent with slightly drier, more steerable root-zone conditions.
Mid-cycle monitoring improves decision-making. Height and node count are practical early indicators of final yield potential, while stem diameter was not a reliable predictor in this dataset. Tracking these traits allows irrigation and steering adjustments before flowering outcomes are locked in.
In controlled environments, substrate and irrigation should be treated as an integrated system. Small, well-instrumented trials conducted under stable climate and fertigation conditions remain the most effective way to validate cultivar-specific responses.
Conclusion
This trial highlights three practical observations relevant to controlled-environment cannabis cultivation. First, the yield differences observed here appeared to be driven primarily by plant structure and genetic factors, rather than by substrate or irrigation category alone.
Second, strategies associated with higher yield were not necessarily associated with higher THC concentration, underscoring the importance of clearly defining production objectives. Third, morphology-based indicators such as plant height and node count proved useful in interpreting treatment responses and may offer a practical framework for guiding irrigation decisions in comparable production systems.
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