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Rockwool and Pre-Harvest Flush: Effects on Cannabis Yield, THC and Terpenes
Inside a controlled indoor trial across two F1 varieties and eight treatment groups: how pre-harvest flushing affected trimmed yield, THC and terpene results, and which protocols delivered the strongest outcomes.
Key Takeaways
- This controlled CRIC Labs trial tested how pre-harvest flushing affects yield, waste, potency, and terpene expression in Quantum Terp F1 and Gaia F1.
- Flushing increased average trimmed yield by around 7% and greatly reduced post-harvest waste, with Quantum Terp F1 delivering the highest output.
- Standard feeding preserved stronger terpene levels, making it the better strategy for growers prioritising aroma, potency, and premium flower quality over maximum yield.
Contents:
Growers often choose their rockwool blocks and flushing strategies based on experience, routine, or personal preference. To move beyond intuition and gather solid data, Grodan conducted a rigorous, controlled indoor cultivation trial at CRIC Labs in Montréal, Canada, in collaboration with Royal Queen Seeds.
The goal: evaluate how two pre-harvest irrigation strategies (flushing vs. standard feeding) affect plant development, trimmed yield, post-harvest waste, potency, and terpene expression across two of our F1 hybrids: Quantum Terp F1 and Gaia F1.
Key Discoveries At A Glance
- Top Yielding Combination: Our Quantum Terp F1 grown in a Rockwool Block with a pre-harvest flush produced an impressive average of 416.4 g per plant.
- Genetics Matter: Quantum Terp F1 was the top-performing variety, yielding approximately 14.6% more trimmed flower than Gaia F1.
- The Flush Effect: In this trial, flushing was linked to a 7% increase in average trimmed yield and significantly less post-harvest waste.
- Peak Lab Results: Quantum Terp F1 grown with standard feeding achieved the absolute highest potency and terpene levels.
- Terpene Trade-off: Flushed samples showed a consistent 21.9% decrease in mean total terpene content compared to standard-fed samples.
Pre-Harvest Flushing Protocol
For this trial, a pre-harvest flush was defined as gradually reducing the nutrient concentration during the final stage of bloom.
While the standard feed was kept at an EC of 3.0, the trial ran a 9-day flushing phase from December 5 to December 14. During this window, the EC was stepped down first to 1.7 and finally to 1.0 to clear nutrients from the root zone before harvest.
Note: Because flush outcomes depend heavily on starting EC, light intensity, and your specific feed chart, these results reflect our exact parameters and shouldn't be taken as a universal rule for every grow room.

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Materials And Methods
The trial ran from 1 October to 28 December 2025 with a total of 48 plants, split into four treatment groups (12 plants per group) to test two key variables:
- Genetics: Quantum Terp F1 vs. Gaia F1
- Irrigation: Standard Feeding (no flush) vs. Pre-Harvest Flush
Identical environmental conditions were maintained across all groups to isolate the effects of the irrigation strategies. Standardized metrics included height, fresh/dry weights, reject rates, cannabinoid content, and full terpene profiles.


The Data Breakdown
The tables below show the direct impact of our irrigation strategies across the different experimental configurations on plant structure and final yield.
Table 1. Quantum Terp F1 Average Results
| Rockwool Type & Irrigation | Avg. Height | Avg. Nodes | Trimmed Yield (per plant) | |
| QRS | Rockwool Type 01, Standard Feeding | 78.8 cm | 24.5 | 398.5 g |
| QNS | Rockwool Type 02, Standard Feeding | 69.7 cm | 21.8 | 371.2 g |
| QRF | Rockwool Type 01, Flush | 68.0 cm | 21.8 | 416.4 g |
| QNF | Rockwool Type 02, Flush | 73.0 cm | 23.5 | 387.6 g |
| Rockwool Type & Irrigation | Avg. Height | Avg. Nodes | Trimmed Yield (per plant) | |
| QRS | Rockwool Type 01, Standard Feeding | 78.8 cm | 24.5 | 398.5 g |
| QNS | Rockwool Type 02, Standard Feeding | 69.7 cm | 21.8 | 371.2 g |
| QRF | Rockwool Type 01, Flush | 68.0 cm | 21.8 | 416.4 g |
| QNF | Rockwool Type 02, Flush | 73.0 cm | 23.5 | 387.6 g |


Table 2. Gaia F1 Average Results
| Rockwool Type & Irrigation | Avg. Height | Avg. Nodes | Trimmed Yield (per plant) | |
| GRS | Rockwool Type 01, Standard Feeding | 64.2 cm | 20.7 | 317.3 g |
| GNS | Rockwool Type 02, Standard Feeding | 61.8 cm | 19.8 | 336.8 g |
| GRF | Rockwool Type 01, Flush | 65.7 cm | 22.2 | 358.1 g |
| GNF | Rockwool Type 02, Flush | 58.3 cm | 20.8 | 361.1 g |
| Rockwool Type & Irrigation | Avg. Height | Avg. Nodes | Trimmed Yield (per plant) | |
| GRS | Rockwool Type 01, Standard Feeding | 64.2 cm | 20.7 | 317.3 g |
| GNS | Rockwool Type 02, Standard Feeding | 61.8 cm | 19.8 | 336.8 g |
| GRF | Rockwool Type 01, Flush | 65.7 cm | 22.2 | 358.1 g |
| GNF | Rockwool Type 02, Flush | 58.3 cm | 20.8 | 361.1 g |


Projected Yield Per Square Metre
Based on the trial planting density of 5.4 plants per square metre, the projected yields were calculated to show potential commercial scaling:
| Yield per Plant | Projected Yield per m² | |
| QRS | 398.5 g | 2,151.9 g/m² |
| QNS | 371.2 g | 2,004.5 g/m² |
| QRF (Highest) | 416.4 g | 2,248.6 g/m² |
| QNF | 387.7 g | 2,093.6 g/m² |
| GRS (Lowest) | 317.3 g | 1,713.4 g/m² |
| GNS | 336.8 g | 1,818.7 g/m² |
| GRF | 358.1 g | 1,933.7 g/m² |
| GNF | 361.1 g | 1,949.9 g/m² |
| QRS | |
|---|---|
| Yield per Plant | 398.5 g |
| Projected Yield per m² | 2,151.9 g/m² |
| QNS | |
| Yield per Plant | 371.2 g |
| Projected Yield per m² | 2,004.5 g/m² |
| QRF (Highest) | |
| Yield per Plant | 416.4 g |
| Projected Yield per m² | 2,248.6 g/m² |
| QNF | |
| Yield per Plant | 387.7 g |
| Projected Yield per m² | 2,093.6 g/m² |
| GRS (Lowest) | |
| Yield per Plant | 317.3 g |
| Projected Yield per m² | 1,713.4 g/m² |
| GNS | |
| Yield per Plant | 336.8 g |
| Projected Yield per m² | 1,818.7 g/m² |
| GRF | |
| Yield per Plant | 358.1 g |
| Projected Yield per m² | 1,933.7 g/m² |
| GNF | |
| Yield per Plant | 361.1 g |
| Projected Yield per m² | 1,949.9 g/m² |
Deep Dive: Yield, Waste, and Irrigation Performance
Across all 48 plants, the trial brought in 17.68 kg of trimmed flower. Quantum Terp F1 proved to be our heavy hitter, averaging 393.4 g per plant compared to Gaia F1’s 343.3 g.
When looking at waste, the flushing strategy was a clear winner. Our 24 flushed plants produced a mere 25 g of dry waste and 136 g of fresh waste combined. Meanwhile, our 24 standard-fed plants racked up 178 g of dry waste and 619 g of fresh waste. Notably, our highest-yielding group (QRF) had zero recorded waste.


The Chemistry: How Flushing Affects Potency & Aroma
Our laboratory testing revealed a fascinating, consistent pattern: flushing reduced total terpene concentration across the board.
In every single group comparison, the flushed plants had lower total terpene levels than their standard-fed counterparts—averaging a 21.9% loss.
The impact on cannabinoids, however, was less clear-cut, meaning we cannot say flushing directly increases or decreases raw potency.
Top 10 Terpene Concentrations Recorded
The table below shows the absolute highest terpene levels we recorded. Notice that 9 out of these 10 peak concentrations occurred in our standard-fed (non-flushed) plants.
| Terpene | Highest % (w/w) | Variety | Irrigation |
| β-Caryophyllene | 0.496% | Quantum Terp F1 | Standard Feeding |
| Terpinolene | 0.346% | Quantum Terp F1 | Standard Feeding |
| β-Myrcene | 0.211% | Gaia F1 | Standard Feeding |
| d-Limonene | 0.190% | Quantum Terp F1 | Standard Feeding |
| β-Ocimene | 0.129% | Gaia F1 | Standard Feeding |
| α-Humulene | 0.119% | Quantum Terp F1 | Standard Feeding |
| β-Pinene | 0.105% | Quantum Terp F1 | Standard Feeding |
| E-β-Farnesene | 0.094% | Gaia F1 | Flush |
| Valencene | 0.090% | Quantum Terp F1 | Standard Feeding |
| trans-Nerolidol | 0.076% | Quantum Terp F1 | Standard Feeding |
| β-Caryophyllene | |
|---|---|
| Highest % (w/w) | 0.496% |
| Variety | Quantum Terp F1 |
| Irrigation | Standard Feeding |
| Terpinolene | |
| Highest % (w/w) | 0.346% |
| Variety | Quantum Terp F1 |
| Irrigation | Standard Feeding |
| β-Myrcene | |
| Highest % (w/w) | 0.211% |
| Variety | Gaia F1 |
| Irrigation | Standard Feeding |
| d-Limonene | |
| Highest % (w/w) | 0.190% |
| Variety | Quantum Terp F1 |
| Irrigation | Standard Feeding |
| β-Ocimene | |
| Highest % (w/w) | 0.129% |
| Variety | Gaia F1 |
| Irrigation | Standard Feeding |
| α-Humulene | |
| Highest % (w/w) | 0.119% |
| Variety | Quantum Terp F1 |
| Irrigation | Standard Feeding |
| β-Pinene | |
| Highest % (w/w) | 0.105% |
| Variety | Quantum Terp F1 |
| Irrigation | Standard Feeding |
| E-β-Farnesene | |
| Highest % (w/w) | 0.094% |
| Variety | Gaia F1 |
| Irrigation | Flush |
| Valencene | |
| Highest % (w/w) | 0.090% |
| Variety | Quantum Terp F1 |
| Irrigation | Standard Feeding |
| trans-Nerolidol | |
| Highest % (w/w) | 0.076% |
| Variety | Quantum Terp F1 |
| Irrigation | Standard Feeding |
The Verdict: How to Treat Flushing as a Strategic Tool
The trial systematically challenges the idea that flushing is a simple "yes or no" choice. Instead, the data suggests it can be viewed as a precise tuning knob that presents growers with two distinct cultivation goals:
- If your goal is Maximum Yield & Labor Efficiency: Go with a pre-harvest flush. By stepping down your EC in the final days, you will boost average trimmed weight by around 7% and virtually eliminate post-harvest waste (saving hours of trimming labor). For the absolute highest output, pair this strategy with Quantum Terp F1.
- If your goal is Peak Terpenes & Premium Bag Appeal: Stick to standard feeding until the end. Flushing triggers a consistent 21.9% drop in total terpene concentration. To unlock the absolute highest aromatic and potency potential of your crop, run Quantum Terp F1 without flushing.
The trial provides controlled data that can help inform F1 hybrid cultivation, showing how different irrigation strategies can influence yield and terpene expression. Whether you prioritize market-leading weights or aromatic profiles, the results provide a useful reference for understanding how these variables performed under the specific conditions tested.
Further trials will help build on these findings and explore how F1 genetics respond to different cultivation strategies.
