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Rockwool Block Type 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 rockwool block format and a pre-harvest flush affected trimmed yield, THC and terpene results, and which protocols led on each measure.
Key Takeaways
- This controlled CRIC Labs trial tested how rockwool block type and pre-harvest flushing affect 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 in a Standard Block 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, we teamed up with CRIC Labs in Montréal, Canada, to conduct a rigorous, controlled indoor cultivation trial.
Our goal was to evaluate how two different rockwool block formats and 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 Standard 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.
- Block Performance: The Standard Block edged out the New Generation Block by about 2.3% in overall yield, though results varied by hybrid.
- Peak Lab Results: Quantum Terp F1 grown in a New Generation Block 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.
Our Pre-Harvest Flushing Protocol
For this trial, we defined a pre-harvest flush as gradually reducing the nutrient concentration during the final stage of bloom.
While our standard feed was kept at an EC of 3.0, we ran a 9-day flushing phase from December 5 to December 14. During this window, we stepped down the EC 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
Our trial ran from 1 October to 28 December 2025 with a total of 48 plants, split into eight treatment groups (6 plants per group) to test three key variables:
- Genetics: Quantum Terp F1 vs. Gaia F1
- Rockwool Block: Standard Block vs. New Generation Block
- Irrigation: Standard Feeding (no flush) vs. Pre-Harvest Flush
We maintained identical environmental conditions across all groups to isolate the effects of the blocks and irrigation. 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 block choices and irrigation strategies on plant structure and final yield.
Table 1. Quantum Terp F1 Average Results
| Block & Irrigation | Avg. Height | Avg. Nodes | Trimmed Yield (per plant) | |
| QRS | Standard Block, Standard Feeding | 78.8 cm | 24.5 | 398.5 g |
| QNS | New Generation Block, Standard Feeding | 69.7 cm | 21.8 | 371.2 g |
| QRF | Standard Block, Flush | 68.0 cm | 21.8 | 416.4 g |
| QNF | New Generation Block, Flush | 73.0 cm | 23.5 | 387.6 g |
| Block & Irrigation | Avg. Height | Avg. Nodes | Trimmed Yield (per plant) | |
| QRS | Standard Block, Standard Feeding | 78.8 cm | 24.5 | 398.5 g |
| QNS | New Generation Block, Standard Feeding | 69.7 cm | 21.8 | 371.2 g |
| QRF | Standard Block, Flush | 68.0 cm | 21.8 | 416.4 g |
| QNF | New Generation Block, Flush | 73.0 cm | 23.5 | 387.6 g |
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Table 2. Gaia F1 Average Results
| Block & Irrigation | Avg. Height | Avg. Nodes | Trimmed Yield (per plant) | |
| GRS | Standard Block, Standard Feeding | 64.2 cm | 20.7 | 317.3 g |
| GNS | New Generation Block, Standard Feeding | 61.8 cm | 19.8 | 336.8 g |
| GRF | Standard Block, Flush | 65.7 cm | 22.2 | 358.1 g |
| GNF | New Generation Block, Flush | 58.3 cm | 20.8 | 361.1 g |
| Block & Irrigation | Avg. Height | Avg. Nodes | Trimmed Yield (per plant) | |
| GRS | Standard Block, Standard Feeding | 64.2 cm | 20.7 | 317.3 g |
| GNS | New Generation Block, Standard Feeding | 61.8 cm | 19.8 | 336.8 g |
| GRF | Standard Block, Flush | 65.7 cm | 22.2 | 358.1 g |
| GNF | New Generation Block, Flush | 58.3 cm | 20.8 | 361.1 g |
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Projected Yield Per Square Metre
Based on our trial planting density of 5.4 plants per square metre, we mapped out the projected yields to show commercial scaling potential:
| 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 Block Performance
Across all 48 plants, our 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.
When it came to the rockwool blocks, the performance was variety-dependent. Quantum Terp F1 preferred the Standard Block, yielding more under both irrigation setups. Gaia F1, however, performed slightly better in the New Generation Block. This highlights that substrate choices should be paired carefully with specific genetics.


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 | Block Type | Irrigation |
| β-Caryophyllene | 0.496% | Quantum Terp F1 | New Generation | Standard Feeding |
| Terpinolene | 0.346% | Quantum Terp F1 | Standard | Standard Feeding |
| β-Myrcene | 0.211% | Gaia F1 | Standard | Standard Feeding |
| d-Limonene | 0.190% | Quantum Terp F1 | Standard | Standard Feeding |
| β-Ocimene | 0.129% | Gaia F1 | Standard | Standard Feeding |
| α-Humulene | 0.119% | Quantum Terp F1 | New Generation | Standard Feeding |
| β-Pinene | 0.105% | Quantum Terp F1 | Standard | Standard Feeding |
| E-β-Farnesene | 0.094% | Gaia F1 | Standard | Flush |
| Valencene | 0.090% | Quantum Terp F1 | Standard & New Gen (Tie) | Standard Feeding |
| trans-Nerolidol | 0.076% | Quantum Terp F1 | New Generation | Standard Feeding |
| β-Caryophyllene | |
|---|---|
| Highest % (w/w) | 0.496% |
| Variety | Quantum Terp F1 |
| Block Type | New Generation |
| Irrigation | Standard Feeding |
| Terpinolene | |
| Highest % (w/w) | 0.346% |
| Variety | Quantum Terp F1 |
| Block Type | Standard |
| Irrigation | Standard Feeding |
| β-Myrcene | |
| Highest % (w/w) | 0.211% |
| Variety | Gaia F1 |
| Block Type | Standard |
| Irrigation | Standard Feeding |
| d-Limonene | |
| Highest % (w/w) | 0.190% |
| Variety | Quantum Terp F1 |
| Block Type | Standard |
| Irrigation | Standard Feeding |
| β-Ocimene | |
| Highest % (w/w) | 0.129% |
| Variety | Gaia F1 |
| Block Type | Standard |
| Irrigation | Standard Feeding |
| α-Humulene | |
| Highest % (w/w) | 0.119% |
| Variety | Quantum Terp F1 |
| Block Type | New Generation |
| Irrigation | Standard Feeding |
| β-Pinene | |
| Highest % (w/w) | 0.105% |
| Variety | Quantum Terp F1 |
| Block Type | Standard |
| Irrigation | Standard Feeding |
| E-β-Farnesene | |
| Highest % (w/w) | 0.094% |
| Variety | Gaia F1 |
| Block Type | Standard |
| Irrigation | Flush |
| Valencene | |
| Highest % (w/w) | 0.090% |
| Variety | Quantum Terp F1 |
| Block Type | Standard & New Gen (Tie) |
| Irrigation | Standard Feeding |
| trans-Nerolidol | |
| Highest % (w/w) | 0.076% |
| Variety | Quantum Terp F1 |
| Block Type | New Generation |
| Irrigation | Standard Feeding |
The Verdict: How to Treat Flushing as a Strategic Tool
Our trial systematically dismantles the myth that flushing is a simple "yes or no" choice. Instead, the data proves it is a precise tuning knob that forces you to choose between 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 in a Standard Block.
- 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 in a New Generation Block without flushing.
By replacing industry intuition with controlled scientific data, we are establishing a new standard for F1 hybrid cultivation. Whether you prioritize market-leading weights or connoisseur-grade aromatic profiles, you now have the exact blueprint to optimize your grow room.
We are already preparing our next round of data-driven trials to keep pushing the boundaries of what our F1 genetics can achieve in your garden. Stay tuned for our next research update!

