Flush Tracker
Track your end-of-grow flush from last feed to chop-ready. Enter your last feed date, growing medium, and runoff EC readings — get a daily progress verdict, EC drop assessment, and estimated chop date. Covers soil, coco, and hydro with medium-specific flush windows and EC targets.
Track your flush progress
Collect the last 20% of runoff water and test with EC meter.
Your runoff EC from the last nutrient watering. Used to calculate expected EC drop rate.
Tool checks whether flush will complete before your target chop.
Flush window for this medium
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Select a medium to see the flush window.
Common flush mistakes
- Flushing with too little water. Each watering needs to run 20–30% runoff volume to physically move salt load through the medium. Light waterings just dilute the top layer — they don't flush the root zone.
- Testing EC from the first runoff. The first water out carries salts from the top of the medium, not the root zone. Collect and test the last 20% of runoff for an accurate root-zone reading.
- Flushing living soil. If you're running a microbe-heavy living soil, aggressive flushing disrupts the microbial colony that's doing the nutrient work. One week of plain water is typically sufficient.
- Chopping before trichomes confirm readiness. The flush timeline is an estimate — trichome state is the real signal. EC at target doesn't mean ready to chop; it means medium is clean. Check trichomes independently.
Flush progress
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Runoff EC status
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Current
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Target
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Estimated flush complete
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Enter dates to see chop timing.
Today's flush checklist
Track your flush from last feed to chop-ready
Enter your last feed date and medium
The day you gave the last nutrient feed is Day 0 — the flush starts the next watering. If you're starting the flush today, hit "Starting flush today." Your growing medium sets the flush window: soil needs 7–14 days, coco 3–7, hydro 1–3. Input water EC sets the target — the tool calculates how clean your runoff needs to get based on your tap water baseline.
Test runoff EC and enter it daily
After each flush watering, collect the last 20% of runoff water in a clean cup and test with your EC meter. Enter this number in "Today's runoff EC." The tool shows whether your EC is on track, dropping too slowly, or already clean. Update the reading each time you water — the checklist adjusts to your current flush day and EC status.
Follow the daily checklist
The checklist updates based on your medium, flush day, and EC status. Core item every day: water to full runoff with plain pH-adjusted water. If EC isn't dropping fast enough, the checklist flags this and tells you to increase volume. If you're in late flush, the checklist adds trichome inspection — flush timing and ripeness timing should align, and the tool links to the Harvest Window Planner.
Chop when EC is clean and trichomes confirm
The tool says "Complete — ready to chop" when runoff EC hits target. That's your medium-clean signal — but trichome state is the final authority on actual ripeness. Use the Harvest Window Planner alongside the Flush Tracker in the final week. EC clean + trichomes right = chop day. Pro mode lets you enter a target chop date and see whether your flush timeline fits.
The 20% runoff rule — why it matters
The first water that exits the drainage holes comes from the top layer of medium — not the root zone. It reads lower EC than what's actually surrounding the roots because it hasn't traveled through the full salt load. Always collect and test the last 20% of runoff, not the first. In practice: if you're flushing a 5-gallon pot with 1 gallon of water (20% = 200mL), test the last cup that comes out, not the first. This gives you an accurate root-zone EC reading.
Why flush duration differs by medium and how EC predicts completion
Flushing removes accumulated nutrient salts from the growing medium before harvest. The mechanism is physical: plain water dissolves salt deposits and carries them out through drainage. How long this takes is almost entirely a function of the medium's buffering capacity — its ability to hold onto nutrients against the flushing action of plain water. Hydro has zero buffering. Soil has significant buffering. Coco sits in between. See the Environment & Climate Guide for a broader picture of how nutrient management and environment interact.
7–14d
Soil flush window
Organic matter has high CEC (cation exchange capacity) — it physically holds nutrient cations against plain water. Multiple high-volume flushes are needed to displace what the humus has bound.
3–7d
Coco flush window
Coco has moderate CEC (primarily from the fibrous structure) but is inert compared to soil. Salt deposits flush faster, especially when combined with coco's typical high-volume daily watering.
1–3d
Hydro flush window
No medium buffer at all. Swap reservoir to plain water and roots are immediately bathed in a salt-free solution. The hydro flush is essentially a reservoir change, not a medium clearance.
What EC measures and why it predicts flush completion
EC (electrical conductivity) measures the total concentration of dissolved ions in water. Nutrient salts are ionic compounds — when dissolved, they split into cations (Ca²⁺, Mg²⁺, K⁺, NH₄⁺) and anions (NO₃⁻, PO₄³⁻, SO₄²⁻). Higher ion concentration = higher conductivity = higher EC reading. A runoff EC reading therefore directly measures how many dissolved nutrient ions are leaving the root zone with each watering.
As you flush, each successive watering dissolves and carries away a portion of the residual salt load. EC should drop roughly linearly over the flush window. The target EC is your input water EC plus a buffer (typically +0.3 mS/cm) — once runoff EC approaches that target, the root zone is essentially clean. The hard floor target is 0.5 mS/cm regardless of input water quality, because below that threshold, the residual salt concentration has negligible effect on smoke quality.
Does flushing actually improve smoke quality?
This is the most contested topic in cannabis cultivation. The evidence is mixed, but the scientific consensus leans toward yes — with important caveats. Studies comparing flushed vs. unflushed cannabis have found measurable differences in ash color (white vs. black) and subjective smoothness. The mechanism: residual chloride, sulfate, and nitrate salts in plant tissue can affect combustion chemistry and produce harsher smoke. Flushing allows the plant to metabolize and deplete its stored nutrient reserves, resulting in lower mineral content in the harvested tissue.
The caveat: the effect is most pronounced with synthetic nutrients, which leave ionic salt deposits. Organic nutrient growers — especially those using living soil — have less to gain from aggressive flushing because organic nutrients are microbe-mediated and don't leave the same type of ionic salt deposits. One week of plain water at the end of an organic grow is generally sufficient and less disruptive to the microbial ecosystem than a 14-day salt flush.
EC target — where the 0.5 mS/cm number comes from
The 0.5 mS/cm target is a practical threshold rather than a precise scientifically derived value. It represents roughly 2–3× the EC of average municipal tap water (0.1–0.3 mS/cm) and is well below the level at which residual salts meaningfully affect nutrient uptake or plant stress. Some growers aim for input water EC exactly, but the +0.3 mS/cm buffer is a reasonable concession to the fact that complete salt removal is impractical without damaging the plant through extended water stress.
| Input water | Typical EC | Target runoff EC | Notes |
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| RO / distilled | 0.0–0.1 | < 0.3–0.4 | Lowest target — very clean starting point. Calcium deficiency risk on long flushes with RO. |
| Filtered / soft tap | 0.1–0.2 | < 0.4–0.5 | Standard for most home growers. Hit 0.5 and you're done. |
| Average tap | 0.2–0.4 | < 0.5–0.6 | Target is input + 0.3. Hard to get much lower than this without RO pre-treatment. |
| Hard tap | 0.5+ | < 0.8–1.0 | High mineral tap water limits how low runoff will go. Consider RO for the final flush week. |
Common flush questions
When exactly should I start flushing?
Start counting backwards from your expected chop date: soil needs 7–14 days, coco 3–7, hydro 1–3. If you're aiming for peak harvest at week 9 of flower (63 days from flip), start your soil flush at day 49–56. The problem: most growers don't know their exact chop date until trichomes tell them. The practical approach is to start the flush when trichomes are mostly cloudy with the first amber ones appearing — that window is typically 1–2 weeks from peak, which aligns with a soil flush window. Use the Harvest Window Planner to track trichome progress alongside the flush.
My runoff EC isn't dropping — what do I do?
Two most common causes: not enough water volume per flush, or compacted medium blocking flow. For volume: you need enough water to physically move salt load through the medium — for a 5-gallon pot, that's at least 1 gallon of runoff per session, ideally more. If you're only running 10% runoff, you're dissolving but not removing. For compaction: if water is pooling on top or running down the sides of the pot, the root ball may be too dense for water to flow through evenly. Try gently loosening the top inch of medium. If neither helps after 2–3 sessions, increase flush frequency.
Do I need to flush if I'm using organic nutrients?
Not aggressively. Organic nutrients are microbe-mediated — they don't leave the same ionic salt deposits as synthetic nutrients. One week of plain water at the end of an organic grow is standard and sufficient for most set-ups. Living soil growers sometimes skip the flush entirely, arguing that aggressive flushing disrupts the microbial ecosystem that's been cultivated over the grow. If you're running a true living soil with minimal synthetic inputs, a 5–7 day plain-water period is a reasonable middle ground.
Can I flush too long and hurt the plant?
Yes. Extended plain-water flushes in late flower can cause the plant to cannibalize itself — consuming stored calcium and magnesium from fan leaves, causing leaf yellowing and deficiency symptoms. Some leaf yellowing during flush is normal and even desirable (indicates the plant is drawing down reserves). But if the plant is dropping fan leaves, showing interveinal chlorosis, or looking generally stressed, you've flushed long enough. If your EC hits target on day 6 of a 10-day soil flush, chop on day 6 — don't wait for the calendar.
What pH should flush water be?
Same as your regular watering: soil 6.0–7.0, coco 5.8–6.3, hydro 5.5–6.5. Using plain water doesn't mean skipping pH adjustment. Out-of-range pH during a flush can lock out any remaining nutrients the plant needs for final ripening, stress the root system, and potentially cause pH-related salt precipitation that increases runoff EC rather than decreasing it. Always pH your flush water before applying.
Should I use a commercial flushing agent?
The science on commercial flush agents (Sledgehammer, Final Phase, etc.) is inconclusive. They're generally chelating agents or surfactants that claim to help break down salt bonds and improve salt mobility through the medium. Plain water does the same job given sufficient volume and time — the advantage of a flush agent is primarily speed. If you're under time pressure (chop date approaching, flush window tight), a flush agent may help you hit target EC faster. If you have adequate flush time, plain pH-adjusted water works just as well.
How do I flush DWC hydro?
Drain and refill the reservoir with plain pH-adjusted water. Add a small amount of Cal-Mag (0.2–0.3 mS/cm EC) if using RO water — pure RO water without any minerals can stress roots through osmotic imbalance. Run the plain reservoir for 1–3 days, testing solution EC daily (it will rise as roots leach into the reservoir — change water if it climbs above 0.5 mS/cm). Some DWC growers run a "final flush" EC of 0.2–0.4 mS/cm with just cal-mag and a trace of PK, rather than pure plain water, to avoid any mineral deficiency during the last days of bud swell.
What does normal fan leaf yellowing during flush look like?
Large fan leaves yellowing from the bottom of the plant upward is normal and expected — the plant is mobilizing stored nitrogen from fan leaves toward the buds as it completes ripening. This is distinct from deficiency-caused yellowing, which often shows interveinal patterns (green veins, yellow between them) and can appear mid-canopy. Uniform fan leaf yellowing that starts at the bottom and progresses upward during a flush is a healthy sign of senescence. Interveinal chlorosis mid-plant during flush suggests calcium or magnesium depletion — consider adding a small Cal-Mag shot.
Flush and finishing supplies
Flushing requires almost no new gear — but the one tool that makes every flush decision clear is an EC meter. If you don't have one, every flush decision is a guess. A $30 EC meter turns a subjective process into a measurable one.
pH & EC meters
Combo pH/EC/TDS meters ($25–60), dedicated EC pens ($20–40), calibration solutions. Without an EC meter, you can't measure flush progress — you're guessing. A combo pH/EC meter handles both daily adjustments and runoff testing. Calibrate before each use and store probe tip in storage solution.
pH adjustment
pH Down concentrate ($10–20), pH Up ($10–15), pH buffer calibration solutions. Always pH flush water to medium-appropriate range before applying. Out-of-range flush water causes salt precipitation and can raise runoff EC instead of lowering it. One of the most common flush mistakes that slows EC drop.
Cal-Mag for flush
Cal-Mag supplements ($15–30). RO and distilled water users should add a small amount of Cal-Mag (targeting ~0.2 mS/cm EC) to flush water — pure water without minerals can cause osmotic stress and calcium deficiency during the final ripening stage. Coco and hydro growers need Cal-Mag throughout, including the flush.
Post-harvest curing
Mason jars, Boveda 62% packs, humidity meters for cure room. Start gathering jars before the flush ends — you'll need them ready on chop day. Use the Harvest Yield Planner to calculate exactly how many jars your harvest needs before you run out mid-cure.
Trichome inspection
60–100× jeweler's loupe ($10–20), USB digital microscope ($25–60). Flush completion is a medium-clean signal — trichome state is the actual harvest readiness signal. Use both together. A loupe is cheap insurance against chopping too early or too late based on calendar timing alone.
Watering gear
Watering cans with long necks ($15–30), watering wands ($20–40). Flush volume matters — you need to apply water evenly across the full pot surface to avoid channeling (water running down one side without penetrating the root ball). A good watering wand with a rose head distributes water evenly and makes high-volume flush sessions easier.