How to Flush Cannabis Plants: When, Why, and the Right Way

Flushing means running a large volume of plain, pH'd water through the growing medium to dissolve and remove accumulated mineral salts. It's one of the most frequently discussed techniques in cannabis growing — and one of the most misunderstood. Some growers flush religiously. Others never flush at all. The truth is between: flushing is a tool with specific use cases, not a blanket practice.

Two types of flushing

Maintenance flush (problem-solving)

Running plain water through the medium to correct a nutrient problem:

• Salt buildup causing nutrient lockout
• Nutrient toxicity from over-feeding
• pH drift in the root zone
• Medium reset before switching nutrient formulas

This is a corrective action, not a scheduled event. You flush when there's a problem, not on a calendar.

Pre-harvest flush (controversial)

Running plain water for the final 1–2 weeks before harvest, supposedly to force the plant to "use up stored nutrients" for a cleaner-burning, better-tasting final product.

For the science and debate around pre-harvest flushing, see our dedicated flush debate guide. This guide focuses on the maintenance flush technique itself.

When to maintenance flush

Signs you need a flush

Nutrient lockout symptoms:

• Multiple deficiency symptoms appearing simultaneously
• Brown, crispy leaf tips (nutrient burn) combined with deficiency signs on other parts of the plant
• Runoff EC/PPM much higher than input EC/PPM (salts are accumulating, not being used)

How to confirm: check runoff PPM. If your input is 800 PPM and your runoff is 2000+ PPM, salts are building up in the medium and a flush is warranted.

pH drift:

• You're feeding at pH 6.3 but runoff pH reads 5.2 (or 7.5)
• Accumulated salts shift the root zone pH out of the acceptable range
• Flushing removes the salts and allows pH to reset to your input level

Nutrient toxicity:

• Dark green, waxy leaves with downward curling (nitrogen toxicity)
• Severe leaf tip burn (general over-feeding)
• The plant is clearly receiving too much of something and you need to clear the medium

When NOT to flush

• Healthy, normally growing plants — if there's no problem, there's nothing to fix
• During the first 2 weeks after transplant — the root system is establishing. Flushing stresses it unnecessarily
• Immediately before or after another stressor (topping, major defoliation, transplant) — don't stack stress events

How to flush correctly

The process

1. Use plain water, pH'd to your target range:

   • Soil: pH 6.2–6.5
   • Coco: pH 5.8–6.0
   • Hydro: pH 5.5–5.8

2. Volume: run 2–3× the volume of the container through the medium:

   • 3-gallon pot = 6–9 gallons of water
   • 5-gallon pot = 10–15 gallons of water
   • 7-gallon pot = 14–21 gallons of water

3. Speed: water slowly and continuously. Let the water percolate through the medium, dissolving salts on the way through. Don't dump 10 gallons at once — the water finds the path of least resistance and channels through without contacting all the soil

4. Monitor runoff PPM: start by measuring the first runoff. It will be high (the salts being flushed out). Continue flushing until runoff PPM drops to within 50–100 PPM of your input water's PPM. This confirms the salts are gone

5. Let the medium drain completely. Remove all standing water from saucers. The medium should be thoroughly wet but not sitting in a pool

After the flush

• Wait before feeding. Let the plant use the water in the medium for 1–2 days (until the pot feels lighter / top inch of soil is dry)
• Resume feeding at 50% strength. The medium is now clean. A full-strength feeding immediately after a flush can shock the root zone. Ramp back up over the next 2–3 waterings
• Check pH of the first post-flush runoff. This is your new baseline. If it's in range, you're reset. If it's still off, there may be a deeper medium problem

Medium-specific considerations

Soil

Soil has natural buffering capacity (cation exchange, organic matter). It accumulates salts more slowly but also releases them more slowly during a flush:

• Flush volume: 3× pot volume
• Flush speed: slow — soil drains slowly, and fast flooding causes anaerobic conditions
• Recovery: soil microbes may be disrupted by the flush. A light compost tea after the flush helps re-establish biology
• Don't flush living soil / no-till beds — the biology IS the nutrient delivery system. Flushing destroys it. Address problems in living soil through amendments and top dressing, not flushing

Coco coir

Coco has high cation exchange capacity — it holds onto calcium and magnesium and releases sodium and potassium. This means coco accumulates specific salts differently than soil:

• Flush volume: 2–3× pot volume (coco drains faster than soil)
• Always flush with cal-mag water (not plain water) — flushing with zero-cal-mag water strips calcium from coco's CEC sites and triggers calcium deficiency
• Resume feeding sooner (within 24 hours — coco doesn't hold nutrients like soil)
• Flush more frequently than soil (every 2–4 weeks as preventive maintenance)

Hydro (DWC, NFT, recirculating)

"Flushing" in hydro means draining and replacing the reservoir:

• Drain the old nutrient solution completely
• Refill with fresh, pH'd water
• Run for 24–48 hours
• Drain again and refill with fresh nutrients at target strength
• Clean reservoir walls and air stones during the flush

In recirculating systems, salt buildup is continuous. Regular reservoir changes (every 7–14 days) prevent the need for emergency flushing.

Measuring the flush

PPM/EC monitoring

The most objective way to know if a flush is working:

Example: input water is 30 PPM. First runoff reads 2400 PPM (heavy salt buildup). After flushing 3× volume, runoff reads 80 PPM. The medium is clean.

pH monitoring

Common mistakes

1. Flushing when there's no problem — flushing a healthy plant removes the nutrients it's currently using. Only flush to solve a diagnosed issue (high runoff PPM, lockout symptoms, toxicity)
2. Flushing too fast — dumping the full volume at once creates channels in the soil where water flows through without contacting the salt deposits. Flow slowly and evenly over 30–60 minutes
3. Not measuring runoff — flushing without checking runoff PPM is guessing. You don't know if the flush worked unless you measure
4. Resuming full-strength feeding immediately — the clean medium absorbs nutrients differently than salt-loaded medium. Start at 50% and ramp up to prevent shock
5. Flushing coco with plain water — coco's cation exchange sites release calcium when flushed with calcium-free water, creating a calcium deficiency. Always include cal-mag (150–200 PPM) in coco flush water

For the pre-harvest flush debate, see our flush debate guide. For lockout recovery, see our lockout guide.