Cannabis Nutrient Flush Pre Harvest: Step-by-Step Guide

Quick Answer

A pre-harvest flush means feeding plain pH-adjusted water for the final 7 to 14 days so the plant draws down nutrients stored in the medium and tissue. Run water through the container until runoff volume is about one third to one half of the container volume, let the medium dry back, and repeat with each scheduled watering. Check runoff EC to confirm the substrate is trending lower. Flushing is a grower practice with limited published proof that it changes potency. Drying and curing control flavor and burn quality more than the flush does.

cannabis flushing guide

What a Flush Does and Does Not Do

Flushing leaches soluble salts from the root zone. That lowers the chance of salt buildup near the end of the cycle, when plants take up less water and nutrients. It does not strip minerals already locked inside plant tissue, and it does not create THC. If you grow in amended soil, expect a slower drawdown than in coco or water culture.

best flushing schedule for weed

Prerequisites

• pH meter and EC meter, both calibrated
• Plain water at the right pH for your medium
• Container with runoff drainage and a catch tray
• Record of your last feeding date and nutrient strength

Flush Steps

1. Stop all nutrient feedings when trichomes on the bud are mostly cloudy and you are 7 to 14 days from the harvest window.
2. Fill your reservoir or watering can with plain water.
3. Adjust pH to 6.0 to 6.5 for soil and 5.5 to 6.0 for coco, hydro, and other inert media.
4. Pour the water slowly across the entire surface of the medium, not just at the stem.
5. Continue pouring until runoff starts, then keep going until collected runoff reaches about one third to one half of the container volume.
6. Measure the EC of that runoff and write it down.
7. Let the medium dry back to your normal watering point.
8. Repeat the plain-water watering on your normal schedule until harvest.
9. Track runoff EC each time and stop flushing early if it stops falling after two cycles.
10. Water one last time 24 to 48 hours before chop, then let the medium dry slightly to speed the cure.

Timing by Medium

Soil

Start 10 to 14 days out. Amended soil holds salts longer, so expect a slow decline in runoff EC.

more on this topic

Coco and Hydro

Start 7 to 10 days out. These media release salts fast, so two or three plain-water cycles are often enough.

cannabis nutrient flush pre harvest

What to Watch

• Runoff EC should fall cycle over cycle. A flat reading means the medium still holds salt.
• Leaves that yellow from the bottom up are normal at this stage.
• Sudden leaf crispness or wilting signals overwatering, not a successful flush.

Mistakes to Avoid

• Flushing too early and starving the plant during bud swell
• Using unadjusted tap water with high alkalinity
• Skipping the dry-back, which invites root rot
• Treating the flush as a substitute for a slow dry and a long cure