Short answer

Flushing means you stop nutrient feed and give the plant plain water for the last days before harvest. Common windows run 7 to 14 days. Flushing is optional. Trials show little change in cannabinoids, terpenes, or yield. The main effect is on the growing medium, not the flower.

Cannabis Flushing With Plain Water: How Long and How To

What flushing does

Plain water lowers the salt concentration in the root zone. Growers measure this as EC or PPM in runoff. In soil, salts bind to organic matter. In coco coir and rockwool, salts stay in solution. A flush pushes those salts out of the pot.

cannabis flushing hydroponics

Flushing does not remove nutrients already inside plant tissue. Nitrogen, phosphorus, and potassium move through the xylem and phloem. Water alone cannot pull them from buds.

Cannabis Flush vs No Flush: Which to Pick Before Harvest

What research shows

Peer-reviewed data on flushing is thin. A 2019 trial by RX Green Technologies compared flushed and unflushed plants over 10 days. It reported no significant difference in THC, terpenes, or dry weight. Reviews note that most cannabis nutrient studies track yield and cannabinoid content, not flush length.

Cannabis Flushing: Benefits and Drawbacks Before Harvest

How to flush, step by step

  1. Pick a harvest date. Check trichomes with a loupe. Aim for cloudy heads with some amber.
  2. Stop nutrient feed. Use plain water at pH 5.8 to 6.5 for coco and hydro, 6.0 to 7.0 for soil.
  3. Water until runoff. In coco, runoff should reach 20 to 30 percent of input volume.
  4. Test runoff EC. Drop it below 1.0 mS/cm for a heavy flush. Some growers target 0.5.
  5. Repeat every 2 to 3 days. Do not let the medium dry out.

Timing by medium

  • Soil: 10 to 14 days. Organic matter holds salts.
  • Coco coir: 7 to 10 days. Coco holds less than soil.
  • Hydroponics: 3 to 7 days. Water carries salts away fast.

Risks

Early flush cuts yield. A plant without nitrogen stops building biomass. Overwatering during flush can cause root rot. Yellow leaves are normal late in flower. Yellow leaves at week 4 are not.

Convenience products

Bottled flush agents cost $15 to $40 per liter. Most contain water, magnesium sulfate, and sugars. They do not replace plain water. Automated dosing systems let growers drop EC with a timer. Convenience changes the method, not the plant biology.

Summary

Flush if your medium holds salts. Skip it if you run clean hydro and watch EC. Test runoff, keep pH in range, and harvest on trichome color.