Flushing cannabis before harvest is the practice of stopping all nutrient feeding and giving your plants only water for the final one to two weeks. The goal is to force the plant to use up stored nutrients, which some growers believe improves the taste and smoothness of the final flower. However, recent research suggests that flushing may not be necessary and can even reduce yield. This guide explains the process, when to do it, and what to expect.
Why Flush Cannabis?
Growers flush for three main reasons: to remove excess salts from the growing medium, to reduce the harshness of the smoke, and to save money on nutrients. In soil and coco, flushing can prevent nutrient burn and lockout. In hydroponics, it helps keep the root zone clean. But the evidence for improved taste is mixed. Some studies show no difference in cannabinoid or terpene content between flushed and unflushed plants.
When to Start Flushing
Start flushing about one to two weeks before harvest. The exact timing depends on your medium and the plant's needs. In soil, begin 10 to 14 days out. In coco or hydro, begin 7 to 10 days out. Watch your trichomes. When they turn cloudy and some amber appears, harvest is near. Stop feeding at the first sign of cloudiness.
How to Flush Cannabis: Step-by-Step
Follow these steps to flush your plants safely:
- Stop all nutrient solutions. Do not add any fertilizer or supplements.
- Check the pH of your water. Adjust it to the proper range for your medium (6.0-7.0 for soil, 5.5-6.5 for hydro).
- Water the plant with plain pH-balanced water until runoff occurs. This washes away excess salts in the medium.
- Repeat watering as needed, allowing the medium to dry slightly between waterings. Do not let the plant wilt.
- Monitor the plant daily. Leaves may yellow as stored nutrients are used. This is normal.
- Harvest when trichomes reach your desired ripeness, usually when most are cloudy with a few amber.
What to Expect During Flush
During flush, the plant will stop growing new leaves and buds will ripen. Older fan leaves may turn yellow and drop off. This is a sign that the plant is using its stored reserves. Some growers see a slight loss in yield because the plant stops bulking up. If you are growing for maximum weight, you might skip flushing or shorten it to a few days.
Does Flushing Work?
Research on flushing is limited but growing. A 2020 study in Frontiers in Plant Science found that flushing did not affect the taste or chemical profile of cannabis. Another study from NCBI showed that flushing reduced mineral content in plant tissue but had no impact on THC or CBD levels. Many experts now recommend flushing only if you have overfed or if you want to save on nutrients. For most home growers, a short flush of 5-7 days is a safe compromise.