Flushing Cannabis Before Harvest: When to Do It and Why It Matters

Flushing cannabis means watering your plants with plain water, no nutrients, to clear leftover fertilizer salts out of the growing medium and roots. Most growers do it about two weeks before harvest. It's the last step before you chop, but don't let that fool you into thinking it's a minor one: the nutrients you feed during the grow build up in the plant, and if you skip the flush, your harvested buds can taste harsh and nasty and will probably have you coughing up a lung. Watering your cannabis plants with plain water washes the leftover nutrients out of the soil. The sweet spot to start flushing is two weeks before harvest. If your strain has an 8-week flowering period, you'd start the flush in week 6. Another way to tell it's time is to check the trichomes: once they start turning from clear to cloudy, with the first amber heads showing up, it's time to start flushing. The most common time to flush is right before harvest, but you can also do it during the vegetative stage to clear excess fertilizer out of the soil. Too much fertilizer can cause nutrient lockout, meaning the plant can't absorb the nutrients in the soil, and that holds back its growth. There are a few ways to flush. The most common is to cut off nutrients completely and water with plain water to rinse things out. Another is to run water equal to three times the size of the container through the medium: for a 20 L (about 5 gal) pot, that means about 60 L (about 16 gal) of water. The pH of the water you flush with matters a lot. Tap water or bottled mineral water is often in a good range for your plants, but always double-check with a pH meter.

When should you flush cannabis?

Flushing happens at the end of the cannabis grow cycle, usually about two weeks before harvest. It means watering the plants with nothing but water, no nutrients or additives, to get rid of the salts and nutrients that have built up in the growing medium and the roots. Flushing is especially important in indoor grows, where fertilizers and other chemical products are used to feed the plants. Clearing out that residue makes for a cleaner taste and lowers the odds of your buds holding on to off flavors. In outdoor grows, flushing may be necessary if you've fed heavily or suspect excessive salt buildup in the soil. In those cases, it's also best to flush about two weeks before harvest. Keep in mind that flushing has to be done carefully, keeping an eye on the pH and quality of the water you're using. It's also key to adjust your watering during this period so the plants don't get water-stressed.