When and How to Flush Cannabis Plants Before Harvest

Cannabis flushing is an important step in the process of growing and harvesting cannabis, but new or inexperienced growers may not be familiar with the term. As you feed the plant nutrients while it is growing, the plant absorbs some of the excess nutrients and stores them – much like the way a human stores fat. Humans go on diets to force their bodies to burn stored fat, and growers get rid of their cannabis plants’ excess nutrients in a similar way: by removing all of the nutrients in the plant’s soil and giving it nothing but pure water, forcing the plant to get rid of stored nutrients. Simple enough!

But wait – nutrients are beneficial to the plant, right? Why would you want to remove them? If your cannabis hasn’t been flushed, the remaining nutrients can negatively alter the taste and smell, and can make the plant much harsher on the throat and lungs. Fortunately, flushing your cannabis plants is easy, and doesn’t require any equipment that you won’t already have.

When to Begin

The best time to start flushing your cannabis is at the early end of the harvest window. If you miss it by a day or two, though, don’t worry – cannabis’ ideal harvest window lasts for several weeks, so there’s a little bit of room for error here.
The most important thing to avoid is starting the flush too early and taking away nutrients that your plant still needs. To avoid this, wait until you’re sure that your plant has entered the harvest window.

Grow tent full of flowering cannabis

At that point, 40 or 50% of the white hairs on the bud should be darkened and curled-in, and if you use a magnifying glass you should be able to see that the once-clear trichomes are now cloudy or amber, halfway between translucent and milky white. If you see those signs, you’re ready to start the flush.

How to Flush Cannabis

To flush your cannabis, you’ll begin by flooding the soil with pure water, waiting a few minutes, and then draining it and repeating the process one more time to remove the excess nutrients from the soil. Each time you do this, you should use three times as much water as you have soil. If there’s one gallon of soil in the pot, use three gallons of water, making sure not to overflow the pot.
After that, all you need to do is stop providing your plant with nutrients and continue to water it at the same rate as usual. Tap water should work just fine, since it is usually within the healthy pH range for cannabis, but it never hurts to check the pH level and adjust if necessary.

When & How Much Water?

Once the soil is dry, flush your plants with 3L of water for every 1L of soil.  

During the flush, keep a close eye on your plant to make sure that it doesn’t yellow too much. A little bit of yellowing is normal, but you’ll want to harvest before the sugar leaves have yellowed. This will help the cannabis keep longer and deteriorate more slowly in storage.

The ideal length of a flush depends on what the plant is growing in. If it’s growing in soil, the flush should last up to two weeks. If it’s growing in coco coir, the flush should be about a week. If it’s in any sort of hydroponic setup, the flush should only last a few days. If you’re growing in amended soil, you shouldn’t flush at all.

Flushing a cannabis plant before harvesting improves the flavor and smell, and also results in bud that is much smoother when smoked. There aren’t any hard and fast rules about when to start flushing your plant, or how long the flush should last – only guidelines. The rest of it comes down to skill and practice, and you’ll get better at it over time. Happy growing!

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