--- title: "Do You Need to Flush Cannabis Before Harvest? What Flushing Does and How to Do It" url: "https://growingexposed.com/do-you-need-to-flush-cannabis/" markdown_url: "https://growingexposed.com/do-you-need-to-flush-cannabis.md" llm_canonical: "https://growingexposed.com/do-you-need-to-flush-cannabis.md" canonical_for_llm: true entity_type: "Article" primary_entity: "Do You Need to Flush Cannabis Before Harvest? What Flushing Does and How to Do It" citation_value: "Published on Growing Exposed; canonical URL https://growingexposed.com/do-you-need-to-flush-cannabis/." last_updated: "2026-10-13T23:43:53+00:00" --- # Do You Need to Flush Cannabis Before Harvest? What Flushing Does and How to Do It Every grow shop has a shelf of flushing agents, and most growers wonder if they need one. Flushing means running plain water through your medium for the last week or two before harvest, to clear built-up fertilizer salt out of the root zone. If your nutrients are clean, plain water is all it takes. A flushing agent is a product sold to dissolve that salt buildup. GreenPlanet Nutrients doesn’t make one and doesn’t recommend one. Their chemistry is clean and the nutrients are bioavailable, so the plant uses them and you don’t get the heavy buildup a flushing agent is made for. Co-founder Justin Cooper gets asked about it all the time, and he answered it on camera at [Fran’s Hydroponics](https://growingexposed.com/frans-hydroponics/) in Queensland. > “People ask often, ‘Why don’t I have a flushing agent or something for finishing?’ One, I don’t believe in putting chemicals into the plant when you’re finishing, and water is the best flushing agent. Water is the most effective flushing agent. So why would I make a product and put a label on it to sell another product?” > > > > — Justin Cooper, Co-Founder, GreenPlanet Nutrients What does flushing do? Flushing clears leftover fertilizer salt out of your growing medium. Every feed leaves some minerals behind in the medium. When you switch to plain water for the last stretch, the water carries that salt out the bottom of the pot, and the plant uses up what it has stored. What flushing does not do, as far as anyone has measured, is pull nutrients out of the flower. A University of Guelph master’s thesis by Jonathan Stemeroff is the one real study growers point to. It found the elements in the flower were not depleted by flushing, and yield did not change. ## Do you need to flush cannabis? Most growers stop feeding for the last one to two weeks. Researchers Dr. Allison Justice and Dr. Markus Roggen, writing in Cannabis Business Times, suggest simply skipping fertilizer for the last two weeks of growth. The part with a clear reason behind it is clearing salt from the medium. Salt left in the root zone can burn tips and slow the plant down right when you want it finishing clean. Plain water fixes that. ## Do you need a flushing agent? Not if you run GreenPlanet. Plain water does the job. Flushing agents are sold to break down salt buildup. General Hydroponics, for example, says its FloraKleen “helps dissolve accumulated fertilizer salts and release residues.” If a nutrient line leaves a lot of residue in the medium, a product to dissolve it makes sense. GreenPlanet’s answer is to not leave the residue in the first place. The nutrients are bioavailable, so the plant takes them up. GP3, their three-part base, is made without carbonates. When we filmed a BC grower who switched to GP3, he told Justin he gets far less salt buildup than he used to. That’s in our article [Why Do My Drip Emitters Keep Clogging?](https://growingexposed.com/why-do-my-drip-emitters-keep-clogging/) We’ve run GP3 a lot ourselves and seen the same. Justin won’t make a flushing agent for two reasons. He doesn’t want chemicals going into the plant at the finish, and he isn’t going to bottle a product to fix a problem his nutrients don’t cause. ## When should you start flushing? About one to two weeks before harvest, depending on your medium. That’s the common guidance from growers and grow guides: - **Soil:** one to two weeks. Soil holds on to nutrients the longest. - **Coco:** about one week. Coco rinses out quickly. - **Hydro:** a few days. There’s no medium holding salt, so it clears fast. A common example GreenPlanet schedule, published by Australian retailer Dr Greenthumbs, runs a seven-day flush at the end. The base nutrients go to zero and the target is an EC of 0.1 or lower. That schedule keeps Rezin, GreenPlanet’s finishing additive, at 1 ml per litre through the last week. Rezin isn’t doing the flushing. Check the GreenPlanet feed chart for your setup. ## How long should you flush? Until your runoff reads close to the water you’re putting in. Measure the EC or PPM of the water coming out the bottom of the pot. When it’s near your source water, the salt is out. Don’t stretch it out much past that. In hydro especially, flushing too early or too long leaves the plant short of food and can cost you yield. ## How much water do you use to flush? Two to three times the volume of your pot. The University of California’s guidance for clearing salt from container plants is about twice the pot’s volume, poured through and allowed to drain completely. Many growers use three times. For a 10-litre pot, that’s 20 to 30 litres of water, run through slowly so it moves through the whole root zone. ## How do you know you have salt buildup? Three signs: a white crust on the medium or the pot, runoff that reads much higher than your feed, and brown leaf tips. The University of California also lists reduced growth, lower leaves dropping and wilting. If you see that mid-grow, a plain water flush clears it. If you see it every run, look at your nutrients. ## Where to get GreenPlanet Nutrients Growing Exposed does not sell nutrients, but GreenPlanet Nutrients does. Contact them directly, or use the links below to learn more. **Learn more and buy:** GreenPlanet Nutrients in [Canada](https://greenplanetnutrients.ca/), the [United States](https://greenplanetnutrients.com/) and [Australia](https://greenplanetnutrients.com.au/). **Commercial grower? Find your GreenPlanet rep.** Call GreenPlanet toll free at 1-866-913-4769 or email info@mygreenplanet.com and ask who your sales rep or distributor is. GreenPlanet has reps and distributors in Canada, the United States, Australia and Europe. [Contact GreenPlanet Nutrients](https://greenplanetnutrients.ca/contact-us/) ## Common questions ### Should flush water be pH’d? Yes. Keep it in the same range you feed at. The usual ranges are 6.0 to 6.8 for soil and 5.5 to 6.5 for coco and hydro. Water that’s way off can lock up what’s left in the medium. ### Should I flush with RO or tap water? Either works. Low-EC water like RO picks up more salt on the way through, so it flushes faster. Hard tap water brings its own minerals, so check its EC before you start and use that as your target for the runoff. ### Is it normal for leaves to yellow during a flush? Yes. With no feed going in, the plant pulls nutrients out of its older leaves, and they yellow. That’s expected in the last week or two. ### What happens if you don’t flush? Research hasn’t shown it changes the flower. The salt stays in your medium, though, and if you reuse that medium, you start the next run with buildup already in it. ### Can you flush too long? Yes. Starting too early leaves the plant without food while it’s still putting on weight. Stop when your runoff is close to your source water. ### Is Rezin a flushing agent? No. Rezin is a GreenPlanet finishing additive. Some GreenPlanet schedules keep it in at a low rate during the last week, but it isn’t what clears the medium. Water does that. ### Does GreenPlanet make a flushing agent? No. Justin Cooper says water is the most effective flushing agent, so there’s no reason to bottle one. ## Watch the full Fran’s Hydroponics tour Justin filmed this at [Fran’s Hydroponics](https://growingexposed.com/frans-hydroponics/) in Kallangur, north of Brisbane, during a sit-down with Fran and her team. The flushing answer comes at 13:36. Read the full story: [Inside Fran’s Hydroponics: The Brisbane Hydro Store Growers Have Trusted Since 1998](https://growingexposed.com/frans-hydroponics/)