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Flushing is a broad term growers use for removing mineral nutrition from the media and plant near the end of its life. The idea behind flushing is to initially remove excess salts from the medium, letting the plant use up what has been stored over the course of its life. I’ll go over organics and hydro. When to flush is not a set in stone formula based on some set amount of time. I base when and how to flush on the plant’s metabolism and growing style. I’ve found over the years longer flowering plants (10 or more weeks) seem to store more minerals in the bio mass for the long run and can need several weeks of plain water before the chop. While a plant that finishes in 7 to 8 weeks typically has a faster metabolism and may need just a week of plain water to use up what’s stored. Little new growth, swelling calyx, maturing pistols, and milky trichomes are an indication things are coming to an end.

Start of flush.

flush start

Day of chop.

flush end
I’ll go over organic growing in a living soil first. Organic growers don’t flush salts from their mediums because they don’t use them. They amend their medium with organic inputs initially like compost and then just add water, letting the biological life in the living soil provide nutrition to the plant when it asks. The most intimidating part of organic growing is getting started. Knowing how much of what input to use in your soil mix can be frustrating for new growers. I prefer to mix my own because I’ve never been happy with anything store bought. I’m a control freak even when growing organically. I basically don’t want certain minerals available in the medium being released when the plant is finished. No amount of water is going to rinse out minerals from organic matter. How much of what amendment can depend on what cultivar you choose, container size, finished plant size, etc. There are many factors to consider. A super soil requires more thought than permaculture or living soils. You can throw your soil out of balance easy by adding too much of this or that to a super soil mix. Start with someone’s proven formula the first few grows to learn all the variables your plant prefers tweaking the mix each time until you’re happy. A good living soil will not allow your plant to over consume minerals like a human force feeding 2000 ppm of salts in an oxygenated solution onto the roots on purpose or accident. Near the end of a plant’s life a sort of feed back loop begins. The plant stops feeding the microbes so in turn the microbes stop feeding the plant. This is known as the senescence or ripening stage. A sort of natural flush. As a plant starts to ripen, photosynthesis slows to a crawl. The plant has little need for much nutrition and will use up what’s been stored. During this senescence phase, microbes like bacteria will begin to leave the rhizosphere when photosynthesis slows causing fewer sugars to be exuded from the roots to feed them. Mobile minerals will be translocated up the plant causing older fan leaves to yellow and eventually fall off. Now with even fewer leaves, photosynthesis slows further creating even fewer sugars. This loop will continue until there are very few microbes left to feed the plant and the end result will be a smooth smoke.
In hydro, flushing is a multi step process. You flush the medium of all or most of the mineral salts and then wait for the plant to use up whats stored. When I flush my hydro systems I’ll run a decent amount of plain water through the medium initially to help break up any salts and I’ll wait 15 minutes or so before running some more water through the medium until my ppm is under 50 measured in the run off. I use RO water for flushing but it’s not necessary. Using a humic/fulvic acid will greatly decrease the amount of water needed to remove excess salts. I like to think of feeding a plant like a roller coaster. The ride slowly creeps up like your ppm, peaks, then slowly descends for a second until it drops off completely. What I mean is, I don’t feed my plants full strength nutrients and then take them away completely. I’ll lower my ppm to 600-700 from 800-1000 ppm for at least a week before I start my flush. I also run weekly plain water flushes to check my mediums ppm. If the run off ppm is higher than my reservoir ppm, I’m over feeding. Don’t be the person who flushes out their medium at the end and see’s 2000 ppm or higher on the meter. Those big numbers don’t mean you’re winning. 😉 Happy growing DGC and growers <3.