By Jeff Lowenfels

I can’t think of any other subject that creates more debate amongst growers than the “flush vs. no-flush” argument. For many of you, just reading that last sentence sends a chill down your spine and triggers flashbacks of the Kennedy-Nixon debate or, depending on your age, the Lincoln-Douglas.

In any case, for as long as this old geezer can remember, hydroponics growers have been switching to pure water, or in modern times, “flushing or clearing solutions,” a couple of weeks before harvest. According to this theory, plants are forced to use up stored nutrients in order to maximize smokability and taste after curing.

Somewhere along the line, many soil growers adopted the practice; perhaps when bulletin boards began noting that tobacco was flushed before it was cured ostensibly to, again, improve taste. Flushing was said to clear the soil around the roots of nutrients, starving the plant into using up excess nutrients.

Claims of better taste are always the predicate here, along with ash color. The idea is that a harvest of plants that has undergone a flush results in plants that not only taste better, but also produce a white ash when smoked. Unflushed weed is said to taste of the chemicals used to fertilize and adjust pH. Moreover, the resultant ash is said to be black.

I really don’t need to frame the arguments any further. Sure, there are variations, which result in familiar, similar side debates.

So, where is the beef when it comes to flushing? There are only 17 nutrients a plant needs. The rest are filler, but we can go into that at a later time.

Each of these nutrients moves in to and out of plant cells though special channels comprised of proteins that are embedded in the plasmalemma, a barrier membrane that prevents things from moving through plant cells in either direction. Amazingly, each nutrient has its own, specific tunnels and can only move through those.

So, already, flushing is limited by the presence of these tunnels. They don’t just open and shut because clean water or some flush-it solution is running by roots. Embedded proteins in the plasmalemma control what goes in and out of a plant cell.

Next, only sulfur, calcium, iron, zinc, boron, copper, and manganese are immobile in the first place. Once a plant synthesizes these molecules, they are not going anywhere, no matter how firmly you believe in flushing. Adding carbon, oxygen, and hydrogen to the list of unflushables leaves only seven of the plant’s nutrients that even could be flushed out.

  So which are mobile nutrients? These are nitrogen, phosphorous, potassium, magnesium, chlorine, molybdenum, and nickel. Once they are incorporated into the plant, they can be relocated if an area of much greater demand for it develops. But this remobilization is not that easy of a process. It requires enzymes, built with nitrogen, and takes energy, which requires phosphorous (ATP). So, if you flush, how do you get things to move out without using nitrogen and phosphorous?

Can you have it both ways? Not generally.

Are you beginning to see the picture? A plant doesn’t simply dump out excess nutrients just because there is water running by its roots. Sure, that is how it works when you are talking about osmosis in a science classroom. There, the barrier is magically permeable to everything, with no embedded proteins to build, operate, and travel through. When textbooks talk about osmosis, things move in both directions constantly as the material transported through the membrane reaches equilibrium on both sides of it. It just doesn’t work that way in and out of plant cells.

Don’t get upset. Read my book or check out other resources. In addition to new thoughts about flushing your plants, you’ll start to understand what depriving them of nutrients for a week of their life does to your final product. And, don’t get me going on those subjective ash color tests.

Jeff Lowenfels is a national lecturer, former president of the Garden Writers of America, and the author of Teaming With Microbes and Teaming With Nutrients, which describe the soil food web and how plants absorb and utilize nutrients.  

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