To Flush or Not to Flush

Columns - Hort How-To

What exactly is being gained — and lost — in this commonly accepted practice?

Although “that is the question” is the rest of Willy Shakespeare’s original phrase, a majority of growers large and small would likely follow that line with: “There is no question … you have to flush.”

Flushing is the use of pure water during a time period leading up to harvest to leach nutrients from the root zone. It is commonly held to be an essential last step in the cultivation process, and is associated with coaxing flavor and smokeability from the product.

The motivation for understanding flushing actually has to do more with money, not cultivation. A five- to 10-day flush represents as much as 12.5 percent of the plant’s flower time. Plants do not add mass in the absence of nutrients, so the loss of yield at the height of the plant’s ability to produce it makes this flavor-enhancement technique an expensive one.

Also, the longer plants sit in a flower room, the more they cost. The turn rate for a flower room running eight-week plants is 52/8, or 6.5 turns room turns per year. If the plants can be brought in at seven weeks at or near “with flush” harvests, the turn rate for that flower space goes up to 7.4, representing a significant increase in production even though seven-week harvests generally will be smaller than eight-week harvests.

The potential to gain almost a whole additional crop through that flower space each year is sufficient enough not to ignore.

The Flushing Affect

Many ways exist to deliver nutrients from entirely media-sourced nutrients (true organic) or by the use of external nutrient delivery. The flush can have a different effect on different media.

Media has a profound influence on a grower’s nutrient program. Media and nutrients need to be matched for effective nutrient delivery. Some media can hold large reserves of water and nutrients, while others require constant irrigation.

Particles in mineral and organic soils have negative electric charges, which, while small, are strong enough to attract and hold positively charged potassium, calcium and magnesium (ions) among others. Soilless and organic media have the ability to build up nutrient reserves in the media when the plant is given even small amounts in excess of its uptake capability. Those reserves can be drawn on if the plant is not nourished on schedule, but this characteristic of soilless and organic media also can contribute to a buildup of excessive levels of nutrients, which, if left untreated, can result in plants suffering nutrient toxicity at worst and water uptake reduction at best.

The proper reaction to high nutrient concentrations is flushing — used to keep roots and the plant safe from damage by excessive nutrient levels.

Because organic media particles are negatively charged, negatively charged nitrate nitrogen and phosphate ions are repelled from the particles and are the first nutrients to leave the root zone when a flush is applied. Without nitrogen in the root zone, the plant’s biological processes will be starved, and it will turn to scavenging and moving nitrogen, phosphorous and potassium (NPK) and magnesium from older tissue to the current growing points.

Although positively charged nutrients are attracted to the media particles, sufficient amounts of water will dissolve the nutrients off the media particles and remove them from the root zone. Because of the media particles’ propensity for holding nutrient ions, flushing leaves the root zone of an organic media with a hard-to-predict nutrient profile.

Rockwool growing mediums do not have the electrical charge character of organic media and therefore cannot store nutrients other than where nutrient solution is trapped in pockets in the media. A flush of rockwool effectively removes all nutrients.

Plants, however, do not stop growing when they are being flushed. Rapidly expanding buds can be seen even while the flush is removing the nutrients.

High nutrient levels are not typical in nature, so when nutrients are encountered, all are welcome. In fact, at certain pHs, plants can uptake lethal nutrient concentrations. This also brings to mind the fact that once the nutrients have been flushed from the root zone, the chemistry is altered, and the pH is exposed to variations. This can further reduce nutrient absorption.

Under normal growing conditions, when nutrient availability exceeds demand, plants will store nutrients. When availability is low, they will cannibalize older plant material, robbing NPK and magnesium, and moving them to needy growing points.

Think about that. Flushing causes nutrients, especially nitrogen, to be moved into the buds to support growth. So although growers aim to remove nitrogen from the buds by flushing, the plant concentrates nutrients in the buds from other places in the plant. These nutrient concentrations are less than if the plant were nourished during the flush week, but concentrating less in the plant biomass is entirely different than removing nutrients from the biomass.

Now that the plant has found a way around the lack of NPK and magnesium, it needs calcium. But the calcium supply needed to complete potential growth is limited. Calcium is not relocatable; it is laid down in tissue cell walls and, thereafter, immobile. If all external nutrients are cut off, the plant will not be able to put on the new growth that captures water in the biomass, adding the majority of mass to new growth.

Calcium and magnesium are seldom cut off completely, however. Other than reverse-osmosis water, the water used to flush contains some amount of both calcium and magnesium. So even in flush, plants are typically receiving some critical nutrients, which means that plants generally do not totally stop adding weight. But the removal of nutrients does result in a significant reduction in plant growth.

A Simple Test for Growth

A simple test is to put two flower plants (unsupported – no trellis) on bathroom scales and watch how the weight varies over time. Each time the plant is watered, note the weight just before it’s watered. That weight will steadily increase through flower, and slow down or level off following a flush.

Compare that to a plant for which nutrients are not withheld during that last week. The mass of the nourished plant should surpass that of a flushed plant. That is not a guess, it is a certainty, which means that flushing costs yield. The difference between the final weight of the nourished plant and that of the flushed plant represents the yield that is forgone in the name of quality.

We believe most growers recognize they are trading yield in favor of the quality enhancements. But it is not clear they have an appreciation for what exactly that quality is costing them or what exactly is being gained.

To us, the concept that flushing somehow changes the chemistry in plant tissue that has been laid down for weeks requires a scientific explanation because that concept seems akin to claiming that the car engine is cleaner after washing the car’s hood. Nutrients are locked in the plant, and an external flush cannot undo the complex biology that locked them in.

The levels of nutrients concentrated in plant tissue are up to 1,000 times more concentrated than those nutrients in the root zone. One-hundred ppm nitrogen solutions produce plants whose leaves can contain upwards of 10,000 ppm of nitrogen. Since we have established that none of those 10,000 ppm are going anywhere but within the plant, we are looking for explanation of how that concentration is being reduced by a flush to the extent it can affect flavor. We haven’t found it yet.

This is significant cost we’re talking about here, and it should get every grower’s attention and nudge them to run their own trials to get real feedback to see what happens when they push the plants more.

Personal experience is the best teacher. There are thousands of plants out there right now that are just about to be flushed, and anyone can flush one set of plants, while nourishing another during the last week in flower. Keep the two harvested materials separate, and ensure equal drying/curing treatment. This isn’t scientific, but a large number of responses could drive some very interesting results.

A Theory Worth Testing

So if you want to help forward the cannabis-cultivation knowledge base, run that test on some of your plants and get back to us.

It can be dangerous to ask for data when you have laid down a position, since you may get data that doesn’t support that position. But that is science: You make a claim and test it. If the data comes back in support of not flushing, then growers may well want to rethink their growing plan. The impact of this could be significant, as cutting 12 percent off of a plant’s production time (by eliminating the flushing period) would have more impact on cost per gram than anything else we can think of, and it’s not hard to do.

If the data comes back in favor of flushing, we know we have not considered all the variables, and we will set out in search of those.

There is no question that flushing removes nutrients from the root zone, so the key question is whether flushing the root zone has any influence on the sensible qualities of the plant’s biomass — in particular, smokeability and taste control. In the meantime, we all get to exercise our brains a little.

About the Authors: Kerrie and Kurt Badertscher are co-owners of Otoké Horticulture LLC (, and authors of “Cannabis for Capitalists.” They have worked with large-scale cannabis producers for more than 5 years. Kerrie has been involved with plants her entire lifetime and earned certification as a Professional Horticulturalist by the 100-year-old American Society for Horticulture Sciences. Kurt brings his 34 years of corporate experience and operations management skills to bear on the business challenges of cannabis cultivation.

Editor's Note: If you conduct tests as suggested in this column, please share your results with CBT by emailing the columnists at