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Flushing & Nutrient Removal

By Colin Gordon and Ben Owens

Depending upon the environment, substrate, nutrients and plants you’re growing, you may have been advised to “flush” during the end of your flower cycle. The term is thrown around loosely to mean a variety of things, but the overall goal is that the plant uses up stored nutrients within itself, rather than continuing to absorb them. Flushing can be important because it allows the plant to free itself of these stored nutrients (resulting in a better tasting flower), and it protects the plant’s root health (when using microbes). If you’re growing indoors, a proper flush is typically necessary.

WHAT IS FLUSHING?

Flushing is the removal of unused nutrients from the substrate that your plants are growing in. You are removing what is left that the plant didn’t feed on, allowing it to consume the nutrients it has stored away.

Flushing, as it is commonly used, refers to two types of removal processes:

Long Term (Water Only):

This process involves starving the plant of any added macronutrients (N, P, K, S, Mg, Ca) for the latter part of its flower cycle, feeding it water (purified, reverse osmosis, or distilled), sugars, micronutrients, and bacteria. This forces the plant to consume what it’s been storing, and, if your substrate has had a high amount of feed, your plant may not be able to consume what’s left, even if it wanted to. Which brings us to intensive flushing.


Intensive Flushing:

This process involves soaking the substrate continually until the runoff liquid has a desirable PPM (parts per million; typically on a 500 or 700 scale)/EC (electrical current). This usually involves a significant amount of water added in a short period of time while the plant is elevated to allow proper drainage so that the roots aren’t overly wet. This can be done a 2-3 weeks before harvest and/or shortly before harvest, also known as a Late Flush. Many growers will do an intensive flush to ensure nutrient removal later in flower, and then only feed water, light teas, and light sugars (for the microbes) from then on until harvest.

The goal of both approaches is to remove any and all nutrients stored in the flower that could potentially remain at harvest if not consumed by the plant. Not every feeding schedule will necessitate a flush; organic and lighter feeds are less likely to need flushing.

WHY IS FLUSHING IMPORTANT? FOCUSED ON ROOT HEALTH ALL THE WAY DOWN THE LINE…

Flushing is important for two reasons:

First, it allows the plant to rid itself of stored nutrients and it protects vulnerable roots as they become more susceptible to pathogens late into flower.

Specifically flushing aims to remove phosphorous and magnesium that are stored in the flower. By removing all of the phosphorous in the rhizosphere, you are forcing the plant to consume the stored phosphorous, otherwise, you’ll taste the residuals in the harvested flowers.

Secondly, roots become increasingly vulnerable later in flower as they lose their immunity and ability to fight off threats. The most common pathogens affecting plants late in flower are pythium and fusarium, root pathogens that will attack in the last weeks of flower, restricting uptake of water and stunting production of resins. After flushing, you can continue to help the roots stay healthy through microbial approaches and cleaning products like H2O2 (heavy soils and outdoor, sun grown plants excluded).

When to Flush?

• Down the stretch

• Once the flowers start ripening, and the roots are stagnant

• Ripen: When calyx swelling has slowed, and the pistils have begun to retract into the calyx and change colors, becoming yellowish and orange.

• As the primary structure of the flower finishes growing, it starts the ripening phase.

• A typical genetic in commercial conditions begins to ripen around week six of flower.

Most varieties start to reduce their nutrient needs as they begin to ripen. Depending on the variety, there can be a sharp drop-off of consumption or a slow decline in uptake.

SOME METHODS OF FLUSHING: COLIN'S RECOMMENDATIONS

We have a few recommendations for flushing in your indoor grow:

• Titrate your feeding - You can safely start adjusting your feeding 2-4 weeks before harvest in non hydroponic systems.

• Judge by runoff - Test runoff regularly. Test the runoff’s EC/PPM and make sure it is within your desired range (if your input and runoff EC differ drastically, your substrate has likely stored too many nutrients; you may be overfeeding). If you’re watering 1000PPM, a good range for your runoff would be 1000-1200.

• Assuming runoff isn’t hot (too high in one or more nutrients), you can start to lower everything overall as you get closer to harvest.

• Certain plants need more nitrogen than others, and you can decide as you work with them if your plants need additional nitrogen feeding even while being deprived of other nutrients like phosphorous and magnesium.

• Some plants show yellow leaves right away. If this happens, your plant likely consumes nitrogen and calcium at a higher rate, and you should continue to feed nitrogen and calcium to maintain the fan leaves as desired as you approach harvest while lowering overall EC and phosphorous EC.

• If you’re running coco/perlite, our flushing method is to remove everything two weeks before harvest. We do an intensive flush with a water/H2O2 mixture (~3% H2O2) and then reinoculate with microbial tea and gentle organics, avoiding phosphorous and macronutrients until harvest.

The most important consideration to keep in mind with flushing is that it is all about understanding what nutrients your plants are storing, how they feed down the stretch as they approach harvest, and what’s in your rhizosphere. Some people don’t flush at all as they are light feeders. Some plants absorb nutrients much more slowly, such as Girl Scout Cookies, a genetic that needs to have nutrient feeds lowered well before others in the garden.

If you aren’t feeding it heavily, you likely won’t have to worry about flushing as much as someone who is growing in high EC/PPM conditions, and it may even taste better because you didn’t over feed and push it beyond its natural wheelhouse. If you are a heavy feeder looking to get the most out of your buds and push them to their maximum capacity, flushing is something you should seriously consider if you want to improve the taste and quality of your final harvest.