When and How to Flush Cannabis?

I was recently approached by a budding young cannabis grower who wanted me to test a sample of his first grow. As this was his initial attempt growing marijuana (performed in his bedroom closet), I hesitantly accepted his gift, telling him I’d get back to him.

Bracing myself, I only took a small toke initially. The harsh, chemical taste and smell immediately filled my senses, initiating a painful coughing spree. He must not have read the chapter on flushing cannabis plants in his ‘How to Grow Marijuana’ guide.

Anyone can grow cannabis; it’s an easy plant to cultivate. Procuring a smooth, cannabinoid-rich flower, on the other hand, requires some special handling as the plant reaches maturity. One way to capitalize on a marijuana plant’s best performance in taste and quality is a process called flushing.

What Is Flushing?

Flushing is the term used to wash away excess nutrients in a cannabis plant as harvest time nears. It is a simple process of eliminating elements residing in a plant’s root system. Flushing a marijuana plant with only pH balanced water clears out any residual nutrients hanging out in the roots.

Removing fertilizer applications also stimulates the cannabis plant to begin using up all of its stored nutrient reserves. Providing only pH adjusted water keeps the plant alive while clearing out undesirable contaminants. As the plant reaches maturation, flushing produces a clean, pleasing flavour and smell, allowing terpenes and flavonoids to fully mature.

When to Flush a Cannabis Plant

Having grown the beauty from the start, you probably have a timetable of when your marijuana plant will finish. Checking trichome development as the plant nears maturity is one way of determining when to begin flushing. As trichomes start to turn from clear to cloudy, it’s time to rinse with pH balanced water only.

This protocol can vary, though, depending on what type of substrate is used to grow the cannabis plant. Hydroponic and soilless substrates require a much shorter time to rid the plant of nutrients. Usually, a couple of days is all that is necessary for a hydroponic system to eradicate the nuts. Coco and other soilless mediums, on the other hand, typically take between 3-7 days to flush nutrients from the plant.

Soil grown cannabis plants take longer to flush depending on the type of soil. A standard potting soil mix generally comes pre-packaged with enough nutrients to sustain the plant for a few weeks of life in the early vegetative stage. However, additional fertilizer supplementation is required throughout the plant’s life cycle to provide nutrition to the marijuana plant.

Because nutrients harbour longer in these types of soils, a seven to fourteen-day flush is required to clear out any remaining elements stored within the plant. An organic soil, rich in microbial life, does not require synthetic fertilization. Microorganisms present within a living soil break down elements present in the medium, providing the cannabis plant complete nutrition throughout its life stages. Organic, living soils do not require flushing due to the absence of nutrient applications.

Don’t Begin Flushing Too Soon

The timetable of when a cannabis plant reaches maturation is dependent upon strain characteristics and growth development. As the plant progresses through the flowering stage, it is utilizing phosphorus, potassium, and other elements to produce dense, thick buds. If these essential nutrients are not available before the plant has finished flower development, the results are low-quality bud formation. Flushing a marijuana plant too early directly affects quality and yield.

Flushing Due to Nutrient Lock-out

There are times throughout a cannabis plant’s life cycle when you might notice yellowing leaves, burnt leaf tips, and stunted growth development. Often this is a result of excess nutrient and salt build-up in the plant’s root system. Improper supplement dosing or unfavourable pH levels directly affect the ability of a marijuana plant to distribute nutrients. Flushing with fresh, pH adjusted water clears excess nutrient build-up as the plant recovers from the stress.

How to Flush a Marijuana Plant

Flushing is a straight-forward process of applying water to the plant. However, always check the pH of the water source. A pH imbalance can negatively affect the growth development of a cannabis plant at any stage of its lifecycle. High-alkalinity reduces a plant’s uptake of valuable microelements, whereas acidic situations reduce macroelement absorption. Keep pH levels between 5.5 – 6.5 for hydroponic systems and 6.0 – 6.8 for soil growing.

Monitoring Total Dissolved Salts (TDS) is a beneficial practice throughout a marijuana plant’s life. From the time a cannabis seed germinates as the plant begins its vegetative stage, a good source for determining nutrient dosing is by taking parts per million (ppm) readings. This process requires ppm measurements of solutions going into the plant’s root system logged by data taken from excessive run-off captured in a basin.

TDS ratios during plant growth should show a slightly elevated measurement of ppm in the output readings. This result shows the plant is absorbing adequate nutrient levels, shedding a small portion of surplus elements. High ppm measurements, on the other hand, express nutrient overload. Flushing the cannabis plant until ppm levels drop will balance the nutrient levels.

Checking TDS levels when flushing a marijuana plant near the end of its life cycle provides a clear roadmap for harvest. As ppm readings drop with each flush, the window is closing fast. When TDS output is less than 50 ppm, it is time to harvest.

Know Your Cannabis Strain

Researching cannabis strains offers growers a better understanding of how long it will take a particular cultivar to mature. For instance, thick leafed indica plants often develop faster than sprawling sativa varieties. Having a base knowledge of how a strain typically performs provides a cannabis cultivator with a timeline for growth activity.