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New Experiment in Flushing

Flushing is a common practice in the cannabis industry that involves restricting fertilizer application and leaching nutrients from the growing substrate during the final weeks of flowering.

Our researchers performed a series of experiments to determine the horticultural merit of flushing. The study was completed in collaboration with The Cannabis Research Coalition , Dr. Jim Faust’s Flowering Physiology Laboratory to address current industry challenges in cultivation and postharvest. Smoking quality was not assessed because human subject testing is highly regulated in academic research. Instead, we sought to determine how the duration of flushing, or not flushing at all, affected bud yield as well as the elemental and cannabinoid concentrations of flower buds. The flushing durations were 0, 1, 2, 3 or 4 weeks prior to the anticipated harvest date (See Table 1 below.) The “0-week” flushing treatment was a control group that was not flushed and received fertilizer for the duration of the experiment.

Plants of two cultivars, The Hemp Mine Jack and Southern OG, were grown in 1-gallon plastic containers filled with a peat-based growing medium. A liquid fertilizer solution (200 ppm N, 15-5-15 Cal-Mag Special) was applied with each irrigation, and the fertilizer solution had an electrical conductivity (EC) of 1.7 mS/cm.

Figure 1. Flushing treatments were applied to The Hemp Mine Jack plants for 0, 1, 2 or 3 weeks prior to harvest. Plants are ordered from 0 weeks to 3 weeks, left to right. The 0 week treatment (far left) was a control that was provided fertilizer for the duration of the experiment. Flower bud mass decreased with the 2- and 3-week flushing treatments, while the mass of the trim remained the same. Thus, flower:trim decreased, which is typically undesirable.

Flushing Protocol Flushing techniques vary considerably among cannabis growers, so we devised a flushing procedure that was clear, easy to communicate and simple to replicate. Our flushing procedure consists of two parts: reduce substrate EC to less than 0.2 mS/cm and irrigate only with water until harvest.

To reduce substrate EC to less than 0.2 mS/cm, we employed drip emitters to slowly pass clear water through the growing substrate, thereby leaching nutrients out the bottom of the containers. Each drip emitter passed 3 liters of water per hour. For a 1-gallon container, 15 liters of water was typically sufficient to achieve substrate EC of less than 0.2 mS/cm, but we often applied 20 to 25 liters to be thorough. The volume of water needed to flush a container to less than 0.2 mS/cm will largely depend on the fertilizer program, the substrate used, and container size. The method used to assess EC leachate was the PourThru Method (Mattson, 2008). After the initial flush of the growing substrate, the plants received only clear water as needed until the end of the experiment. (Editor’s note: For a detailed description of the PourThru Method, read “Troubleshoot Nutrient Problems Before They Occur” in the August 2019 issue of CBT.)

What We Measured At harvest, each plant was cut at the soil line. Vegetative and flower (inflorescence) mass were separated. The vegetative mass is defined as the sum of the branches and leaves immediately below the inflorescence, while the inflorescence mass is defined as the sum of the flowering shoots starting with the lowest cluster of flowers on each stem. All plant material was dried and stored at 68 degrees F and 50% relative humidity for three weeks. After drying, the vegetative and inflorescence dry weights were recorded. The dried inflorescences were further broken down into flower buds and trim (leaves, bracts and internodes). Flower buds were ground and tested for elemental and cannabinoid analysis.

Figure 2. Flushing treatments were applied to THM Jack plants for 0 to 3 weeks prior to harvest.

Finding 1: Flushing Duration Affects Dried Bud Weight The dried flower bud weight in the control group (no flush) and 1-week flushing group were the same; however, dried bud weight decreased as the flushing duration increased from two to four weeks. For example, dried bud weight in Southern OG was 8%, 12% and 29% lower as flushing duration increased to two, three or four weeks, respectively. These results are expected, as limiting nutrient availability can decrease yield for any crop, (see Evans and Fischer, 1999, in “Further Reading” section on the left).

Flower-to-trim ratio (flower:trim) is a useful measure because it describes the partitioning of mass within the inflorescence, reflecting whether flushing affects flower and trim growth similarly. For THM Jack, flower:trim was approximately 30% lower in flushing durations two weeks or more. Fig. 1 (above) illustrates the smaller buds observed in the 2-week and 3-week flush groups, while the trim is similar across treatments.

Vegetative dry weight decreased in flushing durations greater than or equal to three weeks. These longer flushing times resulted in increased leaf necrosis and abscission (shedding) throughout the canopy (Figs. 2, above, and 3, below). Inducing leaf abscission before harvest could be useful for improving processing and trimming efficiency in post-harvest.

Figure 3. Flushing treatments were applied to Southern OG plants for 0 to 4 weeks prior to harvest.

Finding 2: Flushing Affects Elemental Concentrations in Dried Buds Elemental concentration in dried buds was highly sensitive to flushing treatments. Nitrogen, phosphorus and potassium percentages were greatest in the control group (no flush) and decreased as flushing duration increased. Deficiency symptoms in the inflorescence are shown in Fig. 1.

Tissue analysis frequently demonstrates that calcium concentrations increase as nitrogen decreases and vice versa. In this study, calcium concentrations increased in flushing treatments of three weeks or more. Providing high levels of nitrogen during flowering has been well-documented to increase crop susceptibility to various pests and diseases (Havlin et al., 2013), while high levels of calcium can reduce susceptibility to Botrytis infection in ornamental flowering crops Bennett et al., 2020). However, flush durations that increased calcium levels also decreased dried bud weight, reducing overall yield. The yield loss from a Botrytis infection must be weighed against losing yield from longer flushing durations. Thus, restricting nitrogen and increasing calcium concentration may be beneficial for controlling pests and disease in the final weeks before harvest, but further work is needed to test this idea.

Sulfur concentration of dried flower buds increased in THM Jack when flushed for one week or more. Sulfur-containing compounds are involved in the flavors and odors of various plants, but these are often perceived as undesirable, e.g., skunks, burnt matches, and rotten eggs. For cannabis cultivators and consumers, however, elevated sulfur levels in buds may be desirable.

Finding 3: Cannabinoid Concentration is Affected by Flushing CBD concentration in the flower buds was similar among flushing treatments in THM Jack, but CBD concentration increased with flushing duration in Southern OG. For example, CBD accounted for 13% of dried bud weight in the control group but 16% in the 4-week flush group. However, it is unlikely that flushing is stimulating additional synthesis of CBD. Instead, flushing decreases dried bud weight, which reduces the denominator in measurements of CBD per gram of flower buds. Often referred to as yield dilution, this means the concentration of compounds in plant tissues increases as tissue mass decreases.

Take-Home Message The potential value of flushing largely depends on the cultivation goals and the end-product, e.g., extracts or dried flower buds. For example, flushing for two weeks or more resulted in large decreases in dried bud weight and total yield of CBD. Lower yields are not acceptable when cannabis is grown for extraction, so flushing for two weeks or more would not make sense. However, if visual characteristics, such as bud compactness, are improved with longer flushing times, then lower yields may be acceptable when grown for dried flower. In the 1-week flushing treatment, dried bud weight and CBD percentage were unaffected while elemental concentration in the flower buds decreased. These results seem to align with the fundamental goals of flushing, but how changes in elemental concentration affect smoking quality remain unconfirmed. The trade-off between yield and product quality has been described in many crops outside cannabis. Cultivation strategies that improve yield are not always compatible with cultivation strategies that improve quality, so end-product goals are critical for optimization. Michael Alden, M.S. is a Ph.D. student and graduate research assistant with Dr. Jim Faust at Clemson University. Jim Faust, Ph.D. has worked in the floriculture industry for over 30 years and conducts experiments with industrial hemp. Allison Justice, Ph.D. owns and operates The Hemp Mine, a South Carolina-based vertically integrated hemp company. See Michael Alden Live! Alden will present exclusive findings on flushing, as well as studies testing other common cannabis cultivation and crop steering practices. Find details here:

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