Flushing Cannabis Guide: Everything You Want to Know
SPARC cannabis farm photo by David Downs.
Flushing cannabis plants is a controversial subject
Some growers think its usefulness is a myth while others think it’s an essential process that creates the best buds for smoking and vaporizing.
There are many reasons to question the efficacy of flushing:
1. Eliminating or lowering the availability of essential nutrients slows growth at all stages, including the last weeks of flowering.
2. It is difficult to flush large plants that are grown in bags containing 200-300 gallons (760-1140 l) of planting mix. The plants’ roots have a reservoir of nutrients to draw from. Yet, these plants are savored by connoisseurs.
3. There are no double-blind studies that have been performed to test the efficacy of flushing.
4. Although certain stresses increase trichome production, it is doubtful that nutrient deficiency is one of them. Trichome and oil production is expensive; it requires the plant to expend energy. It seems counter-intuitive that depriving nutrients would increase cannabinoid/terpene production.
5. Some gardens using perpetual harvest techniques as well as aquaponic grow methods have no provision for flushing but produce fine connoisseur buds.
6. Large amounts of Calcium (Ca), one of the mobile nutrients, are required for cell division. To mature, plants must grow new cells. Without a constant supply, maturation slows.
7. Only a small amount of Boron (B), another mobile nutrient, is required for plant growth. Without it in sufficient quantities bud maturation slows or stops. So, long flushing periods are likely to reduce yields.
Despite the lack of peer-reviewed studies regarding efficacy, the overwhelming majority of marijuana growers flush.
The consensus is that depriving plants of nutrients during the last phases of flowering results in a higher quality bud.
The basics on flushing cannabis
There are a multitude of flushing methods, but they share a common goal: to remove most of the nutrients available to the roots, thereby encouraging the plant to use the non-assimilated salts and nutrients still remaining in the plant.
The result is the plants will hold few nutrients in their raw form, and instead incorporates them into their tissues or into phytochemicals released by the roots.
The metaphor I like to use is the human body. We take in food for energy and nutrients. What we don’t use, is then stored in our fat cells. When we reduce our intake of food, our body uses this stored fat for energy.
In order for roots to absorb nutrients, the nutrients must be dissolved in water.
Nutrients that are precipitated, either of a solution or bound in a molecular matrix, are not available to the roots even if they are plentiful.
Flushing with water rinses out the nutrients that are already in the solution.
To rinse, use tepid water (about 75° F/ 24° C) that is adjusted to a pH of about 5.8-6.0, which is the range at which the nutrients are all soluble.
With warmer water flushing, more nutrients dissolve and rinse away. The more you rinse, the more nutrients will be carried away.
Nitrogen is the most soluble nutrient and it is the most likely to affect flowering negatively.
Even using a rinse that drains only 10% of the added water removes some of the nutrients.
Flushing essentials: What you need to know and tools to use
Above are two meters from HM Digital: The PH-200 (left) is a professional grade pH/Temp meter ideal for all pH testing. With an advanced microprocessor and ATC (Automatic Temperature Compensation) it provides highly accurate, dependable readings The COM 300 (right) is a handy combo meter that measures pH/EC/TDS/TEMP. Both are waterproof for durability.
pH, EC and TDS meters are essential tools required to properly adjust water nutrient solutions. When the pH is out of suggested range (that is, too high or too low) nutrients fall out of the solution and precipitate. This means that dissolved substances form solids and drop out of solution.
Plants drink their nutrients rather than eat them, so nutrients that are precipitated are not available to them.
As the flush continues, the PPM of the rinse water drops. This process can be stopped at any time. By leaving some nitrogen in the media, the plant still has some resources for growth, without the abundance that promotes vegetative growth.
Check the PPM of dissolved solids using a meter for both the soil and the water. With larger gardens, it may be more efficient to use just a few plants for trials.
Two flushing techniques to consider:
1. Start with a passive flush
This is when you let the plants use the reserves in the mix. Then perform an active flush close to ripening.
2. Perform a constant flush
Each time the plants are irrigated, add enough water so 10-20% of it drains. The drain water will be rich in nutrients, so there is little salt build-up in the medium. There are few excess nutrients to flush near ripening.
Note:
Some growers encourage vigorous vegetative growth before the plant makes a total switch to flowering. If doing so, leave the plants un-flushed, or even supply a small amount of nitrogen during the first two weeks of flowering and let it deplete naturally.
pH
The solubility of the nutrients in the planting mix is pH dependent. The salts, that is, the nutrients soluble in flush water, are adjusted to 5.8-6 pH. It removes more nutrients than water that is not pH-adjusted.
Water Temperature
Salts are more soluble in warmer water. Adjust flush water temperature to 75° F (24° C) if possible. More nutrients will be flushed.
Cannabis flushing techniques: Passive, active and chemically-enhanced
Cherry Cookie stalked capitate trichome photo Doobie Duck.
Marijuana growers use a variety of flushing theories and techniques. There is no one correct technique. Just because it isn’t mentioned here, doesn’t mean it isn’t helpful.
Flushing techniques can be divided into three main categories: passive, active and chemically-enhanced.
What is the difference between passive and active flushing?
The difference between passive and active flushing is that at some point in the plant’s rush to ripening, either the caretaker or nature makes a decision to help the plant ripen by removing nutrients from the soil. The primary nutrient that is removed in this process is N, although many other water-soluble nutrients are removed.
Plants growing in mix or soil are flushed using water. Dissolved and soluble salts are drained out. Depending on how thorough the flush, much, most, or nearly all of the soluble nutrients are removed. They are often replaced with a new flowering formula that may contain some macro or micro fertilizers as well as hormones, enzymes or sugars.
Plants react quickly to this sudden change in the environment by focusing their energy on maturation of flowers rather than continued production of flowers.
Active flushing
Most modern cannabis plants ripen 7-9 weeks after being forced to flower. Their planting media may include ingredients that gradually release nutrients and is often irrigated using a water/nutrient solution. If the planting media is composed of the usual ingredients such as peat moss, coir, or compost, its copious carbon-containing molecules bind some of these nutrients and is ready to release them when nutrient levels get low.
Depending on the size of the plant and container, the media type and the technique being used, this process can take up to three weeks. Irrigate with nutrient-free water so no new nutrients are provided for the last 1-3 weeks of flowering.
For instance, a nine-week plant won’t be fed after the sixth, seventh or eighth week of flowering.
A vegetative plant with N deficiency. When N is lacking, the mobile nutrient is transferred to the canopy where it is most effective. The bottom leaves lacking N turn yellow.
An example of nitrogen toxicity. The fan leaves are turning yellow. This plant was receiving too much flowering formula and was over-watered.
However, enriched organic soils and planting media, especially if they have been used for more than one season, are likely to contain organically locked nutrients that mycorrhizae and other rhizosphere organisms will immediately begin to unlock; this provides more nutrients to the roots.
Then, as the residual nutrients are used up, the plant taps into its own reserves. Some of the minerals are mobile— nitrogen (N), phosphorous (P), potassium (K), magnesium (Mg), molybdenum (Mo)—and they translocate to the canopy top, where new growth is happening and where the plant’s energy provider, the lights or the sun, is available.
As the leaves lose N and Mg they turn yellow. The loss of P and K results in leaf edge curl and dead spots. After the plant has extracted the valuable nutrients from the leaf, leaving mostly cellulose, it has no further use and it withers and dies.
These are indications that the flushing is working.
The goal is to time the total loss of nutrients with ripening of the buds so nutrient deprivation does not cause appreciable loss.
Calcium (Ca), sulfur (S), iron (Fe), boron (B) and copper (Cu) are immobile and their deficiency symptoms, which usually don’t occur in late flowering, can be seen in the new growth. Iron (Fe) deficiency, though rare, often results in bright yellow leaves around the buds. But the same effect could also be caused by N deficiency late in flowering. Lack of Zn, which is rare, causes twisted atypical growth.
As the leaves dry the buds continue to grow and mature. They use the reserves being drawn from the media, roots, xylem and leaves.
Flowering formula fertilizers contain little or no N. Plants growing in soil or planting mixes use the residual N loosely bound in the media that continues to dissolve. The major nutrient N, which is mobile, translocates from the lower leaves to the upper canopy. The lower leaves turn bright yellow then curl and dry. Hydroponic mixes without media reserves require some N during the first half of flowering (usually 3-4 weeks) and less during the next quarter (10-15 days). The lack of N towards the end of flowering hastens ripening and maturity. This is one of the cues the plant uses to begin ripening.
Here are the cannabis flushing time recommendations by media
Medium Scale Greenhouse; SPARC . Photo by David Downs.
Clay Loam: 15- 20 days.
Soils:
Sandy Soil: Flush for a week. It doesn’t contain very much organic matter to bind the nutrients and it rinses readily.
Porous Loam: Flush for 10-15 days. Some nutrients are held tenuously to the matrix and need a bit more flushing than sandy soils.
Heavy Loams and Clays: Flush for 15-20 days. These soils bind nutrients that are hard to rinse away and must be used up by the plant.
Enriched Soils and Mixes: Soils that were enriched using additives such as plant meals and manures may not require any flushing.
Soil microorganisms dissolve the nutrients locked in organic compounds and provide them to the roots as needed. Most nutrients that are left are still locked up in organic matter. There is probably very little free N. However, if bottom leaves are not yellowing, there is too much nutrient left in the soil and the mix should be flushed.
Planting Mixes: Planting mixes differ in their abilities to buffer or hold nutrients so each should be dealt with in its own manner.
Peat moss and Coco: Flush one week if bottom leaves are green and 3-4 days if they are yellow. These mediums buffer nutrients (nutrients attach to them), but flushing will have a noticeable effect on the crop. The free nutrients are already dissolved and are easily rinsed away.
Medium-free/Hydroponic Systems (aeroponic, deep water culture and some nutrient film techniques): Flush 3-4 days. As soon as the water/nutrient solution is removed and replaced with pH’d water, the roots have no access to nutrients. The plants react immediately, first showing signs in the lower leaves, which turn yellow. The buds also ripen faster.
Medium-based hydroponic and fertigation systems (drip irrigation, ebb and flow, wick, capillary mat, reservoir, manually irrigated nutrient/water): Flush 4-7 days. The roots in these systems are usually anchored in a non-nutritive mix composed mostly of coir or peat moss. Infrequently, clay pebbles or perlite are used. None of these bind tightly to the nutrients so plants respond immediately to the new nutrient-free environment.
What are chemically enhanced flushes?
Beautiful calyxes.
Flushes remove or make nutrients unavailable to the roots so plants are forced to use their reserves for growth. The free nutrients that were in the xylem or dissolved in the extracellular water bind to molecules in the plant’s bio-system, creating a smoother draw.
The most popular flush is plain water. Salts in the media or in hydroponic units are all water soluble, or they’re precipitated, that is, have dropped out of the solution.
Precipitated nutrients cannot be taken up or used by the roots. Other salts are bound to larger organic molecules attached to the planting medium. These are only moderately available to the roots and are made available through mycorrhizae and other organisms in the rhizosphere (the area of the media that surrounds the roots). All other salts are soluble and drain out when flushed.
A few flushes claim that they contain chelates that actually draw nutrients from the plants. This may be true but has not been proven yet.
Once plants are flushed they draw from nutrients within their systems.
First they use the unbound nutrients held in the xylem and the extracellular water channels.
Then the mobile nutrients, nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg) migrate from the lower parts of the plant to the canopy that is getting light.
A large light unobstructed plant will deliver nutrients to the sunlit sides as well as the top of the plant. Rather than only going up, the nutrients travel out, to the growing tips and maturing flowers.
The immobile nutrients, boron, calcium, copper, iron, manganese and zinc remain stationary. Chlorophyll and other mineral-laden organelles in the cells break apart, facilitating the migration of the minerals they contain to the most active areas of the plant. Lacking the macro-nutrients, these leaves lose their green color created by Mg, turn yellow or tan and dry up.
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