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