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Flushing and Nutrient Withholding before Harvesting

Posted May 1st, 2019 by Eric Hopper in ,

After waiting at least eight weeks for cannabis plants to finish their flowering cycle, most cannabis cultivators are chomping at the bit to start harvesting. However, experienced growers know that there is one more important step that needs to be taken prior to harvesting. This last step can have a significant impact on the flavor, odor, and overall quality of the finished product. The final, but quite important, step is flushing. Flushing is essentially a cleansing of the plant’s root zone. This includes the medium itself and/or the hydroponic reservoir. The term “flushing” often refers to a final removal, just before harvesting, of built up minerals in the plants’s roots. “Flushing” can also be used to describe the periodical removal of built up minerals in the plant’s root zone during the grow cycle. Both types of flushing are meant to remove unwanted minerals; however, when the plants are flushed during their grow cycles; the nutrients need to be replenished so the plant still has access to essential elements for growth. On the other hand, the flushing of a crop prior to harvest is designed to remove as much as possible from the root zone. Unlike flushing during the grow cycle, after a final flush, the plants are fed pure water and are not reintroduced to supplemented nutrients.

Flushing Before Harvest

Flushing before harvest is crucial for removing built up minerals in the medium and the plant itself. Cannabis crops that are flushed properly prior to harvest produce flowers that are safer to consume, have smoother flavors, and have more distinct odors compared with those that are not flushed. Think of all the fertilizers,

catalysts, and additives that a cannabis horticulturist commonly feeds to his or her plants during a grow cycle. Most gardeners understand that these minerals can build up in a soil or medium, but many do not realize that these minerals can also build up in the tissue of the plants. If not removed properly, these minerals will remain in the plant tissue and can affect the flavors, smells, and medicinal quality of the harvested cannabis flowers. A simple, widely used technique for flushing cannabis plants prior to harvest is to stop feeding the plants any mineral fertilizers during the last week or two of flowering and providing the plants with fresh (pH balanced) water only. This forces the plant to use up the minerals remaining in the medium and in the plant’s reserves (stored minerals). By basically starving the plant, it is forced to use up the nutrients it has stored in its tissue. Eliminating the feeding program and switching to straight water is an effective way to flush cannabis plants before harvesting. The more stored nutrients the plant uses up, the less that remain in the tissue. This also means there is less to affect the quality of the harvested flowers.

Another option for flushing cannabis plants just prior to harvesting is to use a specific flushing product. There are many different types of flushing products available, each with its own advantages and disadvantages. If this path is chosen, the grower needs to carefully follow the manufacturer’s instructions. The difference between various flushing products is generally how each promotes the flushing process. Some flushing products use hormones, while others may deliberately cause stress to the plant’s roots, forcing the plant to use more of the stored up minerals in its tissue. When to implement a flushing product will also depend on the product itself. Some flushing products are designed to be used a week or two prior to harvest, others are “fed” to the plants up until the day of harvest. The variation between flushing products is just one of the reasons why it is so important for a cannabis cultivator to follow the manufacturer’s instructions.

Flushing Throughout the Plant’s Life Cycle

Although flushing at harvest time is crucial for protecting the integrity of a cannabis plant’s odor and flavor profile, flushing periodically throughout the life cycle of the cannabis plant may be necessary to maintain the crop’s optimal health. When minerals build up in a medium, both the nutrient concentration and the pH of the nutrient solution can get out of whack. This can lead to a whole series of problems. The goal of flushing intermittently throughout the grow cycle is to restore the balance of the nutrient concentration and pH in the medium and/or the reservoir. Cannabis plants rarely absorb nutrients in the exact same ratio as what has been fed to them. If more nutrients are continually added, eventually the left over minerals compound and build up in the medium. Cannabis cultivators who use chemical fertilizers and never flush their plants could end up with hindered growth and diminished yields due to the compounding of nutrients in the medium. Unfortunately, many of these growers never realize that their plants aren’t operating at full potential.

When to Flush During the Grow Cycle

There are basically two times when a grower may want to flush his or her growing medium during the grow cycle. The number one reason to flush during the grow cycle is if there are any signs of toxicity or nutrient deficiencies. If a gardener is experiencing a potential nutrient deficiency, but has been feeding a well-rounded fertilizer, more than likely a mineral build up is causing the problem and a flush should be implemented to rectify the problem. The other time to potentially flush the plants is during a transitional period. In other words, when changing fertilizers from vegetative to flowering or right before the fertilizer regimen is changed during the later stages of flowering. Flushing during transitional periods helps remove minerals that may not be appropriate for the particular stage of growth the plants are entering. For example, the high nitrogen fertilizers used during a cannabis plant’s vegetative growth should be removed prior to initiating the flowering stage. Too much nitrogen during the flowering stage will lead to undeveloped flowers and reduced yields and/or quality.

How to Flush

Whether doing a final flush prior to harvest or a flush during the grow cycle, there are different techniques a cannabis grower can use to flush his or her medium. The type of medium is generally the determining factor as to which technique will be the best. For example, in deep water culture or aeroponic hydroponic systems, flushing is as simple as changing out the nutrient reservoir with fresh nutrient solution. These particular types of systems do not have an actual medium that comes in direct contact with the nutrient solution and, therefore, cannot hold on to minerals in the same way that other media can. Soil, stonewool, coco coir, or other soilless media can physically hold on to minerals. In order to flush these types of media, a solution must be poured through the medium to “pull” out the excess minerals. This can be done with a specific flushing product or a diluted fertilizer. As previously stated, if using a flushing product, make sure to follow the directions from the manufacturer. As an alternative to a flushing product, a cannabis grower can dilute the fertilizer regimen he or she is using to 1/8 the normal concentration and pour this solution through the medium. Alike molecules tend to bond to each other, which is why a diluted fertilizer works to pull the minerals out of the medium. When flushing with a diluted fertilizer, cannabis growers should aim for a high volume of solution to run through the medium. Anywhere from five to ten times the normal amount fed to the plants is a good starting point. This is especially true with soil, which tends to hold on to excess minerals the most.

Flushing for Organics

Organic fertilizers rely on microorganisms to break down organic matter into a usable form for the plants. Because of this method of mineral delivery, flushing in an organic garden is not as imperative as in a garden fed with a chemical mineral fertilizer. When cannabis plants are fed a chemical mineral fertilizer, the microorganisms in the soil are bypassed and the plants can easily end up storing excess minerals in their tissue. With organics, the plants are not fed directly, but, instead, the microorganisms in the medium are fed organic matter which they break down into elements that are absorbed by the plants. Plants fed organic fertilizers tend to take what they need and will end up with less excess minerals in the plant’s tissue. Many organic cannabis gardeners continue to feed the plants up to the day of harvest. Organic or not, I recommend withholding all nutrients for the last week or two prior to harvesting.

Cannabis horticulturists who understand how, when, and why to flush are more likely to have healthier crops and a higher quality finished product. Flushing during a cannabis plant’s life cycle could, in some cases, mean the difference between the success and failure of the crop. Flushing during the critical transitional stages will allow the cannabis plants to initiate flowers more quickly, which, down the road, can contribute to an increased yield. If a cannabis grower truly values healthy plants and producing the highest quality product possible, then flushing the plants just prior to harvesting is essential. Flushing cannabis plants right before harvest is the best way to ensure the harvested flowers will not only be safe to consume, but will also retain their natural flavors, odors, and potency.

Eric Hopper is Editor in Chief for NUGL Media Group. He can be contacted at eric@nugl.com.

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