Flushing is an important, relatively simple, and all too often misunderstood technique that should be used by just about every gardener. A proper flushing could be the difference between delicious or harsh tasting fruits.

It could also save plants from salt toxicity. Horticulturists who grow their own food and/or medicine should be most concerned with dialing in a flushing technique.

Gardeners who are growing ornamentals do not have to be as concerned with flushing in regard to their own health but should still understand the process for the health of the plants.

Flushing, in regard to indoor horticulture, refers to the removal of unwanted minerals from the medium or from the plant itself.

The fact that there are two different types of flushing often causes confusion among horticulturists. To master a flushing technique, a gardener must first understand the difference between flushing the medium and flushing the plants themselves.

Flushing Your Grow Media

Growers use many different fertilizers and additives throughout the plant’s life cycle. When all the different bloom stimulators, catalysts and enhancers used by indoor horticulturists are considered, it is easy to see how these minerals could build up in the medium and the plants.

When I was a hydroponic retail store manager, one of the most common problems my customers faced was nutrient lockout due to salt accumulation in the medium.

As a plant takes in some of the essential elements needed for growth, it leaves other minerals (salts) behind in the medium. Over time these minerals can build up and wreak havoc on an otherwise healthy garden.

Nutrient lockout due to salt toxicity will, in many cases, mimic nutrient deficiencies and, because of this, growers will normally try to rectify the problem by supplementing more additives.

Unfortunately, this will only intensify the problem and cause the plant to become more distressed. This is why, when customers would come to me with a potential nutrient deficiency, I would always recommend “flush first, supplement second.”

Think of flushing as a way to get a clean slate for your plant’s medium. In other words, flushing the medium will allow you to start over and freshly address any potential nutrient deficiencies, without worrying about the minerals that have already accumulated in the medium.

When to Flush the Grow Medium

There are basically two times when a grower may want to flush his or her growing medium.

First, if there are any signs of toxicity or nutrient deficiencies, it is a good idea to do a flush.

If a gardener is experiencing a potential nutrient deficiency but has been feeding with a well-rounded fertilizer, more than likely it is a mineral buildup and a flush should be implemented.

The other time to potentially flush the plants is during transition periods. In other words, it is a good idea to flush when changing fertilizers from vegetative to blooming or right before the fertilizer regimen is changed during the blooming stage (bloom boosters).

Flushing during transition periods helps remove minerals that may not be tailored to the particular stage of growth the plants are entering.

For example, when plants enter the blooming stage, if they are still receiving an abundance of nitrogen, their flower or fruit onset can be slowed, thus increasing the entire duration of the flowering stage and/or reducing the overall yield.

Although some growers swear by it, I rarely flush during the flowering stage. However, I do highly recommend a flush during the transition from vegetative to bloom stage to better prepare the plant for a quick onset of fruits or flowers.

How to Flush the Grow Medium

There are different techniques a grower can use to flush the medium. First and foremost, the type of medium will help determine which technique is best.

For example, in deep water culture or aeroponic systems, flushing is as simple as changing out the nutrient reservoir with fresh nutrient solution.

These particular types of systems do not have a 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.

Stonewool, coco coir or other soilless media can physically hold on to minerals, which makes the flushing technique for these media a little more involved.

To flush these media, a solution must be poured though the medium to “pull” out the excess minerals. This can be done with a flushing product (there are many different ones to choose from) or a diluted fertilizer.

If using a flushing product, make sure to follow the directions from the manufacturer. Some manufacturers will even include directions for flushing the medium and directions for flushing the plants.

Some flushing products contain hormones that trigger a more rapid ripening or chemicals that trigger stress in the roots. These types of products should be avoided for flushing the medium and should only be used for flushing the plants at the end of their life cycles.

As an alternative to a flushing product, a grower can dilute their fertilizer regimen 1/8th the normal concentration and pour this solution through the medium.

When flushing, it is always desirable to have a high volume of solution to run through the medium. A good starting point is anywhere from 5-10 times the normal amount fed to the plants.

This is especially true with soil containers. When flushing a soil container, there will be a great deal of runoff coming out of the bottom of the container.

Remember, it takes a little while for the minerals to be drawn out of the medium so be patient. Soil growers can also use either a flushing product or a diluted fertilizer for flushing.

Why and When You Need to Flush Your Plants

Although learning how to flush the plant’s medium during its life cycle is important, flushing the plant itself can be a make or break situation.

Again, think of all the fertilizers, catalysts and additives indoor horticulturists feed to the plants during a garden cycle.

We have already discussed how these minerals can build up in a medium but many growers do not realize these minerals can also build up in the tissues of the plants themselves.

If not removed properly, these minerals will remain in the plant tissue and can affect the flavors, smells and safety of the harvested plants.

Many indoor horticulturists are drawn to gardening because it is a way to guarantee their food and medicine is healthy. However, if the time is not taken to do a final flush before harvesting, a grower may compromise the very thing he or she set out to do.

Again, there are many different types of flushing products available, each with their own advantages and disadvantages. If this route is chosen, just make sure to follow the manufacturer’s instructions.

The difference between flushing products is generally how they promote the flushing process. As previously mentioned, some flushing products use hormones while others may cause stress to the plant’s roots which will force the plant to use more energy which it has stored up in its tissue.

The simplest and widely used technique for flushing plants is to stop feeding any mineral fertilizers during the last week or two of growth and providing the plants with fresh water only.

By basically starving the plants, they are forced to use up the nutrients they have stored in their tissues. The more stored nutrients the plants use up, the less that will remain in the harvested fruits or flowers.

Is Flushing Necessary for Organics?

Due to the way plants receive nutrients in an organic garden (via micro-organisms), flushing an organic garden is not as imperative as with a garden fed chemical mineral fertilizers.

When plants are fed a chemical fertilizer, they are fed directly and can easily end up storing excess minerals in their tissues.

In an organic garden, the plants are not fed directly but instead the micro-organisms in the medium are fed organic matter that they break down into elements, which are absorbed by the plants.

In an organic garden, it is safe to feed the plants up to the day of harvest, but I still prefer feeding the plants straight water during the last week or two before harvest.

Gardeners who understand how, when and why flushing is important are sure to be more successful than those who do not.

Flushing the medium could, in some cases, mean the difference between life and death for plants.

During the transitional stages, flushing may allow plants to create fruits or flowers more quickly, which can contribute to the size of the harvest. If growing healthy food or medicine is the goal, then flushing the plants at the end of the growing cycle is essential.

In fact, flushing plants right before harvest is the best way to ensure the produce will not only be safe to consume but will also retain the flavors and odors every grower desires.