How to flush weed plants

Knowing when and how to flush weed plants should undoubtedly be in your arsenal of gardening skills if you’re aiming to grow the finest cannabis buds. This article will focus on the popular questions around flushing and hopefully help you to answer the question of if you need to do it or not.

Now, we will not dive into the decade long debate around to flush or not to flush before weed harvesting. Pre-harvest flushing has divided opinions among the cannabis-growing community: some growers believe that flushing can improve the taste, aroma, and overall quality of the buds by removing excess nutrients, others argue that it may not be necessary or can even harm the plants. But aside from pre-harvest period, there may be other times where your cannabis plants will benefit from a good old flush, for example when they suffer from nutrients burn/ lockout, or when you feel the need to take preventive measures before these issues become your worst nightmare. As such, it is still useful to learn about flushing cannabis.

Without further ado, let’s get straight to the point.

What is flushing cannabis plants?

Flushing cannabis plants involves the practice of watering your plants with either pure water free of any added nutrients or specialized flushing solutions. This eliminates any excess nutrients that might have accumulated in your plants’ system during the growing process, ensuring that there are no leftover nasty chemicals in the plant that could be harmful to consume.

If done before harvest, flushing can substantially enhance the aroma and flavor of your buds and even reduce the harshness of the smoke. Flushing can also be applied during the vegetative phase to reset the soil if it becomes too nutrient dense; however, keep in mind that the risk of depriving your plants of nutrients too early or for too long can result in stunted growth and affect the overall health of your crop, so flushing should be short and sweet for the best results.

When to start flushing cannabis plants?

When to flush your weed plants depends on the growth stages they are in and your purpose. Generally, the three times to flush your weed plants are:

1 – Before harvest

Typically, flushing should begin about two weeks before the plants are harvested if grown in soil (3-7 days if grown in coco coir and 2-4 days if grown in hydroponics)

You should check the breeder or seedbank’s guide on your plants’ strain to find out how long the flowering period is. If your plants have an 8 week flowering phase, you can start flushing around 6 weeks into the flowering stage; if your plants have a flowering period of 10 weeks, then flushing can begin around 8 weeks into blooming; and so on.

However, breeders often give the lowest possible flowering times on their website to appeal to growers, so you may want to add 1-2 weeks to what they say.

What cannabis looks like when it’s ready for a pre-harvest flush

One way to determine the best time for flushing is by closely examining the trichomes on the plants. When the trichomes begin to turn from clear to cloudy/ milky, it’s a sign that flushing can begin. At this point you know that you have roughly 1-2 weeks until they will be turned to milky/ amber which is normally the best time to begin harvesting. Check out our guide on how to harvest cannabis.

2 – When there are signs of nutrient burn or nutrient lockout, or before these become an issue

Flushing during the vegetative stage can serve as a method to restore the soil. If you happen to have unintentionally overfed your cannabis plants, as can be detected by discoloration and withering of the leaf tips (typical “nutrient burn” symptoms), flushing the soil can remove the excess nutrients. Although nutrient burn can’t be reversed, it can be corrected and should be done as soon as possible.

On the other hand, excessive amounts of nutrients may result in lockout, a condition in which plants are unable to access the required nutrients. If your plants show signs of stunted growth and pale leaves (typical “nutrient lockout” symptoms) this is most likely a salt buildup and incorrect pH levels. Flushing the plants with plain water can help to restore them to their former health by re-balancing the soil enabling the roots once more to absorb nutrients properly.

It can be quite difficult to distinguish between nutrient burn and nutrient lockout in weed plants so check the pH levels to help diagnose the issues.

You may also consider flushing your plants before either nutrient burn or nutrient lockout becomes your nightmare. Flush your plants once prior to the flowering stage and once again during the middle of flowering can reduce the possibility of nutrient buildup.

Once you have flushed the plants, allow the growing medium to dry out before watering them again to prevent root rot. Flushing is a severe approach at this growth stage, so make sure that the issue is indeed nutrient burn or nutrient lockout rather than other causes (eg. heat, root rot, etc.) before taking action. Do not panic and flush your weed plants if there are only minimal changes.

3 – Transition between the nutrient cycles

It’s can be beneficial to cleanse the soil of old nutrients when transitioning between growth stages, as the nutritional demands of your weed plants vary throughout their stages of growth. Now, don’t get me wrong, a flush may not always be necessary, but when the plant transitions between growth stages, it will have absorbed the nutrients from the soil, so removing the residual minerals and resetting the pH levels to give your plants a fresh start can be beneficial.

This is particularly useful if you use pre-balanced nutrients; as quite often, after the first few applications, your plants may be receiving an excessive amount of one nutrient and not enough of the others. The excessive build-up of a specific nutrient can lead to further complications which is why a preventive flush might be a good idea.

How to flush weed plants?

How to flush weed plants grown in soil

Flushing a weed plant grown in soil requires clean tap water, a bucket or watering can, some pH treatment (only if necessary), and a TDS (total dissolved solids) meter (optional).

  1. Time your flush according to your needs and the types of growing mediums (detailed discussion on this below). If you are doing a pre-harvest flush, check that the trichomes start to turn from clear to cloudy.
  2. Check the pH of the water using a pH meter and adjust if needed; the ideal pH for growing cannabis in soil is 6.0 – 6.8. Warmer water temperature 75° F (24° C) will also help dissolve salts better.
  3. Water the plants with untreated nutrient-free tap water at the time you would normally feed them.
  4. Soak the soil with the maximum amount of fresh water it can hold. Leave for a 5-15 minutes to allow the water to break up the minerals/ nutrients in the soil.
  5. Flush again to wash away the mineral content.
  6. Keep a close eyes on water running off from the bottom of the pot. If you use a TDS meter, you can collect and measure the water runoff. Continue flushing the plant until the TDS reading drops to 50 parts per million, or until it is close to the TDS level of the clean water you use for flushing. If you don’t have a TDS meter to hand, continue flushing until the color of the runoff water lightens up and looks a lot clearer.

How to flush cannabis plants grown in hydroponic grows

Flushing a hydroponics grow is much quicker and easier as the roots are not surrounded by nutrient rich soil but by pebbles or whatever hydro medium you are using.

  1. Test the pH of the water and adjust it accordingly; the ideal pH for growing cannabis in hydroponics is 5.5 – 6.5.
  2. Start emptying the reservoir and draining the system, clean out the container and pipes, then fill it back up again with fresh nutrient-free water.
  3. Begin flushing by turning your pumps on again and allowing the hydro system to do its job.
  4. If you are growing medium-free, flush for 3-4 days; if you are growing with medium-based hydroponics and fertigation, flush for 4-7 days.

During the flushing period, ensure that you water your marijuana plants as per your regular routine, and be careful not to let them become excessively dry or wet.

How long to flush cannabis for?

How long to flush your weed plants for depends on the type of growing mediums you are using.

Flushing cannabis grown in soil

  • Soil in general tends to be very good at absorbing and holding onto nutrients so you will typically need to flush your plants for 1-2 weeks to do a thorough job.
  • Sandy soil rinses easily so you may only need to flush for a week.
  • Heavy loams and clay soil on the other hand would require a little more time, so allow 10-20 days.
  • Amended organic soil or super soil should not be flushed to avoid damages to the beneficial microorganisms that your plants rely on.

Flushing cannabis grown in coco coir

Coco coir doesn’t absorb and hold onto nutrients in the same way that soil does so flushing is a little quicker and should be done for 3 – 7 days.

Flushing cannabis grown in hydroponics

Flushing is a lot more straightforward and less time-consuming for hydroponics grows than soil-based medium.

  • Those growing hydroponically using a medium-free system (e.g. aeroponics, DWC or nutrient film techniques) can simply drain the system and replenish it with plain pH-balanced water. Since hydro plants will no longer have access to external nutrients once the water supply has been replaced, the flushing process typically takes only 2-4 days.
  • Those growing with medium-based hydroponics and fertigation may need a bit more time, typically 4-7 days.

How do you know when flushing is complete?

If you are flushing your plants before harvest

  • For pre-harvest flushing, we are basically timing the depletion of nutrients with the maturation of the buds. And as the residual nutrients are depleted, the plant begins to utilize its own stored reserves, and it will put all of its energy to those delicate, tasty, smelly, big, fat buds. You know the whole flushing period is done when the plant and leaves change to a noticeable lighter shade of green, the buds are mature and ready for harvest while the lower leaves turn yellow.
  • Aside from yellowing, the edges of the leaves may curl and develop dead spots as well; this is due to the loss of nitrogen (N) and magnesium (Mg), phosphorus (P) and potassium (K) and is normal. Remember, though, to harvest before the sugar leaves on the buds also turn yellow.

If you are flushing cannabis plants to fix nutrient burn or lockout

  • For this type of flush, wait for your plant to dry out before feeding again. Follow your feeding schedule as required and give your plants fresh and well-balanced cannabis nutrients.
  • Plants suffering nutrient burn will require a bit more patience. Depending on the levels of nutrient burn and the type of growing medium, keep a close eyes on your plants for recovery signs. Your plants may look sad for a while, but they will eventually recover (unless the damages are beyond repair).
  • If your plants are still quite dark green after a few days, it might mean the excessive minerals/ nutrients buildup haven’t budged and you may want to consider performing another flush, adding enzymes to the water this time or using specialized flushing solutions.

Do you flush cannabis plants every day?

No it is not necessary to flush your cannabis plants every day. The flushing period typically lasts between 2 days to 2 weeks depending your purpose, the type of growing medium, and the stage of growth as discussed above. Flushing can be done once or twice during the entire growing cycle.

What can I flush my weed plant with?

When flushing a weed plant, you can use plain water, or specialized flushing solutions that are designed to remove excess nutrients and minerals from the growing medium, or other products that contain enzymes or beneficial microbes that help break down nutrients in the soil.

Can I flush cannabis with tap water?

Yes, you can flush cannabis with tap water. But bear in mind that the quality of tap water varies depending on your location, and it may contain chlorine, chloramines, or other contaminants that can negatively affect your buds. Using filtered water is often recommended for flushing to ensure the best possible results. Some people also let the tap water sit for 1-5 days to allow all the chlorine to evaporate.

Will buds grow during flush?

Yes, during the flush period, the buds on your cannabis plants will continue to grow!

You see, typically, plants store excess nutrients when they are readily available, but when nutrient availability is limited, they will draw nutrients from older plant material to support growth in areas of greater need. During flushing, the plant redistributes nutrients, including nitrogen, to the buds to promote growth. This is why when you do a pre-harvest flush it is common for the leaves to begin to turn yellow but this does not necessarily affect the quality of the buds.

In fact, your plants will still produce more terpenes and flavonoids in the buds, in its final effort to get pollinated.

Mistakes to avoid

Flushing your plants too early before harvest

A common mistake that growers often make is flushing their weed plants too early before harvest. This will stunt their growth because they will not be given enough nutrients in the final stages where the buds are really packing on size. You will also greatly reduce the quality of the weed produce because the buds will not have time to fully mature for harvest, meaning that your long awaited buds may not have as high THC and terpenes as they could have.

Flushing your plants too frequently

If the nutrients are appropriately balanced, there should be no need for frequent flushing with nutrient-free water.

Some nutrient brands do recommend things such as “flushing should be done at many times throughout your grow”, or “flush every third feeding for best result!”; however, if you come across one of those, you should be suspicious. For a nutrient brand or company to suggests frequent flushing, it probably mean that their nutrient mix or quality is questionable. You may be better off sourcing your nutes elsewhere.

Using water with the wrong pH levels

The combination of nutrient deprivation and wrong pH levels is bad news to cannabis plants. And if this combination happens in a short time period it will be a shock to your plants and will do serious damage. So make sure to check and correct the pH levels during and after flush.

Iris Sideris

Meet Iris, the ultimate cannabis connoisseur! With her green thumbs and a puff puff pass mentality, Iris loves sharing her stash of knowledge on all things cannabis. Whether you're a seasoned smoker or a newbie to the ganja game, Iris is the go-to source for all things 420.

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