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Why, When and How to Flush Marijuana Plants

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Cherry pie strain homegrown
May 13, 2020
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    As with quite a few cannabis growing techniques, flushing your plants can seem counterintuitive. Why would you want to remove nutrients from the soil? Surely cannabis plants need nutrients to survive? On the contrary, there are a number of good reasons to flush your cannabis plants. This article will explore those reasons, answering questions like, do buds still grow during flushing? How do you flush hydroponic plants? Should I pH water when flushing? What happens if you don’t flush your plants? And, finally, is flushing really necessary? Spoiler alert: unless you’re growing in amended or super-soil, it is. 🙂

    What is cannabis flushing?

    Flushing your cannabis plants is, quite simply, the act of replacing nutrient-heavy water with plain water. It ‘flushes’ excess nutrients from the soil and compels the plants to use all of the nutrients in their systems. Rather than seeing the buds slow down, they actually get a little boost from the flush and the entire process leads to cleaner, purer, tastier marijuana.

    Why Flush Cannabis Plants?

    There are three very good reasons to flush your cannabis plants.

    • To ease the transition between vegging and flowering
    • To resolve nutrient burn or lockout
    • To allow the plants to use all systemic nutrients ahead of harvest

    1. Do you flush between veg and flower?

    As your cannabis plants progress through their various stages of growth, their nutritional requirements change. During the vegetative stage, cannabis plants like plenty of nitrogen and potassium. Once the flowering period starts, the cannabis plants need less nitrogen but more phosphorus – potassium remains about the same.

    As the plants transition from one stage to the next, you can flush out the old nutes before introducing the new nutrient mix. This can make it easier for the plants to adjust to the new regime.

    2. Do you flush cannabis plants with nutrient burn or lockout?

    There are lots of things that can impact nutrient uptake: pH level, salt build-up, heavy feeding, plant stress, pathogens… all can cause a cannabis plant to stop absorbing nutrients at a healthy rate. Once the absorption rate drops, the level of unused nutes in the soil increases, often to very unhealthy levels.

    Flushing your marijuana plants is a nice reset for the soil, clearing excess nutrients and any accumulation of salts. It also gives you a good opportunity to adjust the pH balance back to optimum levels.

    3. Do you flush cannabis plants before harvest?

    It’s very good practice to flush the cannabis plants ahead of a harvest, usually one or two weeks before. This encourages the plants to use the nutrients they have already absorbed – meaning purer, cleaner cannabis. You’ll certainly notice the difference in the cure!

    When is the best time to flush your cannabis plants?

    You can only answer when to flush your weed by asking why you are flushing your weed. Let’s put the two together…

    1. Flushing cannabis according to growth schedule

    Some people prefer to flush the plants before flipping to flowering. The gardener will decide on the time-scale (the plants could be underfed and nutrient deficient at the time), but it’s anywhere from a few days to a few weeks ahead of flipping. Whatever you decide, you should keep accurate feeding records, detailing nutrient mix and feeding times – this will come in handy when estimating the changes to schedule.

    2. Flushing to remedy nutrient burn and lockout

    What causes nutrient burn and lockout? What are the signs of nute burn and lockout?

    Nutrient burn can be caused by any or all of the following:

    • Over-feeding your cannabis plants
    • Excessive watering or inefficient drainage, which restricts access to oxygen
    • Excessive use of bloom boosters or growth stimulants

    You’ll see the following physical manifestations:

    • Unusually deep green leaves or some parts seemingly bright with near fluorescent tips
    • The tips of the leaves bend at 90°
    • Some stalks and branches turn deep red, magenta or purple
    • Sugar leaves and calyx tips start to yellow
    • The tips turn crispy and die

    Small, burnt tips are not unusual and not a cause for concern, usually. If the discoloration continues and intensifies further, it’s time to take action.

    A nutrient lockout, on the other hand, can be caused by any of the following:

    • Use of fertilizers with high salt content (usually chemical-based)
    • An excess of one specific nutrient preventing others from being absorbed
    • Improper pH level

    How do the plants tell you if they cannot absorb nutrients?

    • The plant looks flimsy and weak, showing stunted growth
    • Yellowing or curling leaves

    In some cases, identifying a lockout can be challenging. For example, the yellowing leaves and droopy branches are also indicators of under-watering, or a nutrient deficiency.

    If you have burn or lockout, you should make a full assessment of what could be causing it before you flush.

    3. Flushing cannabis ahead of harvest

    Normally, the concentration of nutrients fed to the plants continues to increase from seedling to late flowering stage. Most growers flush their cannabis plants about two weeks ahead of harvest, giving the plants ample time to absorb and use all the added nutes. This isn’t set in stone and a good grower will know to flush later if the plant has been underfed and/or is nutrient deficient.

    If you want to know when to flush ahead of harvest, you’ll also need to know the flowering time of the particular cannabis strain you are growing. There are some fast-flowering varieties that could surprise you with their flowering speed! Always make a note of the date you flipped the plant into flower, so you’ll know exactly when to flush your plant.

    If your seedbank hasn’t given you the flowering time of your cannabis plant, there are two steps you need to take. First, find a better seedbank. Homegrown Cannabis Co. has detailed growing information on ALL cannabis cultivars. The next thing to do is watch the trichomes, when they start turning amber or brown it’s usually time to flush.

    How to Flush Marijuana Plants

    Even if it’s your first time growing marijuana, you shouldn’t find flushing too difficult. Rather than feeding the plants a nutrient mix, you will use plain, untreated water – simple.

    If your tap water has excess chlorine, let it sit in a bucket for 24 to 48 hours. This allows chlorine to dissipate before using on the plants.

    You should also measure the total dissolved solids (TDS) from the runoff. Firstly, take a measurement of the water you will use for flushing. It should be between 30-400 ppm – the recommended range for drinking water.

    Fill the plant pot with water, as much as it can hold. Collect the runoff and take a measurement. The reading should be close to around 1300 – indicating the presence of solid materials (including mostly nutrients).

    Give it some time to drain, and then flood the pot with water again. Once more, collect the runoff and measure the ppm. Keep repeating the cycle until the water coming off the drain holes clear up, and the ppm comes close to the initial reading of the water used.

    All subsequent waterings should use the same, nutrient-free water.

    Essential Things to Know About Flushing

    pH level.

    For a quick recovery after flushing, it’s best to adjust the pH of the water used to 5.8 if it is the vegetative stage, or 6.0 if in the flowering stage.

    Hydroponics.

    Unlike soil, flushing a hydroponics system is so much easier. You merely have to replace the nutrient solution with clean, pH-balanced water.

    Amended organic soil or super soil.

    If you are using these mediums, then you should AVOID FLUSHING as it washes away the beneficial organisms that help sustain healthy plant growth.

    Should You Flush the Plants Before harvesting?

    The benefits of flushing are subjective, except if to remedy a nutrient burn or lockout. Some people, for example, insist there is no need to remove nutrients from the soil ahead of harvest. Others, though, maintain the presence of unused nutrients in the buds creates a bitter taste.

    If you use Kyle Kushman’s Vegamatrix, you won’t need to flush the plants before harvest. These nutrients are exclusively plant-based – no animal-derived organic matter. Kyle is the world’s leading authority on using Veganics to grow cannabis indoors. If there is anything you need to know about growing cannabis, you can check out his page right here.

    I like neutral soil. Gives me all the flexibility to push as hard as I want. I always give plain water for a week. With Veganics, there’s no need to flush because it contains very low salts. Three weeks out from harvest I cut all micro-nutes and reduce bloom to half. Two weeks from harvest – no nutes, just enzyme, tea and water. Then one week, plain water.”

    Kyle Kushman
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