How and When to Flush Marijuana Plants

How and When to Flush Marijuana Plants

Just about all marijuana growers use either chemical or organic plant foods and fertilizers to support healthy growth throughout their crops’ life cycles. All those extra nutrients can have a dramatically positive impact on plant growth and yield, but the nutrients can also build up in the soil.

Mineral or nutrient buildup can cause problems during any stage of a plant’s growth, but it’s particularly problematic when it happens near the end of the growth cycle. Growers who use chemical plant foods and fertilizers often find that their harvested buds retain an unpleasant chemical taste, which can make their products difficult to sell. Flushing the nutrients, minerals, and chemicals from the soil using pH balanced water ensures that the final product will taste great.

Flushing marijuana plants isn’t as simple as it may sound. Pre-harvest flushing requires careful timing, and many growers can benefit from flushing their plants at other points in their growth cycles, as well. Read on to find out everything there is to know about flushing marijuana plants to get this essential process right.

When to Flush

Most experienced growers know how important it is to flush the soil prior to harvesting their plants. What they may not realize is that removing excess nutrients and minerals can be beneficial during other phases of plant growth as well.

Flushing to Address Nutrient Burn

Plant foods and fertilizers contain key ingredients that cannabis plants need to grow and thrive. Unfortunately, it can be difficult for growers to determine their plants’ exact nutrient needs. This can lead to overfeeding.

Overfeeding can cause several problems. First, it can cause nutrient burn. Nutrient burn presents differently depending on the amount of excess macronutrients and/or micronutrients within the soil.

The most common presentation of nutrient burn reflects its name. Growers will notice the tips of their plants’ leaves drying, fading, and curling as if having been exposed to extreme heat. Look for yellow-, orange-, brown-, or black-tipped leaves when identifying nutrient burn.

Nutrient burn occurs with excess fertilizer, and is not to be confused with ‘nutrient lockout’, a condition that presents similarly in appearance but instead of excess fertilizer, it’s deficient macronutrient absorption that causes the plant to die.

Other signs of nutrient burn are less obvious. They include root damage, flower malformation, brown spots, and unusually dark green or blotchy-looking leaves. The best thing for growers to do when they notice signs of nutrient burn is to flush the soil of excess nutrients and minerals in order to dilute the soil’s composition.

Flushing Before Changes in the Feeding Schedule

Many growers flush their plants before switching from vegetative growth formulas to flowering formulas. The reasoning behind this choice is that flushing creates a blank slate. Since plant foods designed for the flowering phase feature different nutrient ratios, adding a liquid food or fertilizer intended for flowering plants to unflushed soil can alter those ratios, leading to overconcentration of some nutrients and minerals and under-concentration of others.

Some growers also flush their plants when they switch from seedling food to vegetative phase food. Novice growers who haven’t gotten a feel for how hardy their plants are at different stages of growth should be careful if they choose to follow suit, though. Seedling cannabis plants have small, poorly developed root systems, so over-watering can cause them to become unstable in the soil.

Flushing Before Harvest

While it’s not strictly necessary to flush plants before switching to a new plant food or liquid fertilizer, pre-harvest flushes are essential. Not only will flushing before harvest improve the taste of growers’ harvested flower by removing excess chemicals and nutrients, but it will also stimulate growth in the flowers.

There are two mechanisms at work behind this stimulation in growth. First, flushing the plants causes mild stress, which tells the plants to invest their energy in rapid growth in an effort to reproduce.

Flushing, or removing, the excess nutrients and minerals from the soil also encourages plants to use up the nutrient stores that have built up in their cells throughout the growing season. The use of those nutrients stores route to the flowers, encouraging a growth boost to the flowers during this final phase. The result will be increased bud growth and a lower concentration of unused nutrients, which is what explains the difference in taste that occurs when growers flush their plants pre-harvest.

When performing a pre-harvest flush, growers need to pay attention to the maturity of the flowers because timing is important. The mature flower will have dark orange or amber pistils and opaque trichomes. Flushing the plants too early will rob them of the nutrients they need to fully develop, while flushing them too late will leave a concentration of excess nutrients in the final product. Instead of following generalized rules of thumb like flushing two weeks before harvest, check on the plant’s trichomes to determine when the time is right.

Plants with mostly clear trichomes are in the perfect stage for flushing. By the time the plants are harvested, the trichomes should be milky and white. By then, it’s too late to flush the soil. For best results, stop feeding the plants and flush them as soon as the first trichomes begin to change color.

How to Flush Plants

Now that growers have an idea of when to flush their plants and why it’s so important that they do so, it’s time to look at the actual process of flushing the soil. The process itself is simple. Instead of feeding the plants according to schedule, give them a large dose of clean, pH-balanced water.

Some growers swear by using distilled water to flush marijuana plants, while others settle for tap water. Tap water is usually fine as long as it doesn’t contain excess minerals that could elevate its electrical conductivity (EC) or alter its pH.

If the water is clean, start the process immediately by pouring as much water into the soil as it can hold. Wait a few minutes to let the water settle, then pour a second round of water in to push the first flush down and out of the plants’ root zone. If water starts pouring from the base, you’ve achieved the goal.

Some growers use total dissolved solids (TDS) meters to determine nutrient concentrations. The meter is used to test the water coming from the bottom of the pots after each flush and repeat the process until it registers 50 parts per million (PPM) or less of minerals and nutrients.

Unique Concerns for Hydroponic Growers

So far, this article has assumed that readers are growing in soil using conventional products. Hydroponic growers need to use a different flushing technique that involves the addition of large mineral particles in the water used to flush their plants, as these particles will bind to smaller mineral deposits around the plants’ roots without being absorbed.

While conventional growers typically flush their plants a week or two before harvest, hydroponic growers should wait until only few days before plant maturity to flush the solution.

Potential Issues

Since flushing depletes minerals and nutrients in the soil, it often leads to nutrient deficiencies. Growers can avoid this problem during the vegetative and flowering phase by quickly feeding the plants according to their specific growth phase.

During the pre-harvest phase, a little yellowing of the leaves is nothing to worry about. In fact, it’s completely normal. Just don’t let the leaves get to the point where they are completely yellow, because yellowed leaves don’t contain the chlorophyll necessary to process energy from the sun.

It makes little sense to apply more nutrients to plants immediately before harvest, so resist the temptation to take this approach. Instead, harvest severely yellowed plants as soon as possible and make a note to flush the plants later the next season. This will keep flowers and sugar leaves looking clean and healthy, although it might have a minor impact on the grower’s yield.

Using Additives

Large mineral salts aren’t the only additive growers use during a flush. Hydroponic growers and some conventional growers also use solutions that contain beneficial root bacteria, salt-leaching solutions, pH balancing supplements, or even blackstrap molasses. These additives aren’t strictly necessary but some, including the molasses, can make the finished marijuana taste sweeter.

The Bottom Line

While there is still some debate among even experienced growers as to whether flushing is truly necessary, anyone who has ever attempted to cure a crop of marijuana grown with conventional plant foods or fertilizers has likely learned from personal experience that it is. Flushing marijuana plants between growth cycles, following occurrences of nutrient burn or lockout, and before the final harvest will improve plant health and the quality of growers’ products.

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