How & When to Flush Cannabis

When it comes to flushing cannabis - timing is crucial! By flushing your cannabis plants before harvest you can avoid a harsh & unsatisfying end-product.

If cannabis plants are not properly flushed, the minerals & nutrients used during the cultivation process are still present. Flushing removes all nutrients & improves the overall quality of your bud. Flushing cannabis is easy! Here’s how:

How to Flush Cannabis

1. Test the pH level of the water (tap is acceptable) to ensure it's in the appropriate range for cannabis plants (between 6.0 & 6.8 for soil grown).

2. Water the plants at the time you would normally feed them without giving them any nutrients or supplements. Don't water your plants more than usual to prevent deficiencies.

3. Repeat 15 minutes later.

4. To make sure the flush was successful, use a TDS reader to measure the total dissolved solids & find out how pure the water runoff is.

It's ideal that the water draining out of the pots is as close as possible to the TDS (total dissolved solids) reading of the neutral water you're flushing with.

5. Keep an eye out for signs of too much yellowing. The pre-harvest flush may cause a plant to lose its color quickly. It's normal to see some yellowing, but it's critical to harvest before the leaves on the buds have yellowed.

Once all the leaves have turned yellow your plant is at the end of maturity & buds will start to deteriorate.

6. After flushing, your plants will be a lot lighter in color & are ready to be harvested.

Flushing cannabis is used to remove buildup from the roots & soil of a plant not. It's been recommended to flush at the beginning or the end of a day when plants can be misted (this lowers their transpiration so that they do not over hydrate).

Indoor grow room
A close-up of a cannabis plant.

When to Flush Cannabis

Finding the best time to start flushing will vary based on how close your plants are to harvest & the knowledge & methods used by the grower.

Generally, flushing cannabis normally takes place two weeks before it is harvested. If the plant has an 8-week flowering period, flushing should start 6-weeks after the beginning of the flowering stage when trichomes begin to form a cloudy white color.

Flushing cannabis removes leftover nutrient buildup from the roots & soil of your plants giving them a fresh start. Flushing effectively allows your plants to absorb any nutrients that are still in the soil.

Flushing cannabis too early & too often will restrict nutrients & restrain the plants from growing & flowering. Flushing too early can also result in yellowing or discolored leaves.

Flushing Cannabis - The 3 Times You Should

1. Pre-harvest flush
2. Sudden change in the nutrient cycle
3. Nutrient Lockout

1. Pre-harvest flush - Flushing is used during this stage to improve the quality & smoothness of cannabis. A flush during pre-harvest will force plants to use up the nutrients stored within themselves while preventing harshness & removing excess chlorophyll.

If the nutrient reserves are not used or broken down they will negatively affect the quality of cannabis buds. This should be done between a week & 10 days before harvest & repeated three days later.

2. Changes in the nutrient cycle - Cannabis has different nutrient requirements depending on what growth stage it's in. Cleansing cannabis of old nutrients is a good way to reset soil while a plant is transitioning into a new stage of growth.

Think of this as a preventative flush. It's not required, but by the time a plant reaches the flowering stage, it has sucked all the nutrients from the soil & could benefit from a fresh start.

Cannabis in the flowering stage.

3. Nutrient Lockout - Growers that understand how to balance nutrients individually don't have to worry about flushing as much as those who rely on pre-balanced nutes.

Using pre-balanced nutrients may offer immediate results, however, after the first few doses, your plant is likely getting too much of one nutrient & not enough of the others creating a deficiency. A deficiency in plants is not always due to a lack of nutrients but can be because a plant has too much of a nutrient causing its system to become unbalanced.

By overfeeding a plant's nutrients, it will absorb what is needed while the access sits in the soil. This leads to a build-up of unnecessary nutrients which is known as Nutrient Lockout.

Generally, when dealing with a serious nute imbalance, the cure is to flush the plants & add a fresh, well-balanced, mild dose of nutrients. It's important to pay attention to the details; don't just flush cannabis because of minimal changes.

If you notice a drastic change in your plants but there are no other issues (heat, root rot, etc.), flushing may be a good idea to remove excess build-up & help restore the soils pH balance.

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