Learning how to flush cannabis plants is a critical step to producing top quality buds. Flushing cannabis properly will result in smoother-tasting buds that burn better and are less harsh to smoke.
Properly flushing cannabis removes available nutrients from the grow medium. Since the marijuana plant can no longer update nutrition from the roots, flushing forces weed plants consume nutrients it has already taken up and is storing in fan leaves.
The best way to flush cannabis plants
The best way to flush cannabis plants is to use lots of plain, PH’d water for 7-14 days prior to harvest using the steps outlined below for each grow medium.
Flushing in soil is similar to flushing in coco, you want to water slowly and let the water slowly drip through the growing medium. Watering too quickly will allow it to just pour through, reducing the amount of nutrient salts that get dissolved as it passes through.
Go slowly and use a lot of water. When flushing you can’t overdue it.
When to flush cannabis
You should plan ahead to identify when to flush plants based on the expected flower time of the strain you are growing.
You need to make sure to plan and allow time to properly flush. Don’t let the harvest window sneak up on you. If you’re running different strains (say an 8-week flowering strain vs. a 10-week strain) then you should flush each strain at the optimal time.
For example, if you are looking for a 14 day flush and you know you are running an 8-week flowering strain, the best time to flush weed with plain water is the start of week 7. This will allow for 2 full weeks.
If you’re growing a new strain that is labelled 8-9 weeks, then be safe and assume 9 weeks and start flushing at the start of week 8. That way if it finishes up and is ready to chop right at the end of week 8 you’ve had a full week after flushing already. And if it runs into the end of week 9 you’ll have had 2 weeks of flushing.
But also keep in mind that there are environmental stresses including excessive heat or insect damage that can extend the expected flowering time for your plants, and if your plants are a bit behind you’ll need to adjust your flush start time accordingly.
This can be hard for new growers to gauge, which is problematic since those stresses are most likely to occur as you’re learning to get your grow environment under control.
Best pH for flushing plants
When flushing cannabis plants the pH of the water is similar to the range you use for normal waterings. This would typically be 5.8 – 6.2 for flushing coco coir, 5.5 – 6.0 for hydro, and 6 – 6.5 for soil.
However, it isn’t always practical to pH water if you’re flushing using a hose or running water source such as your bathtub faucet.
In those cases if you flush with straight tap water, you could always go back a few hours later and give a final light watering with properly pH’d water to restore the balance a bit in the root zone.
Just be aware that if you flush with straight tap water containing chlorine you should to let the water sit at least 24 hours so that the chlorine can dissipate.
How to flush plants in coco coir
To properly flush marijuana plants in coco coir you should use at least enough water to triple the container size. For example if you are growing in 5 gallon smart pots, you would want to use 15 gallons of clean, properly pH’d water per plant.
Make sure to not water too quickly when flushing in smart pots, you want to let the water run through the coco rather than pour out the sides.
Water the plants slowly to saturate the medium to runoff, let it sit for 5 minutes, and then add continue to flush slowly to ensure that you dissolve and remove as many nutrients salts as possible from your medium.
If you’re using a automated watering system set it on a slow setting. Reduce flow to ensure a slow watering. You could set your timer to run the pump for one minute, pause for one minute, and repeat. This will help increase the efficiency of your coco flush.
Since coco is a hydroponic medium the roots get nutrients directly from the watering solution, so flushing weed in coco coir has a quicker effect on the plant than it does in soil.
Immediately after flushing the plant will no longer be able to access nutrients, taking effect much faster than if you flush in soil. Because of this, some coco growers choose the low end range of 7 days of flushing.
Advice for flushing in small grow tents
Multiple flushing cycles a few hours or 1-2 days apart will really clear out nutrient salts from your coco medium.
Taking a multi-stage approach to flushing cannabis plants is helpful for growers in small tents or grow rooms who water using 5 gallon buckets and collect runoff water in plant saucers below their pots.
Since you can’t just pour 5 gallons through a plant without flooding the floor with runoff, instead you can water each plant until the runoff container is nearly filled, use a shop vac to suck it up, and then repeat until you’ve completed your flush.
In a pinch you could also stagger the flush over several days. If you need to use 15 gallons of water for a plant in a 5 gallon pot, you could do a 5-gallon bucket for 3 consecutive days which would achieve a similar end result.
If you have a bunch of old 5 gallon buckets sitting around, you could make a plant flushing assembly line with them if you get a 5 gallon Bucket Head vacuum ($33). Fill each bucket, then water one through your plants and vac the runoff. Then repeat with the remaining buckets, each time transferring the bucket vac head so you can fill up the empty one.
How to flush plants in soil
If you need to flush plants in soil you will need to flush with about 3 to 5 times as much water as the container size you’re working with.
So for a 5 gallon container, you would use between 15 to 25 gallons of water to flush. You can’t really overdo it.
Flushing weed in soil requires you to water slowly so the water doesn’t just pour through the sides of the container.
It takes a lot more water to flush plants in soil than it is in coco or other hydroponics systems. This is because the nutrition needs to be leeched from the soil itself, whereas in coco you just need to rinse it out of the medium.
But can can also do it a pint cup at a time out of a 5 gallon bucket. The method doesn’t matter, the technique does. Go slowly with lots of water.
How long to flush hydroponics
Flushing hydroponics takes effect quickly since the roots are in direct contact with the nutrient solution. Once you start providing plain, pH’d water the effect of flushing will begin.
How long you flush plants in hydroponics should be about 7 days before anticipated harvest. While it can be done all at once, some hydro growers will gradually lower the water’s PPM for a few days to avoid shocking the plants.
When flushing cannabis in hydroponics systems you should use pH’d water. Most hydro growers use RO’d water, but if you’re using tap water let it sit a day so the chlorine dissipates before flushing plants.
How can you tell if weed plants are properly flushed in coco?
The best way to tell if cannabis plants are properly flushed in coco coir is to monitor the run-off water escaping the bottom of the container. Your goal is to see PPM levels that are equivalent to the water you’re putting into it. If it seems higher than the plain water you started with, keep flushing.
If you over-fertilize your plants, you will find that you need to flush more then if you are a light feeder. Measuring the PPM’s of runoff throughout the grow cycle can help you understand this better.
Remember f you growing hydroponically, flushing affects your plants quicker than if in soil since the roots are directly in water. You’ll start to see yellowing in fan leaves and smaller leaves within a few days usually.
If you flush cannabis plants too early your sugar leaves may yellow which affects final bud appearance, but the improvements in flavor, taste, and smokability will be well worth it. Cut those leaves at the stems when trimming and you’ll be fine.
Early flushing can happen if your plants take longer to reach the harvest window due to a schedule miscalculation or problems in the flowering period that extend the flowering times for plants.
Are nutrient flushing and salt clearing products like FloraKleen and Clearex worth it?
General Hydroponics FloraKleen Mineral Salt Clearing Solution and Botanicare CLEAREX Rinsing Solution are salt leeching products designed to help leech excess nutrient salts from your plants. They are marketed as helping flush weed plants event better than pure water.
Having never done a side by side test we can’t attest to this. Most growers we know go with the plain water approach.
But if you want to go the extra mile, or if the harvest window snuck up on you and you don’t have a full 2 weeks to flush then these nutrient mineral salt clearing or rinsing solutions can help you make up for lost time.
What happens if you harvest without flushing?
Sometimes shit happens. Maybe you forgot and flushed too late, didn’t use enough water, or didn’t flush at all. So what happens?
You spend the life cycle of the plant pumping in proper nutrients so it could grow and flower properly. If the plant doesn’t consume those elements, they remain in the plant tissue.
While things like chlorophyll break down during the drying and curing process, those elements aren’t going anywhere and will remain in the buds you smoke.
The goal of flushing is to have the plant try to break them down naturally. But you’ve definitely smoked a bunch of weed that hasn’t been flushed properly.
For many decades the U.S. imported Mexican brick weed that most people smoked. It was junk. Filled with leaves and stems, it wasn’t flushed, and in many cases wasn’t properly dried. Usually packed together while barely dry to maximize weight. It certainly wasn’t treated with the care that you are taking with your plants.
So take the time to grow them right and learn how to flush cannabis plants properly to ensure they burn smoothly and taste great.
Summary: Flushing marijuana plants
Knowing how to flush marijuana plants comes down to a few key things:
- Slowly flush with plenty of clean, pH’d down water
- The amount of water per plant is 3-5x your container size
- Begin flushing plants 10-14 days before harvest for grows in coco coir or soil.