by Nebula Haze
Table of Contents
What is the Point of Flushing Cannabis Before Harvest?
Is Flushing Important to Bud Quality?
How to Flush Cannabis Before Harvest
Flushing Tips – Don’t Make These Common Mistakes!
This flushing tutorial is part of our “how to harvest cannabis” series:
Flushing > Harvest > Trimming > Drying & Curing
What is the Point of Flushing Cannabis Plants Before Harvest?
What if you could significantly enhance the quality of your cannabis buds with one simple change in the final weeks before harvest?
Use Up Nutrients Stored in the Buds
Flushing is free and easy technique that may improve the quality, taste, and smoothness of your cannabis buds before harvest. Sounds good, right? Of course it does! The idea is to allow the cannabis plants to use up any extra nutrients contained in the plant. This may help avoid a mineral or chemical taste in the buds after harvest.
But growers must also be careful with flushing cannabis plants before harvest. If you start flushing too early, you can actually hurt your bud quality and reduce your yields. This happens if you starve the plant of valuable nutrients during the crucial final phase of bud development. Today, I’ll fully explain how to use flushing to your advantage so you get the benefits of flushing without any downsides.
What is Flushing?
“Flushing” before harvest means giving your cannabis plants just plain water (with no nutrients) for a little while before you harvest them. During the flush, growers give plain water instead of water with nutrients in it. Depending on your personal cannabis setup and type of nutrients, this flushing process usually goes on for a few days to about 2 weeks.
“Flushing” means watering cannabis plants with plain water (no nutrients).
So how do you get the best results when flushing cannabis plants before harvest? This tutorial will teach you exactly what to do!
Related Tutorial: What about flushing a cannabis plant because it’s sick?
Flushing Cannabis Before Harvest – Quick Summary
Here’s a quick summary of the flushing process for marijuana growers who want quick answers. Continue reading below to get a complete step-by-step cannabis flushing tutorial, plus answers to all your flushing questions.
1.) Wait until harvest window is almost open (how do I know when plants are close to harvest?)
How long should each type of grower flush their plants before harvest?
- Super Soil Growers: Don’t flush (you weren’t using nutrients anyway)
- Soil Growers: Flush 1 week
- Coco Growers Flush: 3-5 days
- DWC/Hydro Growers: Flush for a few days
More info about flushing each grow medium in the full tutorial below.
2.) Water cannabis plants as normal, except with no nutrients or supplements.
You’re going to do everything you normally do when it comes to watering, except without any additional nutrients or supplements. Don’t give more water at a time than normal, because this increases the chances of your plants getting overwatered and showing deficiencies / symptoms caused by overly wet root conditions.
3.) Watch cannabis plants closely for signs of too much yellowing.
During the flush before harvest, your plant can lose its green color very quickly. Although it’s normal to see some amount of yellowing before harvest, it’s important to harvest before the sugar leaves on the buds themselves have turned yellow. Although this doesn’t affect the quality of the buds, it does affect the appearance and buds aren’t as pretty. Additionally, once all the leaves have turned yellow your plant won’t really mature a whole lot more, and buds can start to deteriorate quickly.
4.) Harvest your cannabis plants when buds appear totally “ripe” or mature.
After the flushing period, your plants should appear a lot more pale than they did at the beginning. It’s time to harvest! Learn more about when and how to harvest plants.
That’s pretty much it! Read on to get the complete tutorial with more detailed step-by-step instructions.
Start flushing when cannabis buds already look ready to harvest. Don’t start flushing too early!
The idea behind flushing is to let your cannabis plants “use up” any extra nutrients contained in the plant and buds. This way there is a much lower level of nutrients left over after harvest, and it’s those extra nutrients that can change the smell, taste or (most often) “smoothness” of your buds in a negative way.
So in soil or coco coir, the grower would give the plant just plain water for days or weeks before harvest time. This lets the plant use up the nutrients in the soil/coco, and then – theoretically – start leaching extra nutrients from the buds.
In a hydro or DWC setup, flushing is even easier. The grower simply replaces all the water in their reservoir with plain water, and the plant doesn’t have access to any nutrients available except for what’s already in the plant.
Flushing is Giving Cannabis Plants Only Plain Water For Days or Weeks Before Harvest
Is Flushing Cannabis Before Harvest Important?
There are a million opinions about flushing. When I first started growing cannabis, I always flushed based on the habits I’d learned from other growers. Yet sometimes I’d skip it altogether for one reason or another. This got me thinking. One of the most common reasons growers tell you to flush cannabis plants before harvest is that not doing so affects flavor. After flushing and not flushing, I personally couldn’t tell any difference when it came to smell or flavor of buds.
In fact, I got lots of compliments on the taste and flavor of my buds when I didn’t flush. My unflushed buds didn’t have a “chemical” taste like others had warned of, and I figured the need to flush was basically a growing myth, or something growers should only do if they’ve given their plant too many nutrients.
One time we decided to do a test. We harvested some buds and then started flushing. I harvested the rest of the buds post-flush. It’s not a perfect experiment since the buds were harvested at different times, but flushing didn’t seem to have an effect on enhancing flavor or smell.
It could be that we always keep nutrient levels on the low side throughout the entire second half of the flowering stage, and don’t use any supplements. As long as plants are green and healthy without nutrient deficiencies, I consider that to be the correct amount of nutrients. This is in contrast to some growers who try to “push” plants to their limits by giving as much nutrients and supplements as they can take. Perhaps maintaining lower nutrient levels prevents any type of nutrient buildup (so to speak) in the buds. It’s possible that the flushed buds from our experiment were a little smoother to smoke and we just couldn’t tell. It’s hard to say with certainty.
So does our test prove anything? No. Flushing could still be helpful, and I still do it out of habit, but I wouldn’t worry too much about it if you forget or realize you need to harvest buds today.
On the other hand, there doesn’t seem to be any true downside to flushing as long as you don’t start flushing too early. Because of that, I recommend every grower try it for themselves to see what they think.
There’s no real downside to flushing before harvest (when you do it right), so I encourage every cannabis grower try it at least once.
How to Flush Your Cannabis Plants Before Harvest – Complete Tutorial
1.) Wait until plant already looks like it’s at the early end of the harvest window – in other words, wait to start the flush until you could pretty much harvest the buds right now if you wanted
The biggest mistake a cannabis grower can make when flushing plants is flushing plants for too long before harvest. Too much time without nutrients can actually hurt bud quality and lower your final yield because you’re essentially starving the plant of nutrients at a crucial stage of bud development.
The Biggest Flushing Mistake is Starting Too Early
Restricting nutrients too early starves a cannabis plant of nutrients while buds are still forming. This reduces bud quality and yield.
That means you shouldn’t start flushing until the buds are already in the harvest window. At the beginning of the harvest window, your buds should already look just about the way you want them to at harvest. When are my plants ready to harvest? This harvest window lasts for several weeks because marijuana buds don’t get “overripe” easily. That means you have plenty of time to harvest your buds even two weeks later after they’ve reached the beginning of the harvest window.
At this point it would be like harvesting fruit a little early; they won’t be at full potential, but they’ll still be pretty good, so it’s a great time to start the flush now so you harvest at the optimum time. On the flip side, if you start flushing when your buds “seem” a week away, instead of already being in the harvest window, chances are you will be starting the flush too early and end up with “under ripe” buds (and smaller yields).
Why avoid flushing buds early? Plants start dying if they run out of nutrients, and you don’t want that to happen while buds are still forming. You also don’t want to put yourself in a situation where you have to harvest buds too early. Harvesting too early not only hurts yields but also bud effects. You can read the full explanation here, but basically, buds harvested on the early side tend to be more “racing” or possibly have a paranoia-inducing effect. Waiting until the right time to harvest means bigger buds, higher THC levels, and overall intensified psychoactive properties of your buds. Waiting even longer and harvesting past “peak ripeness” results in buds that are still potent and psychoactive, but the extra time in the flowering stage also starts to add a more relaxed, “couchlock” effect as some of the THC degrades to CBN. For some growers, this may even be preferred.
At the earliest part of the harvest window, at least 40-50% of the white hairs have darkened and curled in. If you look at buds under a magnifier, you’ll see that the mushroom-like trichomes (which were once clear) are at least half cloudy.
Learn how to tell exactly when the marijuana harvest window has started (with pics!)
A little extra info on harvest time: Cloudy trichomes indicate the highest levels of THC. When half of the trichomes are cloudy it signals the very beginning of the harvest window (the earliest it’s ever recommended to harvest buds). For the most part, a cannabis plant won’t reach the very beginning of its harvest window until 6-8 weeks into the flowering stage, and that’s still several weeks earlier than many strains.
Although buds can be harvested at this point, they will continue to develop and increase THC levels over the next few weeks as more trichomes turn cloudy. Your yields will also increase significantly! If you’re looking for a more relaxed effect, I recommend starting the flush when just about all the trichomes have already turned cloudy.
Harvesting on time gives more potent results and bigger yields than harvesting early, so it’s imperative to avoid flushing too early!
The harvest window lasts for a few weeks and with this method, your buds will still be ready to harvest at the end of the flush, without the chance of having started too early.
2.) Provide only plain pH’ed water to plant until harvest (from a few days to a week)
You’re going to do everything you normally do when it comes to watering, except without any additional nutrients or supplements. Don’t give more water at a time than normal, because this increases the chances of your plants getting overwatered and showing deficiencies/symptoms caused by that problem.
Soil Growers Flush 1 week – Soil growers using nutrients should flush the longest, about 1 weeks. This is because there is still some amount of nutrients left in the soil. For super soil growers who haven’t used any nutrients from seed to harvest, you don’t have to worry about flushing because you’ve been giving plain water from the beginning. Your microorganisms in the soil have fed nutrients directly to your plants as needed, and it’s very unlikely you have any type of nutrient build-up.
Coco Growers Flush 3-5 days – Coco coir does not hold onto a lot of extra nutrients and just a watering or two with plain water will wash most nutrients away. Therefore growers utilizing coco coir should only flush their plants for 3-5 days, depending on how fast the plant is turning yellow.
DWC/Hydro Growers Flush for a few days – When a hydro/DWC grower changes their reservoir to plain water, their plants literally have access to almost zero nutrients immediately. Because of that, a hydro grower should usually only flush their plant for a few days before harvest to prevent early yellowing.
Exceptions to the “No Supplements” rule:
- Beneficial root bacteria – Hydroponic growers should continue using products with beneficial root bacteria like Hydroguard right up until harvest. These products do not contain any nutrients but will protect your plants from root rot.
- Continue to manage the pH of your water as normal – PH Up and Down products don’t contain added nutrients. If the pH is too high or too low, it makes certain types of salts and aluminum more available to your plant roots. You don’t want your plant absorbing that stuff up right when you’re trying to flush the buds.
- “Clearing” or “Salt Leaching” Solutions (below) – These supplements have been specifically designed to help remove extra nutrients during the flush.
Even during the flush, it’s still important to manage your pH. Many growers agree that a few days to a week is a good amount of time to flush. Plants in soilless mediums like coco or hydro can’t be flushed as long as plants in soil because they will run out of nutrients too quickly. For hydro you may only be able to flush for a few days. Flushing too long or starting too early increases the chances of reducing yields and running into unpleasant-looking nutrient deficiencies.
“Clearing” or “Salt Leaching” Solutions
These products are formulated to help remove extra minerals or salts when flushing the plant, which may reduce the chance that these leftover minerals or salts end up altering the smell or flavor of your buds.
They’re meant to be used if you’ve been giving your plants extra nutrients in their water. They aren’t necessary when the plant has been getting all its nutrients from the soil.
- FloraKleen (by General Hydroponics)
- Clearex (by Botanicare)
3.) Watch out for early yellowing
During the flush before harvest, it’s important to keep a close eye on your plants. Your plant can turn yellow almost overnight in certain situations.
Although it’s normal to see some amount of yellowing before harvest, it’s important to harvest before the sugar leaves on the buds have turned yellow.
Try to harvest before the green sugar leaves (small single-finger leaves directly attached to buds) start turning yellow. In the following cases, many of the fan leaves have turned yellow during the flush, but the small leaves on the buds themselves still appear green. They are at the perfect time to harvest!
You can see the sugar leaves are mostly still green, but many of the fan leaves are pale and turning yellow. That is what you’re looking for.
This auto-flowering plant is ready to harvest, too! If you wait any longer the yellowing will spread to the buds.
If the leaves of your plant have turned completely another color like red or purple, that is also a sign the end is near (not talking about when the buds themselves turn pink or purple, as that’s caused by a different genetic trait and doesn’t have anything to do with the leaves). Even for plants where leaves changing colors is normal, it usually only happens when harvest time is coming soon. Kind of like maple trees turning colors in the autumn.
Try to harvest your buds before the sugar leaves and/or the buds themselves actually start turning yellow, or buds appear burnt.
Although discolored sugar leaves don’t affect the quality/potency of the buds, it does affect the appearance (your buds won’t be as pretty). Additionally, once all the leaves are yellow your buds won’t really mature a whole lot more, and buds can quickly further deteriorate in health and lose potency.
That being said, even if buds don’t look picture perfect, they will still be good to smoke, and some growers prefer the more “couchlock” style effects of buds that have been allowed to mature on the longer side. Learn more about how harvesting at different times changes the potency/effects.
If you let the flush go this long, where buds are getting crispy and brown without any sign of green, it means the actual potency of your bud is starting to degrade. This bud needs to be harvested immediately so you don’t lose further potency.
4.) Enjoy your smooth and potent buds!
Woo hoo! You’ve been working so hard, and now you are almost at the end of your journey!
Learn how to harvest and cure your buds
You’ve done everything you could to ensure the smoothest buds, so now it’s time to enjoy the fruits of your labor!
Flushing Tips – Don’t Make These Common Mistakes!
The biggest problem with flushing, especially for new growers, is to start the flush too early. New growers commonly think their plant is just a week or two from harvest, when truthfully it’s 4 weeks or more away from the optimal harvest time. Breeders often understate the length of the flowering stage for strains, giving the timeline for the absolute earliest harvest, so it’s generally a good idea to add an extra two weeks (minimum) to their recommended time for a more realistic estimate of the optimum time to harvest.
When you flush a cannabis plant too early, you’re stunting its growth because buds aren’t getting enough nutrients to develop properly. Without being given the right amount of nutrients during the most crucial parts of the flowering stage, the potency, quality, and weight of your buds can also be lower.
Problems with Flushing Too Early
Stunting – Growth is stunted during the most important parts of the flowering stage, and yields are reduced.
Low THC – Potency may be lower because buds didn’t have the nutrients needed to develop THC properly, and if buds are also harvested early it reduces the potency even further.
Buds get yellow or brown – The appearance of buds can be affected, as a plant left too long without nutrients starts developing nutrient deficiencies that can spread to the sugar leaves on your buds, causing unsightly yellow leaves. Although this doesn’t negatively affect how the bud smokes, taking care of your sugar leaves until harvest is very important to get the best-looking buds (this is also another reason why it’s important to avoid too much nutrient burn)
Flushing too early or too long can hurt the looks of your buds because it can cause the sugar leaves start turning yellow.
All the nutrients were used up in the plant, including the ones in the sugar leaves, causing unsightly yellowing on the buds/sugar leaves that are almost impossible to trim off.
Allowing this to happen gives you buds covered with little yellow parts where the base of the leaves were before they got trimmed.
Example – After the bud was trimmed and dried, there are still yellow spots from the base of each sugar leaf that turned yellow, making buds look lower quality than they are.
This example shows a normal bud where sugar leaves remained green at harvest. No yellow spots.
The bud which didn’t have yellowing has an overall higher quality appearance – more evidence that it’s so important to time your flush right.
I’ve seen so many growers finish a 2-week flush, and realize their plant still has several weeks to go after the flush is over. Do they continue flushing, do they start using nutrients again, or do they just harvest the plant early? It’s hard to say which way is best in a situation like that.
So when it comes to flushing, I personally believe in waiting to flush until the harvest window has already opened. What that means is I won’t start flushing until the buds can already be harvested now, since I know they won’t mature much more during the flush..
Once the buds have hit the harvest window (trichomes are at least half-clear, half cloudy – learn more about trichomes and when to harvest), they still usually have several weeks before the plant reaches the end of the harvest window. I personally like to harvest plants a little on the later side, because THC levels are higher, buds are bigger, and effects tend to be more relaxing and not “racing”. So starting the flush after the harvest window started allows me to time my harvest perfectly.
You won’t be able to flush too early if you follow this principle. If your plant still looks like it’s a few weeks away from harvest, wait! You never know how the plant is going to mature, and you can’t be sure that harvest time is close, until the harvest window has actually opened.
Not pH’ing Your Water During Flush
Another common problem growers have is they stop maintaining pH when they start the flush. Even during the flush it’s important to maintain pH at the roots to make sure your plant can properly use the nutrients available. It can help prevent yellowing and spots on the leaves during the flush; when the pH is too high or too low at the plant roots, it makes certain nutrients unavailable to the plant and increases the chance of seeing nutrient deficiencies. The plant also tends to absorb more aluminum and salts out of the water when the pH isn’t in the correct range.
Maintaining pH throughout the flush will help prevent deficiencies and unwanted salt absorption, while still allowing your plant to use up its extra reserves of nutrients in the buds!
It’s generally recommended to flush plants for a few days to a week. I think this is a pretty good standard to go by. Unless you’re growing in amended super soil, flushing for longer than a week without nutrients is likely to stunt growth of the buds, which we don’t want in the flowering stage.
Many growers agree that a few days to a week is a good amount of time to flush
Soil Growers Flush 1 weeks – Soil growers should flush the longest, about a week. This is because there is still some amount of nutrients left in the soil. For super soil growers who haven’t used any nutrients from seed to harvest, you don’t have to worry about flushing because you’ve been giving plain water from the beginning. Your microorganisms in the soil have fed nutrients directly to your plants as needed, and it’s very unlikely you have any type of nutrient build-up.
Coco Growers Flush 3-5 days – Coco coir does not hold onto a lot of extra nutrients and just a watering or two with plain water will wash most nutrients away. Therefore growers utilizing coco coir should only flush their plants from 3-5 days, depending on how fast the plant is turning yellow.
DWC/Hydro Growers Flush for a few days – When a hydro/DWC grower changes their reservoir to plain water, their plants literally have access to almost zero nutrients immediately. Because of that, a hydro grower should usually only flush their plant for a few days before harvest to prevent early yellowing.
I believe a flush is a generally good idea for cannabis growers using liquid or powder nutrients.
As I’ve said, the exception to flushing would be soil growers where the plants are getting their nutrients primarily from the soil instead of getting it directly in their water. When growers provide nutrients in the water (with liquid or powder nutrients) they’re giving nutrients in the most accessible form possible, and the plant uptakes those nutrients immediately, whether it needs them or not.
This is the type of situation where you want to leach out extra nutrients that the plant may have absorbed into the plant matter.
You only need to flush if you’ve been using powder or liquid nutrients. If you’ve been growing in straight soil without nutrients then flushing isn’t necessary.
Directions: Dyna-Gro Foliage-Pro and Bloom may be the easiest cannabis nutrients. Just give 1 tsp/gallon of Foliage Pro in the vegetative stage, and 1 tsp/gallon of Bloom in the flowering stage. Can be used with any grow medium.
Note: Dyna-Gro was recently purchased by a company called “Superthrive”. That means you may receive bottles of either Foliage Pro or Bloom that say Superthrive, or they may say Dyna-Gro. Either way, the nutrients are exactly the same and used the same.
In a composted soil environment, the nutrients are being broken down at the roots gradually in the soil, making it difficult for a cannabis plant to take in more than it needs.
Plus it makes sense that growers who have only been providing water to their plants the whole grow don’t have any need to flush – what would they do different anyway?
There are no truly definitive answers when it comes to flushing. No one has done the type of vigorous controlled experiments it would take to be able to say with authority the exact best way and time to flush, what the exact effects will be, or if it’s even worth it in the first place.
But in my experiments, flushing for at least a few days up to a week before harvest seems to make for smooth buds without any downsides.
Jump to…
When Do I Harvest My Marijuana?
How To Get To Harvest As Fast As Possible
How To Buy Marijuana Seeds Online
