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Flushing for Better Buds

by Nebula Haze

Table of Contents

What is Flushing?

Is Flushing Important?

How to Flush Your Cannabis Before Harvest

Flushing Tips - Don't Make These Common Mistakes!



What is Flushing?

Flushing is free and easy technique that may improve the quality and smoothness of your cannabis buds. Sounds good, right? Of course it does! But growers must also be careful with flushing before harvest because if you do it too early you can actually hurt your quality (and reduce your yields)!

The process of flushing involves giving your plants just plain water (no nutrients) for a period of time before harvest. An example of flushing would be if a grower gave their plants just plain (pH’ed) water for the last 2 weeks before harvest.

The idea behind flushing is to let the plant “use up” all the nutrients contained in the plant and buds, so there isn’t any left over after harvest.

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 Plants Only Plain Water For Days or Weeks Before Harvest

Flushing your cannabis plants before harvest with plain pH'ed water can improve the quality and smoothness of your buds after harvest


Is Flushing Important?

I used to think that flushing wasn’t that important. I'd still do it most of the time based on good practices, but other times I'd cut the flush short. One of the most common reasons growers tell you to flush is that not doing so affects flavor. I’d tried flushing and not flushing, and I personally couldn’t tell any difference when it came to smell or flavor.

In fact, I got lots of compliments on the taste and flavor of my buds. My unflushed buds didn’t have a “chemical” taste like others have 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.

Flushing your cannabis plants before harvest can help improve the quality of your buds

I became the worst thing a grower can be - a "theory crafter" - someone who bases their ideas on a theory or idea instead of actual evidence or experience. When it came to flushing, I told myself, “How could the plant be removing nutrients out of buds, anyway?” I also figured that a flush wasn’t needed for my grows since I kept nutrient levels low throughout the entire flowering stage. I thought maintaining lower levels would prevent any type of nutrient buildup (so to speak) in the buds.

I still don’t truly know about the science behind flushing or why it works, but after my last grow, my experience has completely changed my opinion on flushing. For my last grow, I harvested different plants at different points in the flushing process. 

While this was definitely not a honest-to-goodness scientific experiment since I was growing different strains without controls, my experiment was enough to change my mind.

We did some testing on our cannabis buds to see whether flushing would make a big difference in the quality

I grew three plants - I had one plant I didn’t flush at all, one that was flushed for a couple of days, and one that got flushed for a little more than a week.

My fellow grower Sirius Fourside grew two plants at the same time and flushed his for 2 weeks.

We both used the same nutrients (General Hydroponics Flora trio) and the same type of grow light (HPS).

So how much of a difference did flushing make?

It seemed pretty obvious that flushing made a big difference. The longer the plant was flushed before harvest, the more “smooth” the buds were when smoked. When I say smooth, what I mean is that the smoke was less likely to irritate the back of the throat or lungs.

The plant which wasn’t flushed at all had buds that were particularly harsh, and caused me to cough every time I smoked it; not that the quality or taste suffered. The ones that were flushed for longer were just plain better.

An unflushed bud can be beautiful, look great and smell great, but is it smooth to smoke?Luckily, you can partially fix harsh buds by curing them for longer - giving buds a little extra time in curing jars will reduce the harshness dramatically, but why not try starting with smooth buds from the beginning?

The two plants flushed for two weeks yielded buds that came out buttery smooth, not irritating the throat at all.

In conclusion, in our insanely informal experiment, longer flushing seemed to create smoother buds, at least up to 2 weeks.

As far as we could tell, the taste and smell seemed completely unchanged, so I stand by the fact that, at least in our experiments, flushing didn’t seem to have an effect on flavor/smell.

So does that prove anything? No. But it is enough evidence to change my opinion and I’ll be doing at least a 2-week flush on all my future harvests. There doesn’t seem to be any true downside to flushing (as long as you avoid common mistakes like flushing too early) and the potential benefits are worth it!


How to Flush Your Cannabis Plants Before Harvest

Flushing is Simple & Free!

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 harvest the buds right now if you wanted

At the beginning of the harvest window, your buds should already look just about the way you want them to at harvest. This harvest window lasts for several weeks - buds don't get "overripe" easily, and you have plenty of time to harvest your buds even if they've already 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 peak time. On the flip side, if you start flushing when your buds "seem" two weeks away, instead of already being in the harvest window, chances are you will be starting the flush too early and end up with "underripe" buds (and smaller yields).

Why avoid flushing buds early? Read the full explanation but basically buds harvested on the early side tend to be more "racing" or possibly have a paranoia-inducing effect, while waiting longer increases THC levels and intensifies the psychoactive properties of your buds. Waiting even longer makes 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. 

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 when the harvest window has started (with pics!).

A little extra info on harvest time: Cloudy trichomes indicate the highest levels of THC. Half cloudy trichomes signals the very beginning of the harvest window, the earliest it's ever recommended to harvest buds. For most strains, the plants won't reach the very beginning of their harvest window until they've had at least 6-8 weeks of flowering, and for many other strains it takes longer. 

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. 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 a little late gives more potent results and bigger yields than harvesting early, so it's imperative to avoid flushing too early! 

The harvest window lasts for 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 (usually about 2 weeks)

Even during the flush, it's still important to manage your pH. Many growers agree that 2 weeks is a good amount of time to flush. Less than that is not long enough, but more than that increases the chances of reducing yields and running into unpleasant looking nutrient deficiencies. Two weeks is a safe amount of time to flush.

3.) Harvest & Cure Your Plants

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

4.) Enjoy your smooth and potent buds!

You've done everything you could to ensure the best buds, so now it's time to enjoy the fruits of your labor!


Flushing Tips - Don't Make These Common Mistakes!

Flushing Too Early

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 the 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 and quality of your buds can also be lower.

Problems with Flushing Too Early

  • Growth is stunted during the most important parts of the flowering stage, and yields are reduced

  • 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

  • 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 Sugar Leaves Turn Yellow

The sugar leaves on this bud were allowed to get yellow as the plant was deprived of nutrients too early in the flowering stage

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’s 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 - The yellow spots on this bud are from the base of each yellow sugar leaf

Yellow spots appear wherever there was a yellow sugar leaf

Example - normal bud where sugar leaves remained green at harvest, no yellow spots

A normal cannabis buds that had yellow sugar leaves 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 their two 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 just opened. What that means is I won’t start flushing until the buds can be harvested now, even if it’s a bit too early for my preference.

Once the buds have hit the very beginning of the harvest window (trichomes are at least half-clear, half cloudy - learn more about trichomes and when to harvest), they still usually have 3+ 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 at the very beginning of the harvest window allows you to time your harvest perfectly for a 2 week flush.

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 pHing 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. Maintaining pH throughout the flush will help prevent nutrient deficiencies while still allowing your plant to use up its extra reserves of nutrients.

Learn how to manage your pH

A pH test kit can be used to manage the pH in your cannabis grow


How Long To Flush?

It’s generally recommended to flush plants for 2 weeks and I also think this is a pretty good standard to go by. With a shorter period of time (like I did in my informal experiment) the plant may not get a full flush. At least in my case, the plant that was flushed for one week was noticeably harsher than the plant that was flushed for two. Longer than 2 weeks without nutrients is likely to stunt growth of the plant, which we don’t want in the flowering stage!

Many growers agree that 2 weeks is a good amount of time to flush


Who Doesn't Need to Flush?

I believe a flush is very important for all the growers using liquid or powder nutrients.

Perhaps the exception to flushing would be growers whose plants are growing in soil, where they’re getting their nutrients primarily from the soil instead of it being included 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. In a composted soil environment, the nutrients are being broken down at the roots, and it’s much less likely for plants to be able to take in more than they need.

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’s 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, and what the exact effects will be.

But in my experiments, flushing for 2 weeks seems to make for the smoothest buds without any downsides, and a 2-week flush is what I plan to do for all my future grows.



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