Is flushing necessary during flowering?

I am growing 3 autos in Coco coir/30% perlite, 3-gallon fabric pots, using Fox Farms Hydro nutes at 1/2 strength, ~EC 1200, pH 6.0. I fertigate twice a day, and get a fair amount of runoff. I monitor the EC of the runoff, and if it is >400 points above the ingoing EC, I give some more to get the runoff to a lower EC.

I’m currently at day 50 since sprouting. Two of the girls are well into flowering, and the third is in early flower.

The Fox Farms Feeding schedule for hydro says to do a “Sledgehammer Flush” every two weeks starting at the end of week 2, and continuing until the end of flowering.

Is this necessary, since I am flushing with nutrients twice a day, down to a safe EC level? I haven’t been doing it, and the plants look pretty healthy - in fact, they are too big for the 3x3x6 tent, which AC Infinity describes as suitable for up to three plants. I know it is important to flush a week or two before harvest, but I’m not there yet.


Flushes are unfortunately necessary if using the Fox Farm Trio. The Trio is very heavy on mineral salts that must be washed out periodically to avoid pH problems and nutrient lockout. It’s particularly important to monitor and manage runoff pH when using the Trio. The first symptom you will see that indicates that a flush is needed is when runoff pH becomes unmanageable.

You can avoid flushes by using a product that is not so salt heavy such as Jacks 321.


I don’t have personal experience with fox farms in particular so it may be a little different and need additional flushing. The only time I’ve ever needed to flush coco was when it goes dry or if I don’t water to 10-20% runoff regularly. Both were reflected in runoff EC. I also noticed the drainage was drastically reduced when I watered. Hope that gives you an idea what to look for.

@MidwestGuy is right it’s unavoidable using a line like FF that’s pure ions. They must be washed out.

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“The first symptom you will see that indicates that a flush is needed is when runoff pH becomes unmanageable.”

I have been fertigating with EC 1200-1300, pH 6.0, and the runoff last time watered was EC 1300-1400, pH 6.4 - 6.5. I assume that is ok, as it is not too different than the input EC and pH?

Runoff should be as follows:

pH: 6.3 to 6.8

PPM: shoot for 1,000, but don’t worry too much if you are off by a few hundred PPM here and there.

Comparing input vs output isn’t really useful.

Is that 1000 PPM on the 500 or 700 scale?

Yes, try and go off the F & F CHART. GOOD LUCK. THERE SO BEAUTIFUL. :metal::v:

500 scale

My runoff is almost always below 900 PPM (500 scale), so I think m good there.

So can someone explain to me why the excess salts wouldn’t be washed away by normal Coco watering? I understand in a soil mix that can go a long time without runoff, but I don’t understand how they could just be stuck with ample water running all the way though the medium. What am I missing? Do they become insoluble somehow?

Water can only hold so many mineral salt ions. The excess solidifies and bonds to soil or coco. It builds up over time. The mineral salts are acidic and ultimately submarine your root zone pH. The chemical process is called salinization.

Watering to liberal runoff helps, but it does not eliminate the chances of a salt buildup when products that are high in acidic mineral salts are used.

The formulations that the nute makers use matters, as some products are more heavy on these mineral salts (e.g. the Fox Farm Trio.)


Thanks for the info. That gave me some points to research and think I have a clearer understanding now. I hadn’t taken into account that different salts are more or less soluble. So the less soluble salts can still be left behind, even when watering often with relatively weak solution.

So then I would want to flush with a slightly acidic low EC/TDS water to remove the salts without creating more salts. Do I understand that correctly now?

The most effective way to loosen mineral salts is to use a product like Sledgehammer for FloraKleen.

If acidic soil is the problem (and it always is with a salt buildup,) then adding additional acids defeats the purpose when attempting to correct a low pH.

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I’m already supplementing the nute solution (at every fertigation) with RAW Yucca Flow, which contains 50% Yucca as a wetting agent. I believe that is doing the same thing as Sledgehammer, but at every watering.

Comparing input tds to output tds certainly is important. Even with liberal runoff, tds with any salty nutrients could rise. Now runoff ph can be ignored as long as input ph is on point with the media type.

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That is a particularly puzzling statement given that the thread was created to address salt buildups. That is when it really matters to monitor runoff pH.

I feed organic in super soil outdoors and no need to flush. Always get a smooth smoke and great tasting weed.

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Well mr midwest. In my grows in coco, the runoff ph will start low at beginning of grow and slowly creep up. Even buffered properly and after a full feed, in the beginning will be around 5.2 to 5.3. By the end of grow leveling out at 6. My tds is the one thing that is always rising. In countless grows, its been the same. The ph in does matter, tds in and out both matters. Coco is not soil. How would you know the amount of buildup without testing runoff? The runoff is related in both readings…but if tds is managed properly then output ph does not matter. Good source of info is cocoforcannabisdotcom. Which also states the same thing i am saying. Just my opinion based on reading and experience.