Saturday, April 28, 2012


Flushing is something that a lot of people seem to be confused about.  Either they don't do it at all, or don't know if they have to do it, or THINK that they already do it when they don't.  There are two different types of flushing:

1. The periodic flushing of your medium, to leach the salts out to avoid salt and nutrient build-up (or the mad flush after realizing that you've already over-fertilized in an attempt at damage-control).

2. Flushing during the final week of the plants blooming cycle to remove synthetic nutrients and salt build-up from the plants themselves, to avoid a chemical taste in your fruits and vegetables.  

Let's start with the first one.  When using a porous medium, such as soil, rockwool, a soilless mix (Pro-Mix, Sunshine, etc.), a periodic flushing is always recommended.  The way plants use nutrients is like a conveyor belt - they use what they need and leave behind what they don't.  Over time, the excess nutrients build up in the medium and can lockout the useful nutrients that the plants want.  Also, most fertilizers contain salt.  When the salt builds up in your medium the plants begin to absorb it along with (or, if it's bad enough, instead of) the nutrients.  The best way to catch it before it's too late is to measure your run-off with a PPM/EC meter.  If you're feeding your plants 1000 PPM, and the solution coming out the bottom of the containers/cubes is measuring higher, you know there are other nutrients or salt building up in your medium.  Unfortunately most people only know after-the-fact, when their plants are starting to show signs of over-fertilization and salt buildup - the leaves of the plants begin to curl upwards, they begin to burn at the tips (or if it's gone even further, the whole leaves start to burn up and get crispy).  This will stunt the growth of the plants and if allowed to go on for too long, the plants can die.

Flushing is not the same thing as just watering your plants with plain water (a very common misconception).  In order to remove all buildup from your medium, you need to either use a salt leaching formula (such as Clearex from Botanicare) that will bind to the salt and help remove it faster, or use a massive amount of water.

If using plain water with a soil or soilless medium, you need to use close to 3x the amount water as the size of the container you're growing in (if you are growing in 5 gallon buckets, you would need to pour about 15 gallons of water over each plant).  If using Clearex, 2x the water is usually enough.  This requires something to catch the water you're flushing with, like a big reservoir or a bathtub, or the most ideal thing would be a floor drain.  Have your PPM/EC meter with you and measure the run-off.  At the end of your flushing your PPM/EC reading on your run-off should be almost identical to that of the water you're pouring in.

If flushing in a hydroponic system with rockwool as your medium, I'd recommend draining the reservoir of all nutrients, filling it with plain water and adding Clearex at its maximum dosage (the bottle says 1/4-1/2 oz. per gallon of water, use 1/2oz./gallon).  Measure your PPM before it comes in contact with the medium, then run your system for an hour or so.   After it's been run, take a fresh gallon of water and clearex and pour it over the top of your rockwool cube.  Measure the run-off and hopefully the PPM's are close.  If not, run the system for another 30 minutes to an hour and measure again.

This should be done about every 3-4 weeks.

Now for the final week flush.  When using synthetic or chemical fertilizers, the flavor of your fruits and vegetables can be affected.  Especially when using a three-part fertilizer (like General Hydroponics' Flora-series), the iron in the Micro can leave an unpleasant metallic taste.  The best way to combat this is to remove all fertilizer from your feeding regimen for the final week of flowering/fruiting before harvest.  Add a FINAL flushing agent (different from a salt-leaching formula) to your water like Final Phase from Advanced Nutrients.  Final Phase has chelates in it that bond to heavy metals and nutrient salts and remove them from the plants themselves, not just the mediums.  Some people like to add a flavor enhancer to their final flush (like Sugaree, Sweet or B. Candy) so they get the benefits of the fruit extracts in those formulas, but Final Phase is designed to retain essential oil production so your fruits' natural flavors won't be lost.  

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