Hello rootspook,
First thanks for the picture, however I was hoping to see the plants. Basically I wanted to see plant size, as well the color of the foliage. Also if I could see any salt buildup on the
growing medium, even how the roots looked if possible. Basically wanted to see if there was anything that would provide clues to a problem.
My best advice is to e-mail general hydroponics directly at:
tech@genhydro.com
Give them your situation, and ask for their advice. Especially on their
nutrients. Just because you don't want a plant to bloom, dosen't mean they don't need the mineral elements in the Flora bloom, they just need them in less quantities. I think your biggest issue is mixing the nutrients, so I think asking the manufacture directly will give you the best advice on that. If the conversion calculator I found on line is correct, and 5cc's = 5mL, and 2cc's =2mL. So If I read correctly your total
nutrient solution consists of:
Gro.....5mL (per gallon)
Micro...2mL (per gallon)
Bloom..0mL (per gallon)
Considering the plant size of a one and half to two foot tall, if I'm correct General Hydroponics will probably suggest you mix the Flora series nutrients something more like this instead:
Gro.......10mL (per gallon)
Micro....10mL (per gallon)
Bloom....5mL (per gallon)
I don't even have a EC/TDS/PPM meter, mainly because they simply cant tell you if your nutrients are balanced. You can literally pour straight baking soda, or bleach in the water and reach the desired EC level. But obviously that wont grow plants. They need a balance of the right mineral elements to do that, and that's just something the meters cant tell you about. So there really only good for telling the relative strength of the nutrient solution (relative because they don't know what's in it), but you still need to know if your solution is balanced (and/or what's in it) for the EC reading to be helpful. Their only balanced when you first mix them, once the plants start using the nutrients, they start going out of balance. How fast depends on which elements they plants are using up more of, how much of them their using, and water volume. Beyond the nutrients, your source water will have elements in it as well that will affect the plants, and EC reading (unless your using distilled water).
If there is salt buildup in/on your growing medium then flushing it with plain water (and/or floraKleen) would be useful/helpful. But you shouldn't flush it for weeks. I would flush it with every nutrient change, and from a couple of hours, to 24 max. The way I do it is after I dumped the old nutrient water, cleaned the
pump and reservoir. Then fill it with plain water and run it through the system for a half hour or so, then drain and dump. I will do that a couple of times, then the last one I let cycle for a few hours. Then dump and add fresh nutrient solution.
I think you said your changing the nutrient solution every two weeks, but I don't know what the "q" means in this part "changed out q 2wks." It sounds like you have a mix of large and smaller plants in the system, and I cant really see what you have growing. But considering that you have two foot tall plants in the system, and I see 17 holes for plants in the picture. You may be pushing the envelope on water volume per plant ratios, and that would through off your nutrient balance also. I would want about 2 gallons of water volume for each 1-2 foot plant, and 1 gallon for the smaller plants. Even then I would probably be changing it weekly (at least to see if there is improvement).
I have grown basil in warm temps, as well as when flowering and I don't remember it being bitter (but it wasn't humid here then either). So I don't know how much affect heat has on that aspect, but again I would ask General Hydroponics about that too. I do know that it will affect nutrient uptake, as well as dissolved oxygen in the water (witch will affect nutrient uptake also). So overall, heat stress could be a issue in bitterness. As well as defiantly an issue in plant health weather it's causing bitterness or not. I would also probably run the nutrients on 3/4 strength in hot weather (that I recommended) due too increased plant transpiration. Basically mix it the same, but then add 1/4 more water. I would also want to use a larger reservoir if I could to reduce fluctuations in nutrient concentrations as the water level changes from plant transpiration, as well as replacing the used water volume.
P.S.
I'm interested in hearing what General Hydroponics reply is.