I grow Jack Flash and Original Misty in Canna coco mats (coir in plastic bags) using a run-to-waste dripper system. I follow the instructions on the Canna coco fertilizer and everything seems fine up until the third or fourth week of flowering. Then the lower fan leaves start to go blotchy all over while the top leaves become very dark green. The splotchy appearance spreads up the leaves eventually reaching the newer ones. By harvest time there is little green leaf remaining.
I feed the plants three times a day and flush with fresh water once a week. The pH is kept between 5.2 - 5.8. I also use a B'Cuzz booster designed for coco. I have tried adding Epsom salts but it made no difference.
The plants are suffering from potassium deficiency as well as other possible deficiencies including nitrogen and phosphorous. The reason is that the salts in the fertilizer are insoluble and unavailable to the roots in the highly acid, low pH water. Adjust the water's pH from 5.2 - 5.8 as it is now, to 6.2 - 6.5. The minerals in the fertilizer will become soluble and available to the roots. New growth on the plant will not be affected by the deficiencies, but the damage to the old growth will remain.
One way to get needed nutrients to the deficient tissue quickly is to make a foliar spray using a liquid kelp extract containing a high percentage (3%) of potassium. The spray will provide micro-nutrients and also plant stimulants.
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