I realized now that the big huge lesson is that if I get this part wrong - I will lose the whole plant species rotting away and another in another weeks time. Also - the plant can appear to be doing just fine within the 2-3 weeks time and you may not notice any difference and suddenly the whole plant rotten away from the base.
At first, I had blamed the wood vinegar and slow release fertilizer (Osmocote) - only now, I realized my error - the mix and match feeding regime seemed to be the culprit. Also they don't fair well when the mix includes in between organic and non-organic types - hence I had just stuck with the NPK Orchid Fertilizer - the powdered form - using a tablespoon for a full bucket of water which I keep it standing overnight and spray over the whole garden the next day.
When I do this on weekly cycle - I find all my plants show good steady growth - however I also note that the strength is quite weak that none of the plants showing any inflorescence or flowers for the NPK Orchid Fertilizer where it was meant for blooms. My main concern was keeping the plants healthy in steady growth and it does achieve that desired purpose.
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for more information on Cane Begonias.