Kostas

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Bananas fold their leafs down from the midrib when they cant satisfy their leafs's water demands. That can be caused from too low humidity,lots of sun,dry soil or even root rot. If the plants are not too excessively big for their pots,from how describe it,i would be more inclined to think root or corm rot if they cant satisfy their water needs. A happy banana will flatten its leafs as the temperatures fall in the afternoon and will have a nice curve in the morning before strong sun hit them. In constant very low humidity maybe they wouldnt flatten them enough but still,they shouldnt remain totally folded.

Generous watering in potted bananas calls for problems,even in summer...Once every 4-5 days is more than adequate in this season,even in hot weather in the 30's Celsius,unless the plants are excessively big for their pots,in which case,transplanting to bigger pot would help.
Musa sikkimensis is a cool weather banana and thus even not than hot temperatures are seen as hot enough.

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