I have two mint plants that have grown without much fanfare. Occasionally I do notice the odd leaf with very dark brown edges, almost black. Some leaves also appear to have a powdery white substance on their leaves, leaving them pale in appearance. It does not rub off on my fingers, though.
I've watered my plants consistently every day except on wet or overcast days. The frequency of blackened leaves has increased recently, although, yesterday, I forgot to water the plant and I noticed all it's leaves had wilted heavily.
These plants get a full day's sun, save for morning time. The weather is generally sunny and mild year round (São Paulo, Brazil).
I may have been too generous with fertiliser than needed when I repotted them about six weeks ago.
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