A sign of a bad Pu Er is opposite leaves. Opposite leaves is when the leaves are bigger than the bud.
Normally leaves have a fat bud coming out. The bud gives the tea the additional flavor and body. This usually makes the tea smoother too.
Any trees that have a little bit of this is ok, but some trees have a lot of the opposite leaves. Some years are particularly bad years. Basically these are malnourished tea trees. It is very telling in the brewed tea leaves as well. So if you see a tea with a lot of the opposite leaves, it is not desired.
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