Znać vs. wiedzieć

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Wiedzieć means 'to have a knowledge of sth, have information about sth, be oriented in sth, be aware of sth, realise sth', e.g.:

Nie wiem, dlaczego miałbym to kupić. 'I don't know why I should buy this.'
Nie wiesz, co to za roślina? 'Don't you know what plant it is?'
On nigdy nie wie, co się dzieje. 'He never knows what is happening.'
Wiem, że kiedyś to przeminie. 'I know it's going to fade away.'

Znać means 'to know [some] facts about sb/sth, or have an image of sb/sth, based on one's experience, notion; to be able to recognise sb/sth'.

Nie znam tej rośliny; nigdy wczesniej jej nie widziałem. 'I don't know this plant. I have never seen it before.'
On znał takich ludzi lepiej niż ktokolwiek. 'He knew such people better than anybody.'

It is not usually possible that a particular word can occur with both the verbs. However, there are some exceptions, and one of them is:

Wiem to. = I know it, because perhaps I've seen/ heard/ smelt/ read about etc. it, or just have a feeling like I know it (it sounds familiar to me).
Znam to. = I have seen/ ... / read about it, I have experienced it somehow earlier, so I know it.

but:

Znam go. 'I know him.' Wiem go.

In general it is noticeable that znać requires a definite object wiedzieć:

Znam coś. Wiem coś. 'I know something.'
Znam to coś. Wiem to coś. 'I know the something.'