A Different Take
Mist/dew might be the right translation. It's not the only option. I am in closer agreement with more recent cognate-language scholarship that would translate (v. 6) [u:1dd80b6cb3]'eed[/u:1dd80b6cb3] (Heb.) as something more substantial like "stream." See for example Harris, Archer, and Waltke, [i:1dd80b6cb3]Theological Wordbook of the Old Testament,[/i:1dd80b6cb3] p. 17.
I believe the point to be made in the text (to include the previous 2 verses) is that this is prelude to the creation of man. From the macro-creation view of Chapter 1-2:3, we zoom in on day 6, Man and the Garden. The plants mentioned in verse 5 most likely refer to [i:1dd80b6cb3]cultivated[/i:1dd80b6cb3] plants (see [i:1dd80b6cb3]Did God Create in Six Days?[/i:1dd80b6cb3] p. 209), not to plants in general (created on day 3). Point being is that this is still the beginning of creation. You wouldn't expect to find fields with crops planted because two things are missing--the cycle of growth marked by annual rain (according to which expectation crops are planted) has not yet begun, AND there is no MAN to till the ground.
BUT, though there has been no rain to wet and prepare the ground in this place (our focal point) where God is ready to plant his Garden, it is [b:1dd80b6cb3][i:1dd80b6cb3]well-watered[/i:1dd80b6cb3][/b:1dd80b6cb3] in any case because either the land is flooded to prepare for it, or copious springs or streams water it. We are told later (v. 10) that so much water is present in the Garden that four mighty rivers have their head in it.
Building on chapter 1's statements, Man's creation is now recapitulated (v. 7), followed immediately by the miraculous [i:1dd80b6cb3]planting[/i:1dd80b6cb3] and [i:1dd80b6cb3]growing[/i:1dd80b6cb3] of the Garden (vv. 8-9), a pre-cultivated, pre-groomed place where Man will be tutored and prepared for his dominion mandate of the untamed wilderness beyond. Man was not involved in the Garden's origin. It did not grow up in "ordinary" length of days/years. It grew up in orderly fashion in a fraction of the time it would have taken ordinarily, even within a day.
I do not think that verse 6 teaches that there was never rain prior to the flood. The statement is contextual and meant to be interpreted in relation to Man and the Garden.