Still there after 7km. And the highest mountains on earth are 7 km high so you couldnt physicly grow them any higher.

Something else i should point out, you are only taking in one factor, that is very variable! differant places on earth at different times have different UV levels! one thing consistant with altitude is CO2 and O2 levels, which would have a great affect on the plant, also the temp would play a hugh roll not to mentions the high winds. Come to think of it you couldnt grow plants at high alltitudeds at all without the help of man made devices.
And do you have any statistics that show the UVb light decreases dramaticly enough over 7km?